microelectrochemical characterization of zn, zno and … · microelectrochemical characterization...

Chapter 0: Glossary

1

Microelectrochemical characterization of

Zn, ZnO and Zn-Mg alloys

with online dissolution monitoring

Dissertation

zur

Erlangung des Grades

“Doktor der Naturwissenschaften”

an der Fakultät für Chemie und Biochemie

der Ruhr-Universität Bochum

vorgelegt von

Sebastian Oliver Klemm

aus Wuppertal

Bochum 2011

Chapter 0: Glossary

2

Chapter 0: Glossary

3

1. Gutachter: Prof. Dr. Martin Stratmann

2. Gutachter: Prof. Dr. Wolfgang Schuhmann

Tag der Disputation: 31.8.2011

Chapter 0: Glossary

4

Gewidmet meinen lieben Eltern,

Elke und Reinhard Klemm

Chapter 0: Glossary

5

Acknowledgement The work presented was carried out at the Max-Planck Institut für Eisenforschung GmbH in

Düsseldorf in collaboration with ThyssenKrupp Steel Europe AG, Dortmund. My deepest

compliment for the excellent working atmosphere and the inspiring environment that I

experienced from the first moment on! I particularly express my gratitude to Prof. Dr. Martin

Stratmann for supervising this thesis, for his constant support and the challenging remarks

that proved to be a reliable guidance at all times. I furthermore thank Prof. Dr. Wolfgang

Schuhmann for kindly accepting to act as second reviewer and the time he invested for me.

Prof. Dr. Achim Walter Hassel deserves my explicit thankfulness for being a friendly and

inspiring mentor, and the constant backup on the scientific parquet. All the best for your new

position in Linz! My new group leader, Dr. Karl J. J. Mayrhofer, showed admirable care and

interest from the day I joined his group and I am deeply grateful for the exceptional benefit he

provided. Big thanks go to Dortmund, in particular to Dr. Bernd Schuhmacher, Dr. Janine-

Christina Schauer, Dr. Stefan Krebs, Maria Köyer and Jennifer Schulz from TKS Europe

for an excellent collaboration, many fruitful discussions, and the financial support. Further

thanks are given to Dr. Sascha E. Pust and Dr. Jürgen Hüpkes from the Forschungszentrum Jülich for a very effective and enjoyable collaboration on ZnO. I express my gratitude to the

excellent staff at the MPIE and thank Bernd Schönberger, Cornelia Arckel, Daniel Kurz,

Eberhard Heinen, Rebekka Loschen, Ulrich Wiebusch, the workshop team, and all the

others. Thanks to Bochum at this point, to Gundula Talbot from the Ruhr-Universität for her

great guidance on the admission side. I furthermore appreciate the brilliant scientific

environment I had the pleasure to work in with all colleagues, group leaders and the

fantastic Electrocatalysis-Group composed of Andrea Mingers, Angel Topalov, Anna

Schuppert, Claudius Laska, Hendrik Venzlaff, Ioannis Katsounaros, Josef Meier, Nicole

Fink, and Jay Srinivasan. One member is missing in the former list, but he happens to be the

best friend one can imagine and therefore deserves special mentioning: Arndt Karschin.

Besides him of course, there are many other great friends to thank at this point: Bastian

Huschens, Benjamin Schulte, Daniel Schiffer, Felix Fuge, Jan Lauckner, Jan Spitzley,

Leif Müller, Nils Koenen, Rüdiger von Dehn, and many more. Finally, my warmest

gratitude goes home, to my dear Julia Lengsfeld, for the unbelievable joy of sharing my life

with you! Also to my little brother Alexander Klemm, who I am really proud of at all times. In

the end, I want to mention my parents Elke & Reinhard Klemm, who gave me the greatest

possible trust and support throughout my life. In deep admiration, I dedicate this thesis to you.

Chapter 0: Glossary

6

Chapter 0: Glossary

i

Abstract The primary aim of this study is to utilize microelectrochemical techniques in combination

with time resolved trace analysis to correlate the electrochemical behavior and the dissolution

rate of zinc based materials in order to provide new insights into corrosion processes. For this

purpose, a fully computer controlled scanning flow cell setup is developed utilizing a two-

compartment capillary cell (theta type) with adjustable electrolyte flow. This setup is coupled to

a UV-VIS spectrometer downstream capable of providing time resolved electrolyte analysis. By

using Zincon as a complexing agent, online analysis of zinc and copper in the electrolyte stream

is achieved with a detection limit around 100 nmol l-1. A very good correlation between

electrochemical and spectroscopic data is demonstrated on the example of zinc and copper.

Furthermore, a detailed parameter screening is performed on metallic zinc, covering the impact

of sulfate and chloride anions on the electrochemical behavior and dissolution rate of zinc.

A focus is set on the effect of the pH value on the corrosion and electrochemical response

in aerated buffered and unbuffered electrolytes. The results on zinc are thereby complemented

by investigations on ZnO substrates with large similarities in borate buffered solutions. It is

shown that the dissolution proceeds through a surface oxide under these conditions, with the

electrochemical behavior mainly determined by the rate of oxide dissolution by proton

transport. The dissolution in unbuffered solutions on the other hand is mainly governed by

changes in the surface pH as a consequence of proceeding corrosion processes.

To take full advantage of the high throughput capabilities and the small surface demand of

the capillary tip (~200 µm diameter), Zn-Mg material libraries are prepared by thermal PVD

and characterized by a variety of surface analysis techniques. Linear scans along the

composition gradient reveal strongly non-linear behavior of the electrochemical and dissolution

behavior in both NaCl and borate buffered solutions. The results indicate that additions of

20 at. % Mg are most beneficial for the corrosion resistance of zinc based coatings in

unbuffered NaCl solutions, while a screening in borate buffer highlights the impact of various

magnesium contents on the surface oxides formed.

The high consistency of the obtained datasets underlines the feasibility to perform high

throughput material optimization with the integrated approach taken in this study. Moreover,

the picture presented emphasizes the importance of downstream dissolution monitoring for

correct evaluation of electrochemical data and provides innovations for electrochemical

corrosion testing methodologies.

Chapter 0: Glossary

ii

Content

Glossary ..................................................................................................................... v

1 Motivation .......................................................................................................... 1

2 Corrosion mechanisms.......................................................................................2

2.1 Zinc ................................................................................................................................. 2

2.1.1 General importance ............................................................................................. 2

2.1.2 Effect of solution pH .......................................................................................... 2

2.1.3 Anodic dissolution kinetics................................................................................. 4

2.1.4 Cathodic counter reactions................................................................................. 6

2.1.5 Mixed potential theory ........................................................................................ 8

2.1.6 The impact of surface pH................................................................................... 9

2.1.7 Effect of electrolyte composition .................................................................... 10

2.2 Zinc oxide.....................................................................................................................12

2.2.1 General importance ........................................................................................... 12

2.2.2 The semiconductor electrode........................................................................... 12

2.2.3 Electrochemical decomposition of ZnO........................................................ 16

2.2.4 Chemical dissolution of ZnO........................................................................... 17

2.2.5 Stability of passive film formed on zinc ......................................................... 18

2.3 Zinc-Magnesium alloys............................................................................................... 19

2.3.1 General importance ........................................................................................... 19

2.3.2 Electrochemistry of magnesium ...................................................................... 20

2.3.3 Magnesium oxide ............................................................................................... 20

2.3.4 Beneficial aspects of Mg for zinc corrosion................................................... 21

3 Questions and approach .................................................................................. 24

4 Experimental techniques ................................................................................. 26

4.1 Microstructural characterization ............................................................................... 26

4.2 Sample preparation ..................................................................................................... 29

4.2.1 Preparation of bulk zinc.................................................................................... 29

4.2.2 RF-sputtered ZnO:Al ........................................................................................ 29

4.2.3 Thermal co-deposition of Zn-Mg.................................................................... 30

4.2.3.1 System design and deposition procedure................................................... 30

Chapter 0: Glossary

iii

4.2.3.2 The cosine law and model fitting ................................................................31

4.2.3.3 Composition mapping of material libraries ...............................................33

4.3 Chemicals......................................................................................................................35

5 Development of the scanning flow cell ............................................................ 36

5.1 State of the art..............................................................................................................36

5.2 Design of a flow system .............................................................................................37

5.3 Downstream analytics.................................................................................................40

5.4 Software development ................................................................................................42

5.5 System characterization ..............................................................................................43

5.5.1 Size of the wetted area.......................................................................................43

5.5.2 Validity of microelectrochemical data .............................................................44

5.5.3 Calibration procedures.......................................................................................45

5.5.4 Time delay and peak broadening .....................................................................46

5.5.5 The flow profile at the capillary tip..................................................................49

5.5.6 Summary of the results ......................................................................................53

6 Corrosion of pure Zn........................................................................................ 54

6.1 Unbuffered NaCl solution .........................................................................................54

6.1.1 Open circuit potential and dissolution ............................................................54

6.1.1.1 Effect of chloride concentration .................................................................54

6.1.1.2 Effect of pumping speed ..............................................................................57

6.1.2 Galvanostatic experiments ................................................................................59

6.1.3 Potentiodynamic sweeps ...................................................................................63


6.2 Borate buffers of various pH.....................................................................................66

6.2.1 Open circuit potential and dissolution ............................................................66

6.2.2 Potentiodynamic sweeps ...................................................................................68

6.2.3 XPS-Analysis.......................................................................................................73

6.2.4 The effect of Sulfate anions..............................................................................76


7 Stability of ZnO ................................................................................................ 82

7.1 Chemical dissolution...................................................................................................82

7.1.1 Unbuffered NaCl solution ................................................................................82

7.1.2 Acetate buffer pH 6.0 – 7.0 ..............................................................................85


Chapter 0: Glossary

iv

7.2 Electrochemical dissolution....................................................................................... 90

7.2.1 Unbuffered NaCl solution ................................................................................ 90

7.2.2 Acetate buffered solution.................................................................................. 97

7.2.3 Surface profilometry .......................................................................................... 99

7.2.4 Summary of the results....................................................................................100

8 Corrosion of Zn-Mg alloys ..............................................................................101

8.1 Surface characterization ...........................................................................................101

8.1.1 Optical appearance...........................................................................................101

8.1.2 SEM imaging ....................................................................................................102

8.1.3 XRD analysis ....................................................................................................104

8.1.4 AES maps..........................................................................................................105

8.1.5 Native oxide thickness.....................................................................................107


8.2 Electrochemistry and dissolution............................................................................109

8.2.1 Unbuffered NaCl solution ..............................................................................109

8.2.1.1 Open circuit potentials ............................................................................... 109

8.2.1.2 Zinc dissolution monitoring ...................................................................... 111

8.2.2 Borate buffer pH 7.4 .......................................................................................113

8.2.2.1 Open circuit potentials ............................................................................... 114

8.2.2.2 Potential sweep experiments...................................................................... 115

8.2.2.3 Zinc dissolution monitoring ...................................................................... 119

8.2.2.4 XPS Analysis ................................................................................................ 121


9 Comprehensive discussion..............................................................................124

10 Outlook............................................................................................................128

11 Bibliography ....................................................................................................129

Appendix ................................................................................................................141

Publications ................................................................................................................. 141

Oral presentations...................................................................................................... 142

Poster presentations .................................................................................................. 142

Curriculum Vitae ........................................................................................................ 143

Chapter 0: Glossary

v

Glossary Common abbreviations

AES Auger electron spectroscopy

AFM Atomic force microscopy

ASTM American society for testing and materials

CE Counter electrode

DC Direct current

dll Dynamic link library

EDX Energy dispersive X-ray spectroscopy

ESCA Electron spectroscopy for chemical analysis

ICP-MS Inductively coupled plasma – mass spectroscopy

ICP-OES Inductively coupled plasma – optical emission spectroscopy

FIA Flow injection analysis

FWHM Full width half maximum

GPIB General purpose interface bus

HER Hydrogen evolution reaction

HDG Hot dip galvanized steel

LOD Limit of detection

LPR Linear polarization resistance

LVDT Linear variable differential transformer

OCP Open circuit potential

OER Oxygen evolution reaction

ORR Oxygen reduction reaction

PEC Photoelectrochemical

PLD Pulsed laser deposition

QMB Quartz microbalance

RDE Rotating disc electrode

RDS Rate determining step

RE Reference electrode

RF Radio frequency

RH Relative humidity

Chapter 0: Glossary

vi

RHE Reversible hydrogen electrode

RPM Rounds per minute

RS-232 Recommended standard 232 (serial interface)

SECM Scanning electrochemical microscope

SEM Scanning electron microscope

SFC Scanning flow cell

SHE Standard hydrogen electrode

SVET Scanning vibrating electrode technique

TCO Transparent conduction oxide

UHV Ultra-high vacuum

USB Universal serial bus

UV Ultraviolet

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction

Z Zinc coated steel

ZM Zinc-magnesium coated steel

Formula abbreviations

c Concentration

C∞ Bulk concentration of a species

d Distance

D Diffusion coefficient

E Potential

Eλ Extinction coefficient

EC Energy level of the conduction band

ECorr Corrosion potential

EF Fermi energy

Erd Rest potential in the dark

EV Energy level of the valence band

F Faraday constant

h Planck constant or hour

h+ Electron hole

hS Height over source

Chapter 0: Glossary

vii

i Current density

iCorr Corrosion current density

iDiss Dissolution current density

I Current

IDiss Dissolution Current

k Oxide formation factor or rate constant

Ksp Solubility product

Kw Dissociation constant of water

M Molar mass

N Mole

Nd Molar deposition

p Pressure

r Radius or roughness factor

R Universal gas constant or resistivity

R2 Coefficient of determination

Rd Deposition rate

t Time

Vf Volume flow rate

z Charge number

Greek letters

α Transfer coefficient

β Sharpness parameter

δ Difference

δN Diffusion layer thickness

η Overpotential

θ Incidence angle

λ Wavelength

ν Frequency or scan rate or viscosity

π Circle constant

ρ Density

χ Molar fraction

ω Angular velocity

Chapter 1: Motivation

1

1 Motivation The interest in the corrosion of zinc dates back centuries, always fueled by new and

optimized applications with superior performance for corrosion protection to utilize this

complex system. The enormous role of zinc in daily life and the number of secrets that zinc

tends to release reluctantly continues to drive every scientist working in this field.

Within the large community dealing with this material, the present study aims to provide

innovative technical developments in the field of corrosion research and improved insights into

degradation mechanisms in aqueous environments. The experimental setup designed for this

purpose combines a microelectrochemical flow cell, downstream analytics and a fully

automated positioning and measurement execution routine. This combination allows a very

high experimental throughput with integrated data processing that effectively transfers

manpower from data acquisition to data evaluation. The variety of substrates investigated in

this system and the ease of implementation of other downstream analytics hopefully extends

the reach of this concept beyond the application for zinc corrosion monitoring and potentially

supports other fields of applied electrochemistry.

The comprehensive system characterization is seamlessly followed by a thorough

investigation of zinc corrosion in buffered and unbuffered solutions of near neutral pH to

highlight the importance of this parameter, utilizing the essential capability to execute large

numbers of experiments with parallel dissolution recording. Since zinc oxide plays an essential

role as a corrosion product under these conditions, bulk samples of Al doped ZnO are

investigated with identical methodology to complement the mechanistic insights obtained. The

findings on ZnO are furthermore of high relevance for tuning its optical properties, e.g. for the

application in thin film solar cells.

In order to contribute to the state of the art in corrosion protection, a material optimization

of Zn-Mg alloys obtained by thermal PVD is described that picks up recent industrial trends

for novel zinc based coating technologies. The extensive datasets are fundamentally discussed

on the basis of the results on Zn and ZnO and demonstrate the beneficial effect of magnesium

as a function of the alloy composition.

In the progression of this work, it quickly became clear that sole electrochemical

measurements are sometimes misleading if not coupled with a complementary analysis

technique. It is occasionally confusing, but more often rewarding if processes are not as simple

as initially presumed.

Chapter 2: Corrosion mechanisms

2

2 Corrosion mechanisms Corrosion is the destructive results of chemical reaction between a metal or metal alloys and

its environment [1] (p. 5). The following chapter gives an introduction into the corrosion

mechanism of Zn, ZnO and Zn-Mg alloys. The fundamental aspects of electrochemical

reactions and chemical dissolution for these substrates are discussed with a clear focus on the

destructive degradation in aqueous media, with an overview about the general application of

each system provided at the beginning of each section.

2.1 Zinc 2.1.1 General importance

Zinc is among the metals with the highest worldwide production, and approximately 50%

are used for corrosion protection [2] (p. 1). It is the primary coating material for steel with a

large variety of coating techniques and subsequent treatment procedures [3]. The sole

dimension of industrial production constitutes a major driving force for research and

development because each improvement scales drastically, and leads to the prospect of a

significant economic gain.

However, the literature on zinc corrosion is remarkably extensive due to the variety of

parameters, coating systems, experimental procedures and the combination of fundamental

studies and application oriented investigations. Understanding the corrosion mechanism and

improving the protective properties with respect to the exposure conditions is therefore a

scientific aim with high actuality despite the extensive efforts invested over the last decades.

2.1.2 Effect of solution pH The most decisive factor for the stability of zinc in aqueous media is the pH value. A

fundamental illustration of this dependency derived from chemical equilibria is given in the

Pourbaix diagram. This diagram describes the relative dominance of species in a particular

potential-pH region. The numbers adjacent to vertical and horizontal lines indicate the

logarithm of the zinc ion concentration in solution. The reaction (a) shows the onset potential

for hydrogen evolution (HER) and (b) the reversible potential of the oxygen evolution reaction

(OER).


3

Figure 2.1: Potential – pH

equilibrium diagram (Pourbaix

diagram) for zinc, from [4].

Potentials refer to the standard

hydrogen electrode.

It is apparent that metallic zinc falls below this stability window of water, but exhibits kinetic

stability due to the low exchange current density of hydrogen on zinc [5]. The shift of the

vertical lines (6) to the left for higher zinc concentration indicates that zinc precipitation occurs

at lower pH values for higher zinc concentrations since the solubility product is reached earlier.

This concentration dependence implies that the passivity of zinc originating from Zn(OH)2 and

ZnO (formed by dehydration of the former [6, 7]) is subject to changes of the zinc

concentration due to dissolution processes as well as pH changes induced by the cathodic

counter reaction in a corroding system (see page 9). A characteristic illustrated in Figure 2.1 is

the amphoteric nature of zinc-hydroxide. Being able to accept protons towards mono-hydroxy

complexes (and ultimately Zn2+ species, reaction 6) or hydroxyl-ions giving [Zn(OH3)]-

(referred to as the dehydrated HZnO2- in Figure 2.1), the dissolution rate is mostly governed by

the formation of soluble complexes and approximately U-shaped along the pH-axis:

Figure 2.2: Dependence between

current density for zinc corrosion

and pH value in de-aerated NaCl

(0.1 M) solution. From [8].

For environmental corrosion, pH values above 12 are unlikely, even though a pH above 10

was reported in the vicinity of the cathodic area (oxygen reduction) in a zinc-steel couple [9].

This leaves the transition from an active to a passive state in near neutral pH regions as the

dominant concern for most zinc applications.


4

2.1.3 Anodic dissolution kinetics Zinc ions exhibit a valence of 2 exclusively in all natural compounds [10] and a redox

potential of -0.763 VSHE. Zn2+ ions in aqueous media prefer tetrahedral coordination with a

hydration number of 10 – 12 [2]. The kinetics of electron transfer and liberation of ionic zinc

species from the surface have been extensively studied in both alkaline [11-13] and neutral to

acidic media [14-16]. All these studies agree on the presence of a monovalent species which is

generated on the surface according to [15]

Zn −+ + eZnads (2-1)

or [13]

−+ OHZn −+ eOHZn ads)( (2-2)

depending on the pH value. The subsequent steps of (1.) further oxidation and (2.) desorption

from the surface are generally rate determining.

The mechanistic considerations in all studies consult the Tafel slopes measured as an indicator

for the reaction pathway. To give an insight into this method, the Butler-Volmer equation is

required [17, 18]

][ RF)1(

RF

0

ηα

ηα

Tn

Tn

eeii−

−−= (2-3)

with a dependence between logarithmic current density and overpotential (at large anodic

overpotentials where the reduction reaction is almost suppressed) given by the Tafel equation:

ηαT

niiR3.2F)log()log( 0 += (2-4)

(Note that the factor of 2.3 reflects the conversion from ln to log)

This equation including the anodic Tafel slope

F

R3.2nTba α

= (2-5)

is only applicable for a one step, one electron reaction. Neither of these characteristics is

fulfilled for the total anodic dissolution reaction of zinc. Therefore, the whole reaction scheme

needs to be taken into account with kinetic considerations for both chemical and

electrochemical equilibria at each individual step.

In order to reduce the large complexity arising from the combination of several subsequent

reactions, the following assumptions are usually made [19]:

(i) The overall reaction is governed by one rate determining step (RDS), (ii) all steps

preceding and following the RDS are in virtual equilibrium, (iii) all energy barriers are

symmetrical, and (iv) the surface coverage with intermediates is very small.


5

For the zinc dissolution via ZnO by Johnson et al. [20] in neutral media, the relevant

reactions including surface (s) and hydrated (aq) species are

OHZn s 2)( + −+ ++ eHZnOH s)( (2-6)

)(sZnOH −+ ++ eHZnO s)( (2-7)

OHZnO s 2)( + −+ + OHZn aq 2)(2 (2-8)

with the last step rate determining. The concentration of water can be assumed as constant and

will be included in the rate constants in all further considerations. Since the chemical

dissolution of ZnO determines the current signal, iZn can be expressed as

22´33 ]][[][ −+−= OHZnkZnOkiZn (2-9)

The concentration of the intermediate ZnO can be calculated by

)RT/Fexp(]][[][ 1

´2

2 η−+= HZnOHkkZnO (2-10)

similar to the prominent example of FeOH(ads) intermediates presented by Heusler [21] and

Bokris et al. [22]. [ZnOH] can be calculated in the same manner giving

)RT/Fexp(]][[][ 1

1́

1 η−+= HZnkkZnOH (2-11)

Assuming [Zn] constant and combining equations (2-9) to (2-11), the overall anodic reaction

rate becomes

22´3

1́

12

´2

23 ]][[)RT/F2exp(][ −+−+ −= OHZnk

kkH

kkkiZn η (2-12)

with a potential dependence of exp(2ηF/RT) and a Tafel slope of 2.3RT/2F (~ 30 mV dec-1)

[20]. Note that a pH dependence exists for the formation of intermediates as well as the

dissolution of ZnO.

In the absence of a slow desorption step, the formation of Zn2+ ions becomes rate

determining [14]. The fundamental reaction scheme then simplifies to

Zn −+ + eZn (2-13)

+Zn −+ + eZn2 (2-14)

which yields the following dissolution current density for zinc:

)RT2/F3exp(´2

2

1́

1 ηkk

kkiZn = (2-15)

(Tafel slope 4.6RT/3F, ~40 mV dec-1). All these considerations are facing three major

challenges:


6

I. Several mechanisms can yield the same Tafel slope and reaction orders [11].

II. A high experimental precision is required to distinguish different Tafel slopes.

III. The electrolyte composition may affect the reaction mechanism.

The last point in particular adds a high degree of complexity since all electrolyte constituents

interacting with the metal/metal ions need to be considered. This issue will be addressed in

section 2.1.7. Please note that all mechanism described require the aforementioned assumption

that the surface coverage with intermediates is small. This implies that the described dissolution

mechanism via ZnO is not valid for oxide covered electrodes, which follow significantly

different dissolution kinetics. A detailed discussion of this situation is given in the respective

section for ZnO dissolution (2.2.3 ).

2.1.4 Cathodic counter reactions Zinc exhibits a remarkably low exchange current density for hydrogen in the range of

109 A cm-2 [23], which both results in low corrosion rates in de-aerated media [24] (pH 4-11)

and the possibility to deposit zinc from aqueous solutions with acceptable faradaic efficiency

despite the low redox potential of zinc. While hydrogen evolution is observed at large cathodic

overpotentials during cathodic polarization, the role of this reaction within the potential limits

present in the corrosion case is negligible except for very low pH values. As a consequence, the

oxygen reduction reaction (ORR) is the dominant cathodic counter reaction in aerated media

of moderate pH. Since environmental corrosion almost exclusively proceeds under these

conditions [1], the significance of the ORR for zinc corrosion is decisive.

Two general characteristics of the oxygen electrode can be stated [18]:

I. Large overpotentials (several hundred mV) are observed even at low current densities, a

fact that accounts for the ongoing and intense research for effective catalysts in fuel

cells [25].

II. Two reaction pathways can be roughly separated by either the presence or absence of

desorbable peroxyl species.

On most metals and metal oxides (including zinc), these peroxyl intermediates are observed

[26]. The corresponding reactions are

−−− +→++ OHHOeOHO 222 2 (alkaline solutions)

222 22 OHeHO →++ −+ (acidic solutions) (2-16)


7

followed by either the further reduction

−−− →++ OHeOHHO 3222 (alkaline solutions)

OHeHOH 222 222 →++ −+ (acidic solutions) (2-17)

or a decomposition reaction without electron uptake.

22 22 OOHHO +→ −− (alkaline solutions)

2222 22 OOHOH +→ (acidic solutions) (2-18)

The distinction between the 2 and 4 electron mechanism is based on the possibility of

peroxyl-species to desorb from the surface and does not imply that the electron transfer

reactions proceed in one step. Contrariwise, it is even reasonable to assume a “one electron at a

time” mechanism in each case [17, 18, 26]. Due to the limited availability of oxygen in aqueous

solutions (especially in solutions of high ionic strength) and the high standard potential of the

oxygen electrode, the reaction rate quickly reaches transport limitations when coupled to

cathodic half cell reactions like metal dissolution. The following figure shows the idealistic

potential-current density relation for transport limited oxygen reduction on zinc, including the

potentially interrupted reaction pathway (equation (2-16)) [18, 27]:

Figure 2.3: Schematic shape of

the current density - voltage curve

for oxygen reduction on zinc

including the generation of

peroxyl- intermediates

The variety of reactions involved in combination with changes of the zinc surface induced

by the applied potential causes the experimental data presented by different authors to deviate

from the idealistic case shown in Figure 2.3. Wroblowa et al. [28], Deslouis et al. [27] and

Hausbrand et al. [29] experimentally demonstrated the presence of two transport limited

regions in the cathodic polarization curve, but observed both non-ideal plateau regions

(showing inflection or inclination). Furthermore, the relation between the limiting current


8

density was seldom reported as 1:2 which would be the theoretically expected value. The role

of pre-treatment prior to the experimental run possibly inducing surface changes of the zinc

electrode, is of particular importance in the case of corroding zinc surfaces where the electrode

morphology, the nature and degree of coverage (oxides, precipitates etc.) and the composition

of the surrounding electrolyte are highly dynamic [24]. The outlined complexity of the ORR on

zinc therefore needs to be carefully considered when a correlation between oxygen reduction

and observed corrosion rates is approached.

2.1.5 Mixed potential theory The mixed potential theory explains the dynamic equilibrium behavior in multi component

systems and can be applied to illustrate the electron transfer processes in a corroding system.

Since the net current at the corrosion potential as measured by external devices is zero, the

overall electron consumption by reduction processes on the surface exactly equals the rate at

which electrons are provided by anodic counter reactions (in most cases metal oxidation). This

simultaneous (thus short circuited) reaction of three redox-couples is illustrated in Figure 2.4.

This scheme was constructed to fundamentally illustrate key aspects of zinc corrosion in

neutral, aerated media. A similar figure was not found in the literature, most probably because

several assumptions need to be taken which are not experimentally accessible. The latter fact is

indicated in the figure, showing that the total current is governed by the local dominance of

different reactions, while the exchange current densities lie well beyond the experimental limits.

Potential / V

2,Ooi

Znoi ,

corri

ZnE

2OEcorrE

ORR

OER

log

(i / A

cm

)

-0.8

0

-1

-2

-3

-4

-5

-6

-7

-8

-9

0.8-0.4

-2

HE2

2,Hoi

(a)

(a)

(b)

(c)

(d)

(e)

experimentallyaccessible

SHE Figure 2.4: Schematic illustration of the origin of the corrosion potential (Ecorr) and corrosion current density

(icorr) in case of zinc corrosion in the presence of oxygen using the mixed potential theory.


9

All three redox couples exhibit an exchange current density and Tafel slope which have

been estimated in the graph according to [5] for hydrogen and zinc in de-aerated sulfate

solutions, while the kinetics of the oxygen reduction are only roughly estimated since only the

transport limits [27, 29] of this reactions are relevant. The standard potentials are calculated for

a pH of 7 and standard conditions concerning the concentration of dissolved species.

The dashed lines (a) indicate reactions which are usually limited by a very low concentration

of the respective species (zinc ions in solution for zinc reduction and molecular hydrogen for

hydrogen oxidation). However, hydrogen reduction (b) becomes dominant in case of high

cathodic overpotentials, constituting the lower potential limit for the experimental conditions.

The transport limitation of the ORR exhibits two plateaus, where the lower one (c)

corresponds to the reduction of oxygen to peroxide and the higher (d) corresponds to the

reduction to water. These reactions are decisive for the corrosion current density and corrosion

potential (e) because the anodic dissolution of zinc shows very fast kinetics with steep Tafel

slopes. Hydrogen reduction is of minor importance for the corrosion rate due to the very low

exchange current density, and is superimposed by oxygen reduction by several orders of

magnitude.

The illustration assumes that the corrosion potential falls within the first plateau region in

the ORR (production of peroxide intermediates, (c)) which appears reasonable for a corrosion

potential between -700 and -800 mV as reported by Deslouis et al. [27]. Futhermore, Boto and

Williams reported the generation of hydrogenperoxide during zinc corrosion depending on the

solution pH and composition [30]. A particularly interesting conclusion was that the formation

of precipitates (at pH > 5.6) promotes the generation of peroxide species besides the obvious

interference with oxygen transport processes.

2.1.6 The impact of surface pH Corroding systems in unbuffered solutions of moderate pH (5-9) induce pH changes in the

vicinity of the electrode by oxygen reduction (pH increase) or the formation of zinc-hydroxide

complexes (pH decrease) because the impact of species generation (corrosion rate) is

significant compared to the low proton or hydroxide concentration. The corrosion reaction

therefore affects itself through a variety of feedback pathways (e.g. precipitate formation, salt

agglomeration, local pH differences etc.). The processes shown in Figure 2.4 are therefore

highly dynamic, as the standard electrochemical potential of all redox couples alter upon

changes in zinc concentration and pH value. The exact concentration gradients of all relevant

species as a function of the distance to the electrode are affected by several mechanisms like


10

diffusion, convection and migration [31] (pp. 361) [17] (pp. 28). Even though the convection

on electrodes can be theoretically well controlled by use of the proper setup (e.g. RDE, Flow

cells), it is still affected by changes of the electrode surface like precipitate formation. This

generation of surface films is furthermore of immediate impact on all transport other processes

and is of high relevance for corrosion phenomena.

In the absence of convection, the diffusion processes at the electrode become dominant (as

migration is usually negligible due to the high electrolyte conductivity and the low electric field)

and cause a highly dynamic diffusion zone which usually not considered in corrosion

experiments in stagnant electrolytes. This complexity is most likely the origin of the challenging

reproducibility of measurements under the conditions described [2, 30].

To illustrate the magnitude of surface pH changes, a very recent study has shown that the

surface pH in case of hydrogen evolution changes from 7 (bulk pH) to approximately 8.3 at

5 µA cm-2 and 8.6 at 10 µA cm-2 [32] as determined by the occurrence of a current plateau

region. The experiment was carried our using a rotating disc electrode at 1600 rpm with a

comparably thin diffusion layer. The magnitude of surface pH changes is expected to be even

higher with less or absent convection.

In case of zinc corrosion, the corresponding reaction would be the oxygen reduction

reaction, which however affects the pH similarly. Please note that 10 µA cm-2 is a low corrosion

current density compared to zinc corrosion in aerated NaCl solutions (see section 6.1.2).

Another origin of local pH changes in the vicinity of a surface is the consumption of

protons or hydroxides by chemical reactions (e.g. proton consumption by the dissolution of

Zn(OH)2). The local depletion of these species is therefore just another expression for a change

in the surface pH, even though the former term appears more applicable to describe the

fundamental process.

2.1.7 Effect of electrolyte composition Besides the solution pH as a decisive factor for precipitation, passivation and dissolution of

zinc in aqueous solutions, further processes arise from the presence of other ionic species that

interact with the metal cations. According to Gerischer [33, 34] and Heusler [21], the anodic

current density for metal dissolution equals

∏ ⋅⋅= ++

lylKl

xMe zccki /RT)Fexp( ηα (2-19)

for a single electron transfer with cMe as the surface concentration of reactive metal ions in the

metal, cKl as the concentration of a complexing agent K, and x and y as the respective reaction


11

orders. Since the total current density is the sum of all partial reactions with individual

complexing agents, it is possible that e.g. a rate determining step requiring OH- can be

superimposed (and therefore essentially replaced) by a parallel sequence including Cl-. The

impact of anions on the reaction mechanism can be estimated from the large spread in Tafel

slopes for zinc dissolution for different media, ranging from around 15 to 120 mV dec-1 [2, 14,

20]. For sulfate and halide anions in particular, the Tafel slopes reported are usually low (15-

30 mV dec-1), resulting in large reaction rates at comparably low overpotentials.

Since the electrochemistry of zinc is strongly influenced by surface film formation except

for either very low or high pH values, the effect of electrolyte constituents needs to be

considered in this respect as well. Two classes can be systematically separated:

I. Ions that promote film dissolution, either by complex formation or by a negative

impact on the stability of surface film. Examples are sulfate ions [35] as comparably

bulky species with a high complex formation constant [2] or chloride ions [36] that

induce pitting of the film.

II. Ions that promote zinc passivation, generally achieved by the formation of insoluble

products like carbonates [37, 38] and chromates [39].

Another characteristic besides the solubility product of zinc complexes is the morphology of

precipitates in the presence of the respective species, especially concerning the ability to

prevent the exchange of electrons or ionic species by a barrier effect of the surface film. This

characteristic can be systematically divided into a hindrance of direct dissolution by the

corrosion products (e.g. by a porous precipitate layer), or a dissolution through the barrier layer,

proceeding indirectly by removal of surface species from the compact film accompanied by

film growth at the metal surface.

An additional aspect arises from the fact that several electrolyte constituents are weak

electrolytes, with protons or hydroxyl-ions involved in the dissociation process. A buffer effect

therefore results from the ability to liberate or consume protons or hydroxyl ions in the pH

dependent chemical equilibrium (e.g. hydrogensulfate – sulfate). In the case of dissolved gasses,

this process may accelerate mass exchange between aqueous solution and gas phase depending

on the solution pH (e.g. increased CO2 uptake in alkaline solution by formation of

hydrogencarbonates).

Carbonates are playing a major role for zinc corrosion due to the variety of insoluble zinc-

carbonate compounds [40, 41]. This fact combines with the inevitable carbonate uptake of

aqueous solutions under aerated conditions and needs further clarification. According to


12

Conway and Kannangara [7], an increased passivity range of zinc along the pH-axis can be

observed in the presence of carbonate as shown in Figure 2.5.

Figure 2.5: Combined

Pourbaix diagrams for

zinc and carbonate

(0.1 mM and 10 mM

respectively) at 298 K.

From [2].

Clearly, the effect of carbonate ions in the region of neutral pH values is critical for

atmospheric corrosion in general and needs to be considered in these cases.

2.2 Zinc oxide 2.2.1 General importance

The electrochemistry and stability of zinc oxide is of major relevance for corrosion science

since most environmental degradation processes of zinc and its alloys are accompanied by the

formation of oxidic surface films [2, 42, 43]. Besides this presence as a corrosion product, zinc

oxide is frequently used as a transparent conduction oxide (TCO) for light emitter applications

[44], varistors [45] or photovoltaic applications [46]. Especially the use in thin film solar cells is

fundamentally related to the corrosion of ZnO as the optical properties are tuned by artificially

roughening the surface in an etching step, principally being desired corrosion [47]. The stability

of zinc oxide in aqueous solutions is therefore of high interest for a variety of applications,

those being spread along several technical fields.

2.2.2 The semiconductor electrode While a metal electrode causes a space charge layer in the vicinity of the surface when

immersed into an electrolyte containing a redox couple of different chemical potential

compared to the Fermi level, this situation is reversed in the case of a semiconductor due to the

lower charge carrier density compared to the surrounding medium [18] (p. 124). This process is


13

illustrated in Figure 2.6 on the example of an n-type semiconductor (with the Fermi level close

to the conduction band) in contact with an oxygen redox couple in solution.

E H O / O , pH 72 2

EF, equil

(a) energy levelsbefore contact

(a)

(a)

(b) energy levelsafter equilibration

(b)

(b)

Space charge layer

Excess ionsin solution

Potential / VSHE

Distance into semiconductor Distance into solution

Figure 2.6: Schematic illustration

of the space charge layer generated

after immersion of an n-type

semiconductor into a solution

containing oxygen as the redox couple.

The electron transfer during

equilibration of the Fermi levels shifts

the band positions from (a) to (b).

Due to the low charge carrier density, i.e. the electrons in the conduction band for an n-type

semiconductor, a local depletion occurs at the surface as the Fermi levels equalize. Electrons

are transferred into the solution leaving a positive space charge behind that reaches 1-100 nm

deep into the semiconductor [48]. The positions of the band edges in contact to the electrolyte

are, in good approximation, not altered as the energy levels inside the semiconductor shift [18]

(p. 125).

This type of illustration is very common when discussing processes at semiconducting

electrodes and requires combining the Fermi energy levels in the band structure and the

electrochemical potential of the redox couple onto a common energy axis. This combination is

of tremendous importance because the magnitude of band bending in the surface charge layer

and the position of the band edges in relation to the redox system are main determinants of the

electrochemical behavior observed. The most practical approach utilizes the standard hydrogen

electrode as the common electrochemical reference and introduces the Fermi scale by

calculating the energy released when an electron from the vacuum level is brought to the

potential of the hydrogen electrode. This value was found to be 4.5 eV, which, in sufficient

approximation [48], allows to transform Fermi energies into a potential vs. SHE as shown in

Figure 2.7.


14

E H O / O , pH 7

Fermi scale /eV

Electrochem. scale /V

0 -4.5

-2.25 -2.25

-4.5 0

-6.25 1.75

SHE

3.9 -0.6

7.1 2.6

0.842 2

Conduction band

Valence band

EF, ZnO

Figure 2.7: Comparison

of the Fermi energy scale

and the electrochemical

potential vs. SHE. The

values given for the

conduction and valence

band are taken for ZnO

[49] while the

electrochemical potential of

the oxygen electrode was

calculated for pH 7.

The energy levels for the conduction band (EC) and valence band (EV) including the band

gap of 3.2 eV are material constants while the Fermi level (EF) is strongly dependent on defects

and dopants [2, 50]. This causes EF in Figure 2.7 to be an artificial value close to EC reflecting

n-type characteristics of ZnO.

The difference between the Fermi level of ZnO and the redox potential of oxygen in

solution ranges around several hundred mV and causes electrons to be injected into the

solution (see Figure 2.6) in case of immersion due to the more cathodic potential of EF

compared to the redox couple. This electron transfer is mainly governed by the limited charge

carrier density in the electrode for semiconductors, which differs significantly from metal

electrodes where the population density of the activated complex is usually rate determining

[17] (pp. 92).

Furthermore, charge is possibly transferred by electrons and holes, where the latter can be

principally considered as a missing electron. The ratio between electron and hole transport

fundamentally depends on the density of these states, causing electrons to be the majority

charge carrier in case of n-type semiconductors and therefore being of dominant relevance in

the ZnO/O2 couple discussed previously.


15

E H O / O , pH 72 2

F, η

Energy distributionfor oxidizedredox species

Energy distributionfor reducedredox species

Potential / VSHE

Distance into semiconductor Density of states

E η

η

η

E CB, η

E VB, η

E F, equil

e transfer(reduction)

e tunneling(reduction)

oxidation

Figure 2.8: Schematic

illustration of electron

transfer processes between

ZnO and oxygen in the

absence and presence of

cathodic polarization (η)

assuming a symmetrical

density of states distribution

between the oxidized and

reduced form of the redox

system for simplicity.

As the degree of band bending and the position of the Fermi level reflect a density of states

concerning charge carriers, it is useful to express the redox couple from a similar perspective.

Figure 2.8 contains the energy of reduced and oxidized redox species with a Gaussian shaped

distribution as a function of fluctuations in the electric field due to the movement of solvent

and species themselves [51]. The identical shape and integral of these distribution functions for

both oxidation states are assumed for simplicity in these theoretical considerations but are far

more complex in real systems [48], especially due to different activities of these species as the

main determinant for the integral.

Two main transfer processes are illustrated in the above figure [42] (p. 238). The first one is

the tunneling of electrons of equal energy level through an energy barrier caused by the space

charge region and the second is the direct transfer at the band edges (which is still based on

tunneling processes, but between electrode and redox species [51]). It is evident from Figure

2.8 that an external polarization η (cathodic in the respective figure) causes the energy levels to

shift and therefore increase the population density of the majority carriers in the region of

charge transfer to the oxidized redox species. This increase in population density scales

exponentially with respect to the overpotential (Boltzmann distribution) and therefore causes

an exponential relationship between applied potential and current, a dependence familiar from

metal electrodes.


16

In case of anodic overpotentials (which manifest in a downwards shift of the energy levels

in the semiconductor in Figure 2.8), the population density of electrons in the conduction band

is decreased, first allowing electron uptake from the reduced redox species and ultimately

leading to an increase of holes in the valence band and a direct hole transfer causing oxidation

of the redox system. Since this process needs to overcome the intrinsic inhomogeneity of the

charge carrier density (excess of electrons) in an n-type semiconductor, a comparably large

overpotential is required, called the blocking region. Due to the dependence of the band

bending on EF, this blocking region is strongly affected by the dopant and defect density in the

electrode [52].

2.2.3 Electrochemical decomposition of ZnO The electrochemical decomposition of bulk ZnO, mostly referred to as film breakdown [2]

(pp. 107), occurs at anodic potentials with the breakdown potential depending on a variety of

parameters (e.g. ionic species, film composition and –structure, dopants).

As pointed out in the former section, anodic polarization shifts EF upwards on the potential

scale (downwards in Figure 2.8 due to an inverted y-axis) promoting oxidation reactions on the

surface [53]. Electron exchange with the conduction band is only possible if an oxidizable

redox species exhibits a significant density of states around the conduction band edge,

requiring rather negative redox potentials [54]. For ZnO in typical aqueous media (e.g.

solutions of NaCl, Na2SO4, etc.), this is not the case.

One oxidation processes is therefore the oxygen evolution reaction (OER) by an exchange

process with the valence band:

++ hOH 22 2212 OH ++ (2-20)

It is important to note that this reaction induces a pH shift on the surface by generation of

protons with immediate effect on the stability of zinc oxide [55] (see Figure 2.1 and section

2.1.6). Another possible pathway proceeds by direct lattice decomposition according to

equation (2-21).

++ hZnO 2 22

21 OZn ++ (2-21)

Both processes proceed in by tunneling of holes and therefore charge exchange with the

valence band as reported by Pettinger et al. [52]. It has been stated that both reactions take

place in parallel, even though the ratio has not been specified.

Due to the photoeffect [56] that occurs on zinc oxide upon irradiation with violet and more

energetic light (387 nm light equals a photon energy of 3.2 eV exactly matching the band gap),


17

a charge separation by excitation of electrons into the conduction band causes both electrons

and holes to be available for charge exchange with the solution. This photoelectrochemical

(PEC) effect is principally capable of lattice decomposition (photo dissolution), but requires

comparably high irradiation intensities (several mW cm-2 UV-irradiation) to proceed at

significant rates [57], which is why this effect has been neglected in the laboratory environment.

2.2.4 Chemical dissolution of ZnO ZnO is a polar crystal and exhibits different bonding states of surface atoms depending on

the crystal orientation. This effect originates from the tetrahedral coordination of atoms and is

illustrated in Figure 2.9.

O

Zn

O

Zn

O Odipole

moment

Zinc terminated surface Electron acceptor

Oxygen terminated surface Electron donor

Zn Zn

Oxygen:Always 3bonds up,1 down

Zinc:Reversedcase

Figure 2.9: Origin of polarity in

III-V compounds (zinc-blende

structure) for a <111>

crystallographic orientation and the

resulting surface termination for

different facets. The continuous

stacking of oxygen and zinc layers in

the crystal body between the surfaces is

indicated by a broken vertical bond.

The figure illustrates that zinc directs three of its four bonds into the direction of the oxygen

terminated surface throughout the crystal. Since all bonds are polar, this unequal distribution of

bonds along the vertical axis results in a dipole moment.

A further consequence of the directional distribution of binding orbitals is the surface

termination [58]: Because each atom directs three bonds into one direction and only one in the

other along the dipole moment, the surface is composed of the species which are triply

“anchored”, that is oxygen in the lower and zinc in the upper surface in Figure 2.9. Even under

continuous etching, a removal of stable surface atoms (e.g. zinc) results in an exposition of the

second, therefore unstable species (e.g. oxygen) which is only held by a single bond and

therefore easily removed subsequently. As a consequence of the different surface termination

with either zinc or oxygen, both surfaces react strongly different to chemical etching because of

the presence (oxygen) or absence (zinc) of dangling electrons [59]. The former is generally

susceptible to electron acceptors (e.g. H+) while the latter is primarily etched by electron donors


18

(e.g. OH-). It is to mention that all ZnO surfaces are dissolved by H+ and OH- due to the

instability of zinc in extreme pH regions, but the rates may differ significantly [60].

In case of polycrystalline materials with a large degree of intrinsic heterogeneity like

sputtered [61] ZnO or grown surface films on zinc [2] (p. 115), the individual etching behavior

of different crystal orientation looses significance. Gerischer and Song [62] performed

experiments with sintered ZnO pellets and found a linear relationship between pH and

logarithmic dissolution rate in acidic media below pH 5. Fruhwirth at al. [63] discussed a linear

dependence between dissolution rate and pH on the basis of the flat band potential, even

though the chemical dissolution in the dark is supported by a comparably thin data set (e.g.

linear fit over two data points, Fig. 2 in the respective publication). The pH dependent

chemical conversion of zinc oxide into soluble species can be expressed as [4]: +2Zn +)]([ OHZn ZnO −])([ 3OHZn −2

4 ])([ OHZn (2-22)

Besides this pH dependency, the ionic species in the electrolyte need to be considered since

soluble or insoluble zinc complexes might result from the presence of specific anions [64] as

previously described in section 2.1.7.

2.2.5 Stability of passive film formed on zinc Oxides grown on metals constitute a special case since they are polycrystalline or

amorphous and do not have a homogeneous composition [60]. Furthermore, a dissolution

process at the interface oxide-electrolyte occurs simultaneously with an oxide growth at the

metal-oxide interface, resulting in a net transport of metal cations through the film. In order to

obtain electrical neutrality, the counter reaction, i.e. reduction of an electron acceptor, needs to

proceed at equal rates. This is accomplished by a potential difference between metal and

solution, being composed of two potential differences at the interfaces metal-oxide and oxide-

electrolyte, and a potential drop across the oxide layer. In case of a corrosion current density

independent of the applied potential, i.e. the ideal passive case, the potential drop at the oxide-

electrolyte interface is unaffected by the applied potential as shown by Wagner on passive iron

in acidic solutions [65]. Therefore, the potential drop is mostly located within the oxide itself,

causing the thickness to increase to maintain steady field strength. This situation is comparable

to the oxide growth mechanism on valve metals where the oxide thickness is a function of the

applied potential [66].

It is to note that the electrode kinetics at the oxide-solution interface, with the release of

metal ions in particular, is affected by several factors. Complexing agents (e.g. Cl- [65]) as well

as transport limitations (e.g. H+ [67]) may alter the dissolution rate significantly, while the latter


19

becomes dominant in case of precipitation reactions near the surface. Thus, the steady-state

corrosion current density is completely determined by the reactions at the outer interface [68].

For passivation of zinc in KOH solution, Dirkse conluded that the criteria for passivation is

the ability of the surrounding electrolyte to dissolve ZnO or Zn(OH)2 formed upon anodic

polarization [69]. If the solution is unable to take up Zn-complexes equal to the formation as a

function of the applied potential, the electrode passivates. If high anodic potentials are required,

the resulting films are metastable and dissolve if the applied voltage is interrupted [2]. Since the

solubility of zinc is the determining factor in these considerations, the electrolyte composition

is of uttermost importance (see section 2.1.7) and the corrosion current density a function of

the chemical dissolution of the oxide under these conditions [69-72].

It is important to note that reprecipitation, i.e. formation of ZnO or Zn(OH)2 from the

solution, does not passivate the surface due to the porous nature of the film formed [7, 70].

This surface reaction is denoted as type II film growth, while the passivating type I requires

direct oxide formation on the surface. Because film dissolution needs to be sufficiently small

for a type I formation, it usually occurs after Type II has been formed since the solution in the

vicinity of the electrode is essentially saturated in this case.

2.3 Zinc-Magnesium alloys 2.3.1 General importance

With the aim to improve the protective or mechanical properties of zinc based coatings as

part of the optimization process, various alloying elements were introduced. Common

candidates for this purpose are cobalt [73], aluminium [74], manganese [75], nickel [76] and

magnesium [77].

The improvement of the corrosion resistance is thereby mainly based on the formation of

more stable corrosion products as compared to pure zinc, resulting in a significantly higher

resistance of the coating against dissolution. Magnesium is a very promising metal as several

studies reported superior corrosion resistance of Zn-Mg coatings in combination with the

suppression of paint delamination processes [77-79].

Because an improvement of the protective properties can be generally used to reduce the

coating thickness, it is of special relevance for the automotive industry as this technical

application includes a variety of deformations, welding procedures and corrosion sensitive

geometries (e.g. sections and crevices between panels) [77]. It is furthermore of economical

interest as Mg itself is not an expensive metal and available in large quantities.


20

2.3.2 Electrochemistry of magnesium Magnesium is a very active metal with a low redox potential of -2.372 VSHE and a corrosion

potential in neutral solutions around -1.5 VSHE [80]. The electrode surface is usually covered

with a surface film of low conductivity, therefore limiting the corrosion rate in rural

environment to values between Al and low carbon steels [81]. Even though magnesium ions in

solution exhibit a valence of 2, several authors proposed the release of monovalent ions as part

of the dissolution process in conjunction with a subsequent reaction with water [81, 82].

Independent of the reaction mechanism, the cathodic counter reaction is the reduction of

water/protons, being largely unaffected by the oxygen concentration [83]. The corrosion rate in

aqueous environments is therefore dependent on the stability of the surface film, which is itself

highly sensitive against corrosive anions like chloride, sulfate and nitrate. Makar and Kruger

[83] showed a strong influence of alloying elements on the stability of this surface film, which is

particularly interesting for this study as zinc was shown to strongly increase the corrosion rate if

added up to 20 wt. %, while further additions up to 30 wt. % reduced this effect. The

experiment was carried out in borate buffer of pH 9.2, which fulfills the stability criteria of zinc

but not magnesium. This illustrates the fragility of the surface film as the stability is decreased

by alloying an element itself passive under the experimental conditions.

2.3.3 Magnesium oxide Magnesiumoxide is an insulator in the native state and exhibits a large band gap between 7.4

and 7.9 eV [84]. This band gap is however influenced by the surface orientation [85] and is of

limited relevance for aqueous solutions and humid environments because of the transformation

to the thermodynamically more stable Mg(OH)2 [4] and MgCO3 [86]. The surface films formed

in typical laboratory environment (~50-65 % rel. humidity) are several nm thick, of hydroxidic

nature, and grow slowly but continuously upon sustained exposure [87].

The thermodynamic stability of Mg in solution is dependent on the stability window of

Mg(OH)2, which causes passivation of metallic magnesium at pH values above ~11 [4] without

showing amphoteric behavior at high pH values.

An interesting compound concerning Zn-Mg alloys for corrosion protection is the

corresponding mixed oxide. Even though the solubility of MgO in ZnO is very low in the bulk

form (~4 %), a significantly higher content while conserving the hexagonal crystal system of

zinc was achieved for thin films [88]. In general, the Mg doping of zinc oxide yields higher

band gaps [89] in conjunction with structural changes gradually deviating from Wurtzite as the

MgO content is increased [90]. This tendency was confirmed for natively formed oxides by


21

Hausbrand et al. showing that the surface film formed on MgZn2 exhibits a Fermi- and

conduction band level shifted cathodically on the electrochemical scale by approximately 200-

300 mV [50] (especially chapter 6.7, also [29]).

It is, however, important to note that the different chemical stability of ZnO and MgO

induces compositional changes within the layers formed in aqueous environments (e.g. surface

enrichment of ZnO at pH 10 and MgO in pH 12) , which adds a high degree of complexity to

the passivation behavior of binary Zn-Mg alloys.

2.3.4 Beneficial aspects of Mg for zinc corrosion The protective properties of zinc coatings are fundamentally based on the bulk resistance

against dissolution, the capability to provide cathodic protection in case of exposed steel areas

and, furthermore, the ability to cover cut edges with insoluble corrosion products to suppress

the oxygen reduction reaction and therefore reduce the resulting sacrificial dissolution.

If an organic coating is applied on top, the corrosive delamination of a certain polymer from

the zinc based coating becomes an additional parameter. It appears useful to structure the

impact of magnesium along these aspects:

Cathodic protection: Pure magnesium itself does not provide cathodic protection due to

the strong tendency to form surface films of low conductivity [91]. As previously pointed out

(section 2.3.2), this situation changes drastically in case of alloys. In case of magnesium as a

minor alloying element in technical coatings, it is usually included in the intermetallic

compound MgZn2 [77, 92] within the coating, which shows a corrosion potential very close to

pure zinc in immersive tests [29, 50]. Other possible intermetallics like Mg2Zn11 were shown to

display the same behavior [79]. The cathodic protection provided by the coating in case of Zn-

Mg alloys is therefore comparable to classical zinc coatings, but of course benefits from the

longer durability of the coating itself as the reduced amount of bulk dissolution of the coating

is immediately available for sacrificial protection [77].

Bulk resistance against dissolution: Zn-Mg coated steel (ZM) has been reported to show a

significantly higher stability in salt spray [92], climate [77] and cyclic corrosion tests [93] as

compared to standard hot dip galvanized steel (Z). General consensus exists on the fact that

this behavior originates from the higher stability of the corrosion products formed. The

following table provides an overview of the most common precipitates formed during the

testing procedures mentioned before:


22

Name Formula

Brucite Mg(OH)2

Hydrozincite Zn5(OH)6(CO3)2

Magnesite MgCO3

Magnesium oxide MgO

Simonkolleite Zn5Cl2(OH)8·H2O

Smithonite ZnCO3

Zinc hydroxide Zn(OH)2

Zinc oxide ZnO

Table 2.1: Typical corrosion products

with relevance for ZM coatings.

Presently, there are two possible stabilization mechanisms discussed in the literature. The first

one considers a direct increase of the blocking properties of the surface film by the insulating

character of magnesium corrosion products (especially oxidic films, see section 2.3.3) as

reported by Prosek et al. [79]. Other authors proposed an indirect mechanism where

magnesium assists the formation of Simonkolleite which is considered as one of the most

protective corrosion product [92, 94]. An interesting observation was made by Ishikawa et al.,

demonstrating that Mg2+ ions do not lead to the preferred formation of Simonkolleite during

precipitation of ZnCl2 from solution [95]. However, the situation on a corroding surface in a

spray test is significantly different. First, the surface pH is strongly increased due to oxygen

reduction and comparably small electrolyte volumes, possibly causing zinc dissolution due to

the amphotery of Zn(OH)2. Secondly, the carbonate content is high due to the continuous

contact to air and the high pH causing massive uptake of carbon dioxide. With focus on these

two parameters, Hoskin et al. proposed magnesium to act as a buffer towards both, i.e.

scavenging OH- and carbonate species that are capable of destabilizing Simonkolleite [77]. This

buffer concept was supported by experimental and theoretical results presented by Volovich et

al. [96]. It is to note that the conditions described are present in technical corrosion tests (e.g.

spray tests) that are most comparable to marine environments [97]. A transfer of the results to

conditions of less humidity on the one side or immersive tests as the other extreme is therefore

hardly possible. It appears valuable to clarify the impact of magnesium under various

conditions since the stabilization of particular zinc corrosion products may only be valid under

certain environmental conditions.

Self healing: The term self healing refers to the coverage of exposed steel surfaces by

insoluble corrosion products and is limited to comparably small areas (like scratches and cut

edges) [98]. The concept is that the sacrificial dissolution of the coating causes the blank steel


23

to act as a cathode, thereby causing a pH shift upwards by oxygen reduction (up to above 11

[99]). The impact of magnesium on suppression of oxygen reduction by precipitate formation

was investigated by Hausbrand et al. showing a strong decrease of the oxygen reduction current

density by magnesium ions alone [100] and in conjunction with zinc [50]. It needs to be

pointed out that the effect of magnesium is minor compared to zinc for typical oxygen

reduction rates on iron when cathodically polarized due to the high pH values required to

trigger precipitation of Mg. The exact effect of magnesium in the self healing process is

therefore an open question and the literature is not clear so far.

Chapter 3: Questions and approach

24

3 Questions and approach As outline before, the literature on zinc corrosion is remarkably extensive and characterized

by a large variety of methods employed. However, there is still a significant gap between

technical corrosion tests (e.g. salt spray tests, climate tests) and laboratory experiments (e.g.

immersive electrochemistry) concerning comparability and mutual relevance. Even though the

dissolution kinetics of Zn and the semiconducting properties of ZnO have been

comprehensively investigated under well defined laboratory conditions, it still appears unclear

how these insights can be transferred into real applications to translate into a broader profit.

This issue was approached by taking the most common and intuitive parameter for

corrosion, the loss of material, and combining it with a variety of electrochemical and surface

analysis techniques. This way was considered promising to describe the correlation between

real dissolution rates and electrochemical behavior under various conditions, possibly allowing

to quantify the effect of different parameters (e.g. ion concentrations) on both aspects.

Therefore, the initial work of this study focuses on the design of an electrochemical flow cell

and the integration of trace analysis of dissolution products with high time resolution. The

system characterization and optimization primarily followed three key aspects: (i)

Reproducibility and comparability of electrochemical data, (ii) miniaturization and automation

of the electrochemical cell to ease sample preparation and allow high throughput

experimentation, and (iii) reliability and sensitivity of the online zinc analytics.

The following application of the integrated system utilizes pure zinc substrates to clarify the

following questions:

I. Is it possible to calculate the corrosion current density from the dissolution profile?

II. What is the correlation between the dissolution rate and electrochemical behavior (e.g.

potentiodynamic sweeps, open circuit potential measurements)

Furthermore, a detailed investigation of different parameters was required to assist the

comparability of relevant literature which suffers from a lack of standardization concerning the

testing conditions. The most fundamental distinction was made between unbuffered NaCl and

buffered borate solutions which are both widely used for corrosion testing. The effect of

parameters like ion concentration (NaCl and Na2SO4) and flow speed was investigated as well

because these exhibit the highest spread within the relevant literature.

Chapter 3: Questions and approach

25

With regard to the different corrosion mechanisms of zinc, primarily either direct

dissolution or corrosion through an oxide, it was required to compare the results on metallic

zinc to those on zinc oxide. For this purpose, aluminium doped ZnO:Al thin films were used

because the polycrystalline and conductive nature of these substrates compares to passive film

grown on oxides. With respect to the corrosion of metallic zinc, the following questions were

to be clarified:

I. What determines the dissolution rate of ZnO in both NaCl and borate buffered

solutions?

II. Can this dissolution account for the corrosion rate of zinc in the respective medium?

III. What processes occur during film breakdown events at high anodic potentials?

The final aim of this study was to investigate if the new methodology can assist material

development procedures on protective coatings. In close collaborations with ThyssenKrupp

Steel Europe AG, an optimization of the composition of binary Zn-Mg systems for corrosion

protection of steel was attempted in a high throughput approach utilizing laterally graded PVD-

film. The production and surface characterization of these material libraries constitutes an

important part of this study, complemented by a large dataset covering electrochemical

behavior and dissolution profiles as a function of the material composition. As pointed out in

section 2.3.4, the role of Mg in protective Zn-Mg coatings in not fully understood and subject

to controversial discussions. It was to be clarified whether the use of both buffered and

unbuffered systems in comparison allows for a deeper insight into the stabilization mechanism.

Accordingly, the following questions were addressed:

I. Is it possible to perform fully automated, high throughput screening on material

libraries?

II. What composition exhibits the most promising results? Does this data compare to the

literature?

III. Does the use of a buffer system alter the results? Does this allow conclusions about the

role of Mg?

All individual chapters are designed to contribute to a broad perspective on the use of

downstream analytics to evaluate the corrosion behavior of zinc based coatings as a

complementary technique to classical electrochemical experiments. Especially the potential

synergy emerging from the parallel use of both methods constitutes a major scientific aim of

this study.

Chapter 4: Experimental techniques

26

4 Experimental techniques The aim of this experimental section is to give a brief introduction into the surface

characterization methods employed in this study as well as to provide details about the sample

and electrolyte preparation. Furthermore, a cosine based evaporation model for thermal co-

deposition is developed and used to allow a precise correlation between position and

composition on graded material libraries.

4.1 Microstructural characterization The following tools were used for structural investigations:

Atomic force microscopy: The AFM technique was initially described by Binnig in 1986 [101]

and serves as an essential tool for investigations of surface topographies or adhesion forces.

Topographic images, as the relevant application of this technique within the study presented,

are based on profiling the surface with a cantilever attached to a mechanical spring whose

bending is directly proportional to the applied force and topographic inhomogeneities.

Quantification of the magnitude of spring bending is achieved by detecting changes of the

reflection angle of a laser beam focused on the cantilever. By either recording those values or

minimizing bending changes by height adjustment, topographic images can be obtained of

areas with dimensions up to 100x100 µm2 with a resolution in the angstroms range [64]. The

microscope used in this study (JPK NanoWizard I, JPK Instruments AG, Berlin, Germany)

was equipped with a silicon cantilever (CONTR obtained from BudgetSensors, typical tip

radius <10 nm).

Surface profiling: The profilometer (Dektak 6M Stylus Profilometer, Veeco, Santa Barbara,

USA) moves the sample under a diamond coated tip (tip radius 12.5 µm) that rides the surface

similar to an AFM in contact mode, but with lower lateral resolution and higher travelling

distance. The vertical translation of the tip (resolution: Sub nm) is recorded by a linear variable

differential transformer (LVDT) and the data is leveled by substracting a linear background.

Auger Electron Spectroscopy: Auger electron spectroscopy is a surface sensitive method

based on the ejection of low energy electrons (= core shell electrons) by an exciting beam. The

hole generated is reoccupied with a higher shell electron and the energy difference between

both involved states causes an additional electron to be emitted with a characteristic energy,

called the Auger effect [102]. Since three electron shells are involved (shell of first ejection,

shell of the filling electron and shell of auger electron), the peak nomenclature involves three


27

letters (e.g. KLM). The energy of the auger electron is independent of the exciting beam and

only affected by the energy difference of the element specific relaxation process. Furthermore,

the escape path of auger electrons through the sample is very short [103] which accounts for

the high surface sensitivity of this technique. Combined with a translation of the excited beam

over the surface, the scanning auger technique provides elemental maps of surfaces with very

high lateral resolution (several nm). Figure 4.1 illustrates the emission processes caused by an

exciting beam.

Figure 4.1: Schematic illustration of the

excitation zone generated by a primary electron beam

and the corresponding processes leading to the

emission of electrons or radiation.

The scanning Auger microscope used in this study (JEOL JAMP-9700F) was equipped with

a hemispherical analyzer with multi-channel detector, providing a theoretical resolution down

to approximately 8 nm.

X-Ray photoelectron spectroscopy: Replacing the electron beam from Figure 4.1 by a focused

X-ray beam (aluminium anode, Kλ=1487 eV), an excitation zone is created in the vicinity of the

surface (1-3 nm) where electrons are ejected with a velocity equal to the energy difference

between exciting beam (with an energy equal to hν) and their binding energies [104]. With a

sufficiently high energy resolution of the detector, these photoelectrons carry detailed

information about their original binding energy in the surface and therefore provide

information about the nature and chemical state of surface atoms. A PHI small spot (100 µm

spot size) ESCA Quantum 2000 system in an UHV-cluster (p < 10-7 Pa) was used in this work,

equipped with an Ar sputter gun for depth analysis. The sputter rate was calibrated by etching a

100 nm thick SiO2 film and was found to be 1.87 nm min-1 for 1 kV and 4.8 nm min-1 for 2 kV

accelerating voltage. Detailed spectra were recorded with a step size of 0.3 eV and pass energy

of 23.5 eV, and Casa XPS was utilized to quantify the elemental composition. Due to the


28

semiconducting nature of oxidic film on magnesium and zinc, slight shifts in the whole spectra

can occur due to a charging of the surface. If necessary, corrections were made taking the C1s

peak position (285 eV [29, 105]) as a reference.

Energy dispersive X-ray spectroscopy: The principle of this technique is based on the emission

of photons by exposing the specimen to a focused electron beam utilizing a scanning electron

microscope [106]. By ejecting an electron from a low energy core level by the exciting beam, a

hole is generated and filled with a higher shell electron as described earlier. In this case however,

the detector is designed to detect X-ray radiation instead of auger electrons. The energy

dispersion of this radiation can be used to extract characteristic peaks for different elements to

determine the chemical composition of the specimen. As evident from Figure 4.1, the volume

of characteristic X-rays is comparably large and yields high intensities, causing EDX to be the

least surface sensitive, but quickest method among all electron excitation methods presented

here. The system used in this study consists of a Zeiss LEO 1550 VP scanning electron

microscope with nitrogen cooled Si(Li) X-ray detector. The INCA software was used for data

processing.

Scanning electron microscopy: As evident from Figure 4.1, secondary electrons are emitted

from a surface excited by a focused electron beam by inelastic scattering of the primary beam

electrons at electrons bond in the sample atoms [107]. Detection of these secondary electrons

in a raster pattern reveals the topographic characteristics of the sample and can therefore used

to image the surface. The “in-lens” detector of the setup used (Zeiss LEO 1550 VP scanning

electron microscope) is capable of highly resolved images even at low excitation energies and

was used for all images shown.

X-Ray diffraction: The scattering of X-ray radiation at crystal lattices results in an

interference pattern being constructive in the case described by Bragg´s law [108]:

θλ sin2dn = (4-1)

With a known wavelength on the incidence light (CuKλ radiation in the system used), the

constructive case for a given crystal structure is fixed to defined angles between source and

detector. Since the penetration depth of the radiation is angle dependent, a small incidence

angle is chosen (5°) and fixed throughout the experiment, whereas the detector angle is variable.

This grazing incidence technique [109] is most suitable for thermally evaporated thin films as

investigated within this study due to the small penetration depth of the radiation, even though

the strong increase of the illuminated area with a given slit aperture (600 µm) needs to be taken

into account. The XRD system used (Bruker AXS D8) was equipped with a parabolic Göbel

mirror and SolX detector.


29

4.2 Sample preparation 4.2.1 Preparation of bulk zinc

Zinc specimen were prepared from 2 mm thick Zn foil (99.99 %, Alfa Aesar GmbH & Co

KG, Karlsruhe, Germany) by cutting the sample to the desired dimensions and applying the

following grinding/polishing procedure:

I. Grinding with Si:C grinding paper of 250, 1000, 2000, 4000 grit.

II. Polishing with 5 µm diamond particle suspension

III. Rinsing with ethanol

IV. Final polishing with 50 nm SiO2 particle suspension

V. Thorough cleaning with ethanol

The samples were stored in a desiccator under dry atmosphere.

4.2.2 RF-sputtered ZnO:Al Approximately 800 nm thick polycrystalline [46] ZnO:Al films were deposited on a cleaned

(10×10) cm2 glass substrate (Corning Eagle XG) using radio frequency magnetron sputtering in

a vertical in-line system (VISS 300, VON ARDENNE Anlagentechnik GmbH, Dresden,

Germany) from a ceramic target consisting of ZnO with 1 w/w% Al2O3 (Cerac inc. Milwaukee,

WI, USA). The deposition was carried out at the Forschungszentrum Jülich with a substrate

temperature of 300 °C, a discharge power of 2 W cm-2, and an Ar pressure of 0.1 Pa. Details

about the process were published elsewhere [110]. The following figure shows a SEM image of

an as-deposited ZnO:Al surface at 300 k magnification:

Figure 4.2: SEM image of a

ZnO:Al thin film on glass

prepared by RF-magnetron

sputtering. The individual grains

are clearly visible.


30

4.2.3 Thermal co-deposition of Zn-Mg

4.2.3.1 System design and deposition procedure The thermal PVD-system used for co-deposition of Zn-Mg graded samples consists of a

bell jar deposition chamber (Oerlikon Balzers AG, Balzers, Liechtenstein) evacuated by a liquid

nitrogen baffled diffusion pump with a rotary vane fore-vacuum. The base pressure is

approximately 2 x 10-4 Pa and rises to around 4 x 10-4 Pa during the deposition procedure.

Three tungsten baskets (B12A 3x.030W, Testbourne ltd., Basingstoke, UK) for thermal

evaporation are powered by two 900 W DC- and one 1800 W AC-Power supply. To evaporate

zinc, one basket was replaced by a tungsten coil with boron-nitride crucible (B8A 3x.025W and

C1-BN, Testbourne ltd., Basingstoke, UK) due to the high need for constant and gentle heating

given the high vapor pressure of zinc. Each source is located 80 mm above the base plate with

a spacing of 110 mm with respect to the neighboring sources. Within this triangle, the substrate

is placed 120 to 300 mm above the base plate, yielding aspect ratios between spacing and

substrate height above the source from 2.75 to 0.37. Figure 4.3 shows a 3-D render (generated

with blender 2.49b) of the PVD unit used in this study [111].

Figure 4.3: 3-D render

showing the geometry of the

PVD-unit used for thermal

co-deposition.

The deposition rates were constantly measured using quartz microbalances (QMB) for each

specific source, each of them well shielded from the others. The tooling factors for the three

QMB were determined by placing a fourth one centered and about 60 mm above the sources,

using the quotient of the central and the individual QMB as a factor for further measurements.


31

Steady deposition rates were achieved by using a LabView based program, reading the QMB

through a SQM-242 interface card (Sigma Instruments, Fort Collins, USA) and controlling the

power output of the power supplies utilizing a proportional-integral regulation. The setpoint of

the deposition rates from each individual source was adjusted according to the target gradient

(e.g. high Zn and low Mg evaporation to obtain generally zinc-rich material libraries), and falls

between 1 and 10 Å s-1 in all cases.

Metals used for evaporation were obtained from Alfa Aeser (Alfa Aeser GmbH & Co KG,

Karlsruhe, Germany) with a high purity of at least 99.995%.

4.2.3.2 The cosine law and model fitting The key parameter of combinatorial libraries is the precise correlation of the position on the

substrate and the corresponding composition [112, 113]. While this function can be

approximated by subdividing the surface into individual sectors and determining the

composition of these sectors [114], the size of these sectors is still in the mm-range and subject

to measurement errors of the analysis method (e.g. EDX). The material libraries prepared are

comparably large (100 mm) and allow for a very high compositional resolution because of the

small wetted area (around 200 µm), as only a small fraction of the gradient is covered with each

measurement. This resolution though demands a composition mapping with a very low error

(< 0.3 at. %). The approach taken in this study was to develop a fitting model, yielding a

surface function f(x,y) that allows calculating the composition for every possible location on

the substrate. The fundamental considerations behind the model presented are of geometrical

nature as illustrated in Figure 4.4:

Source

x ydy

dx

y

x

xy

r

Figure 4.4: Schematic

illustration of the

geometric parameters for

thermal co-deposition.

Given a point source, a plane in the distance hS being the substrate and a point directly

above the source being x0,y0, the deposition-rate Rd(x,y) at a particular point is anti-proportional

to r2 because a constant mass flow per angle covers a greater area with increasing θ. This is a

direct consequence of the fact that the distance between x0y0 and xy in Figure 4.4 is equal to


32

hS··tanθ with the incremental increase equal to the derivative hS·cos-2θ. Since the deposition rate

is antiproportional to the area exposed with a fixed flux, the deposition rate at every point can

be calculated from the rate at the surface normal according to

θθ 200, cos)( ⋅= yxdd RR (4-2)

and therefore

22

02

0

200,

)()(),(

s

syxdd

hyyxx

hRyxR

+−+−= (4-3)

This equation is only valid for a point source that emits species in all directions at the same

rate. In thermal PVD systems, this is not feasible.

Fundamental studies concerning the angle dependent deposition of metal vapour on

surfaces and the angle dependent emission from sources were performed by M. Knudsen in

1916 [115] and 1917 [116]. It was shown that the rate of metal deposition onto a flat substrate

from a small emission source shows an angular dependency besides the geometrical

considerations, called the cosine law. The reason for this angular dependency is the existence of

a preferential direction of emission [117] which needs to be implemented in the model. Adding

another cosine dependence to equation (4-2) yields a cosine cubed function as used by Stella et

al. for E-beam evaporation of Tantalum from small rods [118]. Since the source geometry is of

higher complexity when using baskets or crucibles, the exponent of cosine was taken as a

variable β. Equation (4-2) therefore transforms to

θθ βcos)( 00, ⋅= yxdd RR (4-4)

with β=3 for a point source with preferential direction towards x0y0 according to the cosine

desorption model. The reason for β taking different values for different sources can be

suspected in many effects:

The sources body may emit atoms in a preferential direction not following the cosine rule.

This exponent was described as the sharpness parameter for the angular distribution of

repulsive desorption [119]. Furthermore, depositions at acute angles cause an equal impact

angle of the atoms on the surface. The sticking parameter, being the fraction of atoms

adsorbed after hitting the surface, depends on the atoms energy, the surface properties [120]

and the ability to transfer kinetic energy into the surface. The latter was reported to be cosine-

shaped [121]. Finally, some source parameters like the temperature distribution within the

glowing coil and the exact geometric shape are not known.


33

Even though these parameters can not be quantified individually, the determination of β is

still possible with high accuracy using a least square fit on the thickness distribution of copper

evaporated on silicon:

The thickness of the film (df) along one axis can be described as

222

0

00,

))(()( β

β

s

syxdf

hxx

thRxd

+−= (4-5)

with the evaporation time t. A least square fit of the experimental thickness data of copper

(determined by AFM) using equation (4-5) with fixed geometric parameters yields Figure 4.5

[122]. The simulated thickness distribution for β=2.4 matches the experimental data very well

and proves the experimental determination of the sharpness parameter to be feasible.

0 20 40 60 80

100

150

200

250

300

350 Simulated (β = 2.0) Simulated (β = 2.4) Simulated (β = 3.0)

position / mm

thic

knes

s / n

m

Thickness measured

Figure 4.5: Least square

fit of the thickness of a

copper thin film along the

substrate using equation

(4-5). 0 mm corresponds to

the location directly above

the source position.

The diagram demonstrates the opportunity to calculate the thickness at every possible

location using the mathematics presented and is therefore of high value for thickness resolved

screening experiments [123].

4.2.3.3 Composition mapping of material libraries In the case of co-deposition of two or three elements simultaneously, the chemical

composition of the resulting film can be calculated by converting the deposition rates of the

individual elements into molar numbers, giving the composition as quotient. The following

equation is used for this conversion concerning element A:


34

A

AdAAd M

RN 7

,, 10

ρ= (4-6)

This conversion is required to calculate the atomic ratio between two elements on each

position on the substrate. The factor 107 reflects the conversion of nm to cm. The density ρ in

equation (4-6) always needs to have the same value as the value used by the QMB for the

specific source (needed during calibration of the QMB). Additionally, the substrates rectangular

shape demands the use of both, x and y coordinates to map and simulate the composition.

Using the sharpness parameter from equation (4-4) and x0 and y0 as the position closest to

source A, equation (4-5) translates into

2220

20

7

,,

))()((10),( β

βρ

sA

AdAAd

hyyxxM

hRyxN

+−+−= (4-7)

Plotting the quotient of NMg and NZn or NAl, NCu and NMg in percent against the xy-

coordinates, a three dimensional surface graph was obtained. Figure 4.6 shows this surface fit

in combination with EDX maps and reveals a very high accuracy for binary and ternary

systems. The RMSD around 1 % is assumed to mainly originate from the experimental error of

the EDX analysis.

Figure 4.6: EDX maps (surfaces) and composition fit (spheres) for a Zn-Mg binary (left) and a Ag-Cu-

Mg ternary (right) system.

With these model fits, the composition of every possible location on the substrate can be

calculated automatically and was therefore implemented into the data processing system. This

software handles the data points obtained by the SFC and calculates the corresponding

composition from the position of the translation stage using a model fit database including all

gradients.


35

4.3 Chemicals All chemicals used in this study were p.a. grade except for the sulfuric acid (suprapure) in

the experiments on platinum. Water was purified using a PureLab Plus system (Elga, Celle,

Germany) with a specific conductivity below 50 nS cm-1.

Buffers were prepared from their respective acid followed by adjustment of the pH by

addition of NaOH under pH control (E-632 Digital pH-Meter, Metrohm AG, Herisau,

Switzerland). The concentration of buffers always refers to the sum of both protonated and

deprotonated species.

Chapter 5: Development of the scanning flow cell

36

5 Development of the

scanning flow cell The following chapter describes the concept and experimental realization of a micro

electrochemical capillary cell capable of maintaining a steady flow of electrolyte over the

substrate under investigation. In addition, the implementation of downstream analytics for

dissolution monitoring and a comprehensive system characterization are presented.

5.1 State of the art The concept of miniaturized electrochemistry experiences increasing interest since the 1970s

where local confinement of classical electrochemical techniques was achieved by either

embedding small wires into resin [124] or masking the surface using photolithography [125].

Along with the development of scanning methods like scanning vibrating electrode

technique (SVET) [126] or scanning electrochemical microscope (SECM) [127, 128], the

microcell approach was extended by a positioning system and a capillary based housing, which

allows adjustment of the wetted area by the capillary size. This development was particularly

driven by T. Suter [129, 130] and M. Lohrengel/A. Moehring [131-133] with the focus on

either local corrosion (Suter) or anodization (Lohrengel). Numerous examples demonstrating

the potential of locally confined investigations can be found in the literature, with examples of

an extensive experimental survey of Al-Cu intermetallics by N. Birbilis [134], the investigation

of structured oxide films on aluminium by A.W. Hassel [135] or grain dependent passivation of

zinc by C.J. Park [136].

While the cell design regarding the type and placement of reference and counter electrode

differs between each individual setup, the confinement of the wetted area is in all cases

achieved by either a free droplet shaped by its surface tension between tip and substrate [137],

or by sealing the area by a silicone gasket on the tip in contact mode [138].

According to these two principles [139], the step towards a flow system was realized by

either a coaxial system where the free droplet of an inner capillary is drained by an outer

capillary by K. Fushimi [140] or a theta capillary approach in contact mode where the

electrolyte is allowed to stream from one compartment of a theta capillary into the other


37

through an opening right above the substrate by M. Lohrengel [133]. In both cases, high flow

rates were applied due to the use of a vacuum drain or large pumping speed (> 800 µl s-1).

The use of complementary electrolyte analysis downstream was demonstrated on the

example of UV-VIS spectroscopy in the latter study but required high current densities to

exceed the detection limit (LOD) of released species due to the large flow rates. For the

investigation of corrosion phenomena at rather low corrosion current densities however, a very

high sensitivity is desired. In addition, a very stable flow is necessary even when the cell is lifted

from the substrate to allow a purging step between individual measurements to remove residual

dissolution products. A system that fulfills these requirements has not been reported so far and

therefore constitutes the major driving force behind the developments presented in the

following sections.

5.2 Design of a flow system The requirement for a new flow system as described in the former section is fundamentally

driven by the following aspects:

Detection of dissolution products – The most obvious feature of the transport process is the

ability to feed the released species into downstream detection.

Control of concentration gradients – Given the small electrolyte volume of the microcell, local

accumulation of species (up to saturation) proceeds quickly. With an integrated flow system,

the concentration gradients in the vicinity of the surface is brought to a steady state comparable

to the rotating disc electrode [17, 18]. This feature is of particular importance in potential

sweep experiments that can only be considered stationary if the experiment history (e.g. the

potentials applied prior to the actual potential) has a minor impact compared to the process

taking place during data acquisition.

Stability of the flow – Since a dead volume of electrolyte located between tip and detector is

unavoidable in downstream settings, purging of the cell is required after each measurement to

correctly quantify the species released. In the case of spontaneous dissolution (e.g. corrosion),

this process needs to be executed while the cell is lifted from the substrate to avoid continuous

release of species. This purging between individual measurements allows a sequence on

different locations and assures identical initial conditions because residues from previous

location have been removed.

Low flow rates – The flow rate of the macroscopic pumping system needs to be sufficiently

low to adapt to the very small wetted area and the dilution of the analyte in order to increase

the detection limit significantly.


38

High throughput analysis – A capillary cell operating in contact mode can be fully computer

controlled given a precise xyz-positioning unit and a force sensor with feedback loop. By

synchronization of all positioning and measurement devices, it is possible to automatically

execute measurement sequences on an array of previously programmed locations, including

data acquisition and –handling.

According to these aspects, the new SFC concept was based on a theta capillary (World

precision instruments, Sarasota, USA) in order to achieve a stable meniscus at the capillary tip

at all times. The exact procedure of preparing the microcell body is composed of the following

steps:

I. Pulling the capillary using a PC-10 capillary puller (Narishige, Tokyo, Japan)

II. Grinding the tip (EG-400 grinder, Narishige, Tokyo, Japan) to an opening diameter as

desired

III. Mechanical removal of the separating wall at the very tip using a small needle. The

removed area should be roughly equal to the capillary opening or slightly larger. In the

latter case, the application of another grinding step can be used to equalize the areas.

IV. Dipping the tip into a RTV-118Q acetoxy curing silicone (Momentive, New York,

USA) followed by purging with nitrogen for 20 minutes to form the silicone gasket.

This procedure reduces the previously adjusted capillary opening and needs to be

considered beforehand.

An optical image of a microcell tip prepared by the described method is shown in Figure 5.1.

Figure 5.1: Optical image of the capillary

tip, side view, perpendicular to the plane of the

separating wall.

A 4 channel peristaltic pump (REGLO digital MS-4/12-100, VWR international GmbH,

Darmstadt, Germany) was selected and works the electrolyte flow via Tygon®-tubing of


39

0.38 mm inner diameter to ensure that electrolyte is supplied to one compartment at a flow rate

matching the drain in the other compartment.

In order to complete the 3-electrode setup required for half-cell electrochemical

investigations, a platinum counter electrode (50 μm Pt-wire, 99.99 %, Goodfellow, Bad

Nauheim, Germany) was inserted in the drain compartment and an Ag/AgCl [141] or

Hg/Hg2SO4 [142] micro-reference electrode was placed in the supply compartment close to the

capillary tip. The placement of the counter electrode was carefully adjusted to be close to the

working electrode, but inside the stagnant electrolyte area above the drain capillary ending to

ensure that species are not electroplated on the counter electrode and therefore escape

detection. This arrangement is schematically shown in Figure 5.2. The 350 µm micro capillaries

connecting the tubing to the inner capillary are obtained from Microfil® syringe needles

(World precision instruments, Sarasota, USA).

Figure 5.2: Schematic illustration of the

scanning flow cell setup including the Y-

connector downstream where mixing of

analyte and complexing agent takes place.


40

5.3 Downstream analytics Since classical electrochemical measurements access reaction rates by the current generated

within this process, it is not possible to distinguish directly between the dissolved and

precipitated form of ions (e.g. Zn2+(aq) vs Zn(OH)2) or between different dissolution processes

proceeding in parallel (e.g. dissolution of Zn and Mg in alloys).

The use of spectroscopy as a complementary technique to electrochemical measurements

for distinguishing the contribution of different species was shown in numerous examples in the

literature, like in situ ATR [143, 144], in-situ Raman spectroscopy [38, 145] or downstream

dissolution monitoring by ICP-OES [146] or UV-VIS spectroscopy using flow injection

analysis (FIA) [133, 146, 147]. For an initial validation of the flow concept and the possibility to

implement downstream detection, a UV-VIS spectrometer was utilized within this work.

As most metal ions itself are not detectable by UV or VIS spectroscopy, a complexing agent

is required that selectively forms complexes with a high extinction coefficient (Eλ) in the

spectral range provided by the spectrometer. The following table provides an overview about

metal ions and complexing agents, including the limit of detection (LOD), suitable for FIA

UV-VIS detection:

Analyte Reagent λ/ nm LOD / ng ml-1 Ref.

Al(III) Chromazurol S 546 10 [148]

Cu (II) Zincon 600 2.7 [149]

Fe (II) 1,10-phenanthroline 512 35 [150]

Fe (III) Sulfosalizylic acid 530 100 [151]

Mg (II) 1-(2-Hydroxy-3-sulfo-5-chloro-1-

phenylazo-)-2-naphtol-3,6-disulfonic acid 527 200 [152]

Zn (II) Zincon 612 0.8 [153]

Table 5.1: Reagents for detection of selected metal ions in the visible region.

Zincon (2-Carboxy-2'-hydroxy-5'-sulfoformazylbenzene sodium salt) is a highly sensitive

complexing agent selective for copper and zinc ions with a diminishing sensitivity for other

metal ions [154] and is therefore highly suitable for the study presented. The Zincon-metal

complex shows an absorption maximum around 600 nm, seen from a series of spectra with

different concentrations of ZnCl2 in Figure 5.3. In addition, a drawing of the custom made

transmission flow cell is given.


41

400 500 600 700 800-45000

-30000

-15000

0

15000

2 x 10-5 mol l-1

1 x 10-5 mol l-1

5 x 10-6 mol l-1

abso

rptio

n / c

ount

s

λ / nm

increasing Zn2+

concentration

Isosbestic point

Figure 5.3: (left) Absorption spectra of

Zincon-Zn2+ complexes for different

concentrations of ZnCl2 and (top) the design of

a custom made transmission flow cell in Z-

geometry (SMA=Connector for optical cables).

The presence of an isosbestic point is of particular importance for FIA analysis since it

proves a transition between two states (as free species or ligand in a complex) with the absence

of intermediates and side reactions. It therefore allows quantification of species using the

height of the respective peaks, in the present case the high absorbance at 590 nm originating

from the Zincon-Zn2+ complex. As previously mentioned, the complexing agent was injected

into the drain tubing by a Y-connector using another channel of the peristaltic pump equipped

with Tygon® tubing of 0.19 mm inner diameter (compared to 0.38 mm for the cell flow). The

difference in flow rates between analyte and complexing agent was therefore fixed at 4:1 to

reduce the dilution of the analyte. The following table summarizes the settings finally chosen

for the VIS-spectrometric detection of zinc:

Component Specification

Complexing agent Zincon (10-4 mol l-1) in borate buffer pH 9.25 (0.2 M)

Transmission Cell Custom made, see Figure 5.3, 10 mm optical pathlength

Mixing length The tubing between Y-connector and transmission cell was 50 mm

Optical cables 1 m length, SMA connectors, 600 µm inner diameter

Spectrometer EPP 2000 / Black Comet, (StellarNet Inc., Tampa, USA)

Lamp SL-1 tungsten halogen lamp (StellarNet Inc., Tampa, USA)

Table 5.2: Experimental details on the spectrometric arrangement for downstream zinc detection.


42

5.4 Software development The whole software developed for the scanning flow cell is exclusively based on LabView

(National Instruments, Austin, USA). Communication with all devices is executed by

processing string commands (RS 232, GPIB) or calling dynamic link libraries (dll) provided by

the manufacturer. The programming concept follows a sequential order of background

processes (force measurement, UV-VIS data acquisition etc.) executed at low priority if no high

priority case (user interaction) is active. This structure is implemented using a LabView event-

structure with background processes stacked in the timeout-case.

Figure 5.4 shows two pictures of the setup assembled within the scope of this work. The

cameras (SMX-M83 C, Sumix Corporation, Oceanside, USA) are equipped with a low- and

high-magnification optics (top and side view) and streamed as active-X objects via USB. The

XYZ-positioning system (DC-Motors with C-844 motor controller, Physik Instrumente GmbH

& Co.KG, Karlsruhe, Germany) is controlled via GPIB and is in constant feedback with the

force sensor (KD45 2N force sensor, ME-Messsysteme, Hennigsdorf, Germany) to allow the

software to programmatically control and adjust the force applied.

Spectroscopic data is acquired via USB (dll based) approximately once every second and

processed as illustrated in Figure 5.4.

Figure 5.4: Labeled pictures of

the scanning flow cell setup.


43

Figure 5.5:

LabView

processing of the

spectroscopic

data.

5.5 System characterization 5.5.1 Size of the wetted area

One key parameter of a microelectrochemical capillary cell is the reproducibility of the

wetted surface, since all current readings during electrochemical experiments need to be

normalized to the surface area as they are presented as current densities. To determine the

geometrical surface area, anodization of Hf or Ta and subsequent optical determination of the

colored oxide was shown to be a highly precise method [138]. For a detailed review on the

growth mechanism of oxides on valve metals, the reader is referred to [66].

Figure 5.6: Optical recognition of the capillary contact area

on anodized Hf and automated size calculation by LabView.

The equivalent radii are given in µm.

Figure 5.6 shows a Hf-thin film anodized to 10 VSHE in 8 independent measurements in

acetate buffer of pH 6.0 before (top) and after (bottom) processing by a house developed,

color threshold based LabView program. The equivalent radii for each measurement display a

very high reproducibility and are in full agreement with results presented by other authors [155].

The determination of the wetted area for regular maintenance purposes during the lifetime of a


44

cell can also be performed based on the current density during the anodization process. The

optical inspection can be skipped when the obtained graph matches the data initially recorded

during the calibration process; due to the direct relation between the plateau current density at

a fixed scan rate and the surface area [156].

5.5.2 Validity of microelectrochemical data It is not intuitive that macroscopic and microscopic electrochemical systems are directly

comparable, especially since the size of the wetted area can differ more than three orders of

magnitude. Valve metals in general are ideal candidates to validate the comparability of the

current readings because of the almost 100 % faradaic efficiency for oxide growth and the

homogeneity of this process on the complete sample surface [66, 135]. The current density for

Al, Hf and Ta in the passive region during potentiodynamic scans in anodic direction during

oxide growth is described by equation (5-1) [155]

dtdE

Mrzkiox ⋅

⋅⋅⋅⋅=

ρF (5-1)

with k as oxide formation factor (nm V-1) and the roughness factor r. Figure 5.7 shows a series

of measurements performed on thermally deposited Al thin films (~ 300 nm); used to calculate

the oxide formation factor from the plateau current density between 5.1 and 5.5 VSHE according

to equation (5-1) (rearranged to solve for k).

0 1 2 3 4 5 6 7 8 9 100

50

100

150

200

250

300

350

400

i / μ

Acm

-2

E / VSHE

4 Measurement series are shown

Figure 5.7: Cyclic

voltammograms recorded during

the stepwise potentiodynamic

oxidation of an Al thin film at a

scanrate of 100 mV s-1.

Hf and Ta thin films (sputter deposited [155]) were investigated using the same method, the

results on all samples compared to the literature are summarized in Table 5.3.


45

Oxide M / g mol-1 ρ / g cm-3 z k / nm V-1 Literature Ref.

Al2O3 101,96 3,5 6 1,63 1,6 [135]

HfO2 210,79 9,68 4 2,20 2,3 [157]

Ta2O5 441,89 8,1 10 1,72 1,8 [158] Table 5.3: Properties, experimental oxide formation factor and literature values for different valve metals.

It is to note that the roughness factor was approximated with 1, due to the smooth film

obtained by PVD processes [156] and the inherent smoothing that occurs by the oxide

formation. It is evident from Table 5.3 that the values measured with the SFC are of good

comparability to the literature, which is a weighty prerequisite for SFC-based experimental

series.

5.5.3 Calibration procedures Two main calibrations are required for the SFC system presented: The flow rate needs to be

determined as a function of the RPM of the peristaltic pump, and the concentration of zinc in

solution as a function of absorbance in the UV-VIS system.

The former is easily done by pumping ultrapure water of 22 °C and correlate the pumped

volume (as determined by weighting using a precision scale) and the time elapsed at different

pumping speeds. The resulting pumping speed for the system presented is 7.8 µl min-1 RPM-1.

It is to note that this value differs from the value stated in the manual (9.0 µl min-1 RPM-1) due

to pressure drops in the rather complex tubing system.

0 400 800 1200 1600 2000 2400 2800 32000

10000

20000

30000

40000

10-5 mol l-1

5 x 10-6 mol l-1

10-6 mol l-1

2 x 10-5 mol l-11 x 10-6

increments

abso

rptio

n / c

ount

s

time / s

2 x 10-7

increments

0.0 1.0x10-5 2.0x10-5 3.0x10-5 4.0x10-5

0

10000

20000

30000

40000

0.0 2.0x10-6 4.0x10-60

2500

5000

7500

10000

abso

rptio

n / c

ount

s

concentration / mol l-1

abso

rptio

n / c

ount

s

concentration / mol l-1

linear region

slope:1.82 x 109 counts mol-1

Figure 5.8: Absorption at 590 nm versus time plotted during a calibration series with ZnCl2 stock

solutions (left). Data analysis is performed by a linear regression to determine the proportionality factor

between absorption and concentration (right).


46

The calibration curve for the spectroscopic system can be obtained by recording the

absorption of stock solutions with different concentrations of ZnCl2 (in HCl pH 2.0). Figure

5.8 shows the absorption at 590 nm plotted over time while the system is fed with stock

solutions of different concentrations. The resulting calibration curve is shown aside. The

indicated linear region in the calibration graph covers at least 2 orders of magnitude, which is

sufficient for the application presented. The non-linear region at high analyte concentrations is

a consequence of non-quantitative complexation.

5.5.4 Time delay and peak broadening Characteristic for all downstream detection systems is a retention time required for species

to travel from the site of generation to the detector. While this effect can be easily corrected

given an exact calibration of the travel time, diffusion and mass exchange processes inevitably

occur during this period leading to a broadening of the detection peak. The most common

sources for peak broadening with high relevancy for FIA are longitudinal diffusion [159], eddy

diffusion [160] and mass exchange processes [161]. Due to the slow laminar flow in the tubing

(in the range of mm s-1), eddy diffusion is considered to be of minor importance compared to

longitudinal diffusion and mass exchange processes.

To specify the effect of longitudinal diffusion, the solution of Fick´s second law

)( 2

2

xcD

xc

∂∂

=∂∂ (5-2)

for the root mean square displacement of diffusing species yields

21

21

2 )2( Dtx =>< (5-3)

(for a detailed derivation see [162]).

This flattening of concentration gradients in the tubing system during the travel time is a

major source for broadening of a signal obtained.

To investigate the relevant diffusion processes and determine the dead time from the SFC

to the detector, galvanostatic pulses were applied to a Cu thin film (~200 nm) thermally

evaporated on a silicon wafer. The electrolyte was chosen as 0.1 M HCl at a flow rate of

15.6 µl s-1. Since copper does not significantly dissolve at the OCP in the respective medium,

the release of copper ions can be easily controlled using the galvanostatic technique. Moreover,

copper also forms complexes with Zincon that are detectable in the VIS-downstream analytics

presented (See Table 5.1). Figure 5.9 shows two independent, highly reproducible measurement

series of a sequence of 3 galvanostatic pulses applied at t=0, t=200 and t=400 s. The y-axis is

separated into two corresponding scales, on the left side the concentration as determined by


47

the detector and on the right side the current calculated from this measured flux of charged

species according to

F⋅⋅⋅= zcVI fDiss (5-4)

with Vf being the flow rate in l s-1 and z taken as 1 [146, 163] (since the primary ion formed is

Cu+ in HCl solutions).

0 200 400 600 800 1000

0

1

2

3

4

5

6

7

8

0

25

50

75

100

125

150

175

200

0 4 8 12 160

4

8

12

16

n (C

u) *

F / μ

C

applied charge / μC

z = 0.95 3 μA5 s duration

2 μA5 s duration

I Dis

s / nA

Spot 1 Spot 2

1 μA5 s duration

td

[Cu]

+ / μm

ol l-1

time / s

200 s Ranges for numeric integration

Figure 5.9: Copper

dissolution profiles and

calculated corresponding current

(2 measurement series) during

galvanostatic pulses. The inset

shows the numeric integrals of

the peaks multiplied with the

Faraday constant plotted over

the applied charge.

Figure 5.9 carries a lot of information, which can mainly be summarized in three main

conclusions:

I. The dead time can be reproducibly determined as 157 ± 5 s at a flow rate of 15.6 µl s-1.

II. The current calculated from the downstream analytics shows very good agreement with

the literature charge number for Cu in HCl (0.95 measured compared to 1 from the

literature) as seen from the numeric integrals of the peaks as a function of the charge

consumed during galvanostatic pulses (see inset of Figure 5.9). This is very important

since it proves the calibration to be valid and shows a quantitative transport of species

into the detector.

III. The peaks itself were generated using a 5 s pulse but show a significant broadening to a

FWHM around 70 s. The peaks from Figure 5.9 can be plotted individually over t1/2

and log(t) to reveal the mathematics behind the asymmetry observed. As evident from

Figure 5.10, the peak asymmetry vanishes for a square-root-t plot which points at

diffusion processes with t1/2 dependency to be responsible for the peak shape. The

longitudinal diffusion of copper ions during the dead time of 157 s can be calculated

from equation (5-3) using a diffusion coefficient of roughly 7 x 10-10 m2 s-1 [164]. At a

flow speed in the tubing of approximately 0.1 to 1 mm s-1 and a mean displacement


48

below 1 mm by longitudinal diffusion in the tubing, this can not solely account for the

observed peak broadening.

0 2 4 6 8 10 12 140

50

100

150

200

250

0 20

50

100

150

200

250

inte

nsity

/ a.

u.log (time / s)

0 50 100 150 2000

50

100

150

200

250

inte

nsity

/ a.

u.

time / s

inte

nsity

/ a.

u.

squareroot (time / s)

Figure 5.10: Initial copper peak from Figure 5.9 plotted over different time axis.

The other aspect concerning peak broadening is mass transport in the carrier stream. In a flow

system, mass exchange processes can occur with the system walls or a stationary electrolyte

volume, either as a stagnant layer in the vicinity of the walls or as pores and cavities. While

stagnant electrolyte volumes in the flow profile broadens the signal via diffusion controlled

exchange of species between layers of different velocity, mass exchange processes with the

system walls are based on adsorption or precipitation. Since Zn ions are not expected to adsorb

on the tubing walls, the solubility of zinc in the respective carrier is of dominant importance

[165]. The most important parameter affecting this solubility in near neutral media is the pH

value as discussed in section 2.1.2. Figure 5.11 shows an experiment where two solutions of

ZnCl2 in 0.05 M borate buffer pH 7.0 and 0.01 M HCl were injected subsequently into the FIA

system with an intermediate purging period (2000 s). A major difference between the initial

behaviors of both signals is evident even though the final values level at the same concentration.

0 100 200 300 400

0

1

2

3

4

2.5 μmol l-1 ZnCl2

in borate buffer pH 7.0

[Zn2+

] / μ

mol

l-1

time / s

2.5 μmol l-1 ZnCl2

in HCl pH 2.0Figure 5.11: Zinc signals obtained

in an experimental series with first

borate buffer and following HCl as

carrier medium with equal Zn2+

content. The tailing in neutral media

is clearly visible.


49

In the case of a buffered solution of pH 7.0, a tailing of the signal manifests in a delayed

increase to the final value followed by a broadening of the decay during the purging period

(with 0.1 M NaCl). This effect is contrary in acidic media where an initial peak indicates the

removal of residual zinc from the system by the acidic front travelling the tubing. In addition,

the signal decay during the subsequent purging period is steeper and the baseline is reached

within a shorter time. These results pronounce the importance of zinc precipitation for peak

broadening even at concentrations well below the solubility limit (2.5 µmol l-1 in the experiment

vs. 497 µmol l-1 solubility at pH 7.22 [166]).

However, the signal decay in Figure 5.11 for the acidic solutions still exceeds the magnitude

expected from lateral diffusion, while precipitation reactions can be excluded in this case. A

major source for peak broadening therefore originates from pH insensitive exchange processes,

most probably cavities and non-laminar flow profiles within the system. It is to note that this

aspect has been subject to optimization efforts, and the peak broadening was significantly

reduced by lowering the complexity of the tubing system, especially concerning the UV-VIS

cell and the connectors. Furthermore, it is likely that the flow profile in the capillary is

inhomogeneous at the tip, thus amplifying local diffusion processes by e.g. a lower local flow

rate. This effect will be discussed in following section. Nevertheless, a pH independent peak

broadening appears inevitable so far and needs to be considered in all experiments.

5.5.5 The flow profile at the capillary tip As evident from Figure 5.1, the flow geometry at the capillary tip is approximately U-shaped

with the samples surface located at the bottom. Lohrengel et al. published a finite element

simulation of the flow velocity profile in a capillary based microcell for high flow rates (see

section 5.1) as shown in Figure 5.12:

Figure 5.12: Finite element simulation of the flow

velocity profile in a capillary microcell at high flow rates

(10 m s-1). Figure taken from [133].


50

The simulation qualitatively reflects the inhomogeneity of the velocity distribution within

the cell. Two series of experiments were performed to illustrate the impact of the electrolyte

flow on the removal of species from the surface (chemical etching) and the transport towards it

(oxygen reduction reaction ORR).

Figure 5.13 shows an AFM topography scan performed along a hole chemically etched into

a zinc surface by exposing the sample to an electrolyte stream (15.6 µl min-1, borate buffer pH

6.6) for 3 h at the OCP (dissolution mechanism see 6.2.1). It is evident that the removal of

species appears very homogeneous with the maximum etching depth reached within several µm

distance from the capillary walls.

0 1 2 3 4 5 6 7 8 9 10-5

-4

-3

-2

-1

0

1

heig

ht /

μm

x position / μm

silicone gasket

Root mean square roughness of the polished surface:Around 18 nm

Figure 5.13: AFM cross

section showing the boundary

area of a pit chemically etched

into pure zinc.

The transport of species towards the surface can be estimated utilizing a transport

controlled reaction like the oxygen reduction reaction (ORR) on polycrystalline platinum

(mirror polish) in aerated H2SO4 solution (0.1 M, suprapure). A set of 6 CVs was measured for

each pumping speed with the last cycle shown in Figure 5.14. The potentials refer to the

reversible hydrogen electrode (RHE, ERHE = ESHE -0.059V pH-1), which allows to compensate

for the pH dependence of the ORR and therefore provides data comparability between

different electrolyte systems.


51

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Oxygen reduction

Oxygen evolution

Non faradaic region0.10 0.15 0.20 0.25

-1.9-1.8-1.7-1.6-1.5-1.4-1.3

i / m

Acm

-2

E / VRHE

(b)

(a)

0 10 20 30 40 50-1.9-1.8-1.7-1.6-1.5-1.4-1.3

i / m

Acm

-2

pumping speed / μl min-1

i / m

Acm

-2

E / VRHE

Increased pumping speed

Figure 5.14: Cyclic voltammogram (100 mV s-1) of poly-Pt for different pumping speeds in aerated 0.1 M

H2SO4. The inset (a) shows a magnification of the forward cycle between 0.07 VRHE and 0.27 VRHE. Inset

(b) displays the arithmetic mean of the current densities from (a) plotted over the pumping speed.

As evident from the data shown, a high reproducibility of the non Faradaic, anodic region

(oxide formation) was achieved with the oxidation current density in agreement with the

literature (~0.14 mA cm-2 at 1.2 VRHE [100 mV s-1] measured compared to ~0.15 mA cm-2 at

1.2 VRHE [100 mV s-1, removed roughness factor] [167, 168]). The onset of oxygen reduction is

dependent on the scan direction due to either reducing or oxidizing preconditioning of the

surface [169], and is in good agreement with RDE measurements obtained by other authors

[25] (~0.94 VRHE compared to ~0.97 VRHE taking the forward cycle). Even though the transport

controlled region as magnified in inset (a) appears very noisy, the arithmetic means of the

diffusion limited current densities clearly increase with the pumping speed as shown in inset (b).

The mathematical description is provided by the Levich equation for a channel electrode [170],

which simplifies to

31

32

32

lim F43.5 fanalyteanalyte VxDcnI = (5-5)

for an annular electrode in a cylindrical channel [171]. This equation suits well for a rough

estimation of the expected currents. The following values were taken: n=4,

canalyte=0.3125 mol m-3, calculated from an estimated oxygen content of 10 mg l-1,


52

Danalyte=2.1·10-9 m2 s-1, x(length of the annulus)=1.5·10-5 m, and Vf=2.6·10-10 m3 s-1

(15.6 µl min-1). The length of the annulus was calculated by assuming an electrode area equal to

the channel diameter. Equation (5-5) then yields Ilim=1.71 µA, or 1.73 mA cm-2 with an

electrode area of 98960 µm2. This value is surprisingly close to the measured values, especially

since two opposing processes are assumed to take place: The first is the diffusion from oxygen

into the cell, especially through the silicone sealing, which shifts the limiting current density

upwards for all flow rates. The contrary process is the decay of the flow velocity in the vicinity

of the electrode, lowering the oxygen supply from the solution. Since the magnitude of both

effects is unknown and given the number of assumptions, it is impossible to compare the

measured and theoretical values beyond stating that both range in the same order. However,

the flow rate dependency can be accurately confirmed as shown in the following figure.

2.0 2.5 3.0 3.5 4.0-1.9

-1.8

-1.7

-1.6

-1.5

-1.4

-1.3

R2 = 0.99982

i / m

Acm

-2

(Vf / μl min-1)1/3

i = -0.226Vf1/3 - 0.968 Figure 5.15: Plot of the limiting current

densities during ORR in 0.1 M H2SO4

solution over the cube root of the volume flow

rate. The resulting linear equation and the

regression coefficient are given as inset.

Figure 5.15 demonstrates the limiting current density to be a linear function of the cube-

root of the volume flow, thus showing that the proportionality from equation (5-5) is valid for

the SFC. Nevertheless, the intercept with the y-axis does not cross the origin and is of high

value compared to the flow-dependent current density observed (e.g. an increase of the flow

rate by a factor of 6 did not increase the current density by a factor of 1.82 [=61/3], but 1.25

instead). This effect most probably originates from oxygen diffusion into the cell (shift of the

intercept) and the complex flow profile at the tip (reduced slope). Assuming that the intercept

of -0.968 mA cm-2 from Figure 5.15 reflects a flow rate independent background current

density by an external oxygen supply, the flow rate dependent reduction current density at

15.6 µl min-1 equals 0.562 mA cm-2.


53

A rough comparison to the RDE can be made by using this limiting current density and

calculating the diffusion layer thickness according to [18] (p. 192)

lim

02

2F

ic

Dn OON =δ (5-6)

giving 45.1 µm. This corresponds to a rotation rate of approximately 430 RPM using

31

61

21

261.1 ON Dνϖδ −= (5-7)

assuming a viscosity of 10-2 cm2 s-1. However, more work is required to characterize the flow

profile at the tip and allow quantitative comparison to the RDE and other electrode setups

utilizing forced convection.

5.5.6 Summary of the results In this chapter, a fully automated microelectrochemical scanning flow cell setup was

developed. The comprehensive system characterization revealed a functionality which has not

been reported so far, especially concerning the integration of downstream analytics with high

sensitivity (LOD around 100 nmol l-1). The electrochemical data obtained on Al, Cu and Pt is

of high reproducibility and comparability to the literature, while full computer control enables

measurement series without user interaction. The transport characteristics of the system, as

investigated by means of oxygen reduction on platinum, revealed a clear flow rate dependence

with high comparability to a classical channel electrode. For a flow rate of 15.6 µl s-1, as mostly

used in this study, the transport limited current density for the ORR was found to be

1.53 mA cm-2.

Chapter 6: Corrosion of pure Zn

54

6 Corrosion of pure Zn The following section deals with the electrochemical behavior and dissolution of pure zinc

samples in a variety of media. The aim is to provide a comprehensive dataset demonstrating the

relationship between dissolution rate and electrochemical data and quantify the impact of

different parameters (e.g. pH value) on both properties. It is to be clarified if the corrosion

current density of zinc can be calculated from the dissolution profile, and how this value is

affected by the use of buffered systems and aggressive anions.

6.1 Unbuffered NaCl solution 6.1.1 Open circuit potential and dissolution

The corrosion of zinc in unbuffered NaCl solutions is mainly governed by zinc oxidation

and corresponding oxygen reduction as described in sections 2.1.3 and 2.1.4. Precipitation of

zinc influences both reactions by the inhibition of anodic dissolution [72] and reduced oxygen

transport [24] through the surface film. Due to its fundamental importance, film formation and

film dissolution are primary determinants in corrosion experiments. Since both processes are

dynamic and dependent on the experiment time and prehistory, monitoring of the corrosion

rate is only accessible by methods that do not introduce major disturbances by external

polarization. In the open-circuit case, the measured current is zero by definition, but the half-

cell current for dissolution can be determined by the zinc concentration detected downstream

according to equation (5-4). It is important to consider that precipitation products escape

detection, but the impact of this process can be estimated from the time resolved dissolution

rate: Assuming that the precipitation layer does not grow infinitely, a steady state between film

formation and dissolution will be finally reached.

6.1.1.1 Effect of chloride concentration As chloride accelerates zinc corrosion by interference with surface film formation [2, 36], it

appears useful to include the chloride concentration as an additional parameter in the

experimental series. This serves the purpose of selecting a specific electrolyte composition for

further experiments and allows classification of these results with respect to the large variety of

electrolytes used in the literature. The corrosion potential as measured over 4 ks of continuous

flow are shown in Figure 6.1.


55

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0-800

-775

-750

-725

-700

-675

-650 0.025 M NaCl 0.05 M NaCl 0.1 M NaCl 0.4 M NaCl 1 M NaCl

Pote

ntia

l / m

VSH

E

time / ks

Corrosion potentialshifts cathodically

Figure 6.1: Corrosion potential of

pure zinc in aerated NaCl solution of

various concentrations. Flow rate:

15.6 µl min-1.

The values obtained are in good agreement with the corrosion potentials reported by other

authors (-770 mVSHE in 0.1 M NaCl [79], -785 in 0.5 M NaCl [50]) and exhibit a high degree of

stability throughout the experiment. Except for the initial period of around 200 s where a

cathodic drift of the corrosion potential with time is observed for all concentrations, the steady

state value fluctuates in the range of few mV. Figure 6.1 further demonstrates that increasing

chloride concentrations shift the corrosion potential cathodically. The following figure shows

the dissolution profiles during the experiments:

0 1 2 3 4 5 6

0.0

0.5

1.0

1.5

2.0

2.5

0.05 M NaCl

0.1 M NaCl 0.4 M NaCl

1 M NaCl

0.025 M NaCl

purging

[Zn]

2+ /

μmol

l-1

time / ks

OCP scan 4 ks

Figure 6.2: Zinc dissolution profiles

during 4 ks OCP scans in NaCl

solution of various concentrations.

Flow rate: 15.6 µl min-1.

An interesting observation is that all concentrations up to 0.1 M show a plateau

concentration reached after approximately 2-3 ks, presumably reflecting the steady-state

corrosion rate. This behavior is not observed after further increase of the chloride content.

Instead, the dissolution rate increases steadily throughout the experiment with only minor

indication of a leveling in the case of 0.4 M NaCl. Remarkably, the dissolution in 0.4 M NaCl


56

falls below the 0.1 M counterpart in the initial 3 ks. It is worth noting that the reproducibility of

corrosion experiments in aerated, unbuffered NaCl solution is complicated [24, 79] due to the

very complex nature and interactions of the processes involved (see section 2.1.7). This effect is

pronounced at higher chloride concentrations, which may reflect the ability of chloride to both

form soluble complexes and be incorporated in specific corrosion products (e.g. Simonkolleite

[Zn5(OH)8Cl2·H2O] among the dominant precipitates). The stoichiometry of the precipitates, in

particular the chloride content, is both dependent on the pH and the chloride activity. W.

Feitknecht [172] for instance described a transition from ZnCl2·6Zn(OH)2 to ZnCl2·4Zn(OH)2

between 0.1 and 1 M Cl- under neutral conditions. Since in-situ spectroscopy is not available

within the scope of this study, this issue can not be finally resolved (ex-situ or UHV techniques

are not applicable due to dehydration and surface changes).

However, it is evident from the data that the final values in the dissolution profiles increase

with increasing chloride content. The oxygen reduction reaction is transport limited at all

measured corrosion potentials (see mixed potential theory section 2.1.5), and the rate of oxygen

reduction needs to increase with increasing zinc dissolution because it constitutes the only

significant counter reaction. The cathodic potential shift is however contrary to the simple

assumption that only the transport limit of oxygen shifts upwards due to reduced blocking

properties of the surface film, because this would result in an anodic shift of Ecorr. It appears

likely that the anodic Tafel slope is affected by the chloride concentration (see page 10), or that

chloride may shift the equilibrium potential of zinc cathodically due to a decrease in the activity

of zinc ions in the vicinity of the electrode as a result of complex formation. The following

figure shows anodic potential sweeps to investigate possible changes in Tafel slope.

-0.80 -0.75 -0.70 -0.65 -0.60 -0.55 -0.50

10-5

10-4

10-3

0.40 M - 19 mV dec-1

0.20 M - 22 mV dec-1

0.05 M - 23 mV dec-1

i / A

cm

-2

E / VSHE

0.01 M - 23 mV dec-1

increasing NaCl concentration

Tafel fit

Figure 6.3: Anodic potential sweeps

(5 mV s-1) starting from the previously

recorded OCP (1000 s) on pure zinc in

NaCl solution of various concentrations at

a flow rate of 15.6 µl min-1. Linear Tafel

fits are indicated with the respective slopes

listed in the figure.


57

The Tafel slopes display a slightly increasing trend, resulting in lower overpotentials required

to increase the current density by one decade. However, this change in the anodic dissolution

kinetics may not account for a shift of Ecorr covering nearly 100 mV. It is therefore concluded

that the effect of NaCl on the corrosion potential mainly originates from the effect on the

reversible potential of the zinc electrode under these conditions.

Since the dissolution kinetics do not change significantly with changing NaCl concentrations

and with respect to the uncertainty regarding the origin of the unsteady dissolution profiles

observed above 0.1 M NaCl, a concentration of 0.1 M NaCl was selected as a standard for all

further experiments. This concentration exhibits a high but stable corrosion rate and is

considered as appropriate to clarify the impact of other parameters (e.g. pumping speed,

magnesium addition etc.).

6.1.1.2 Effect of pumping speed The flow rate of the electrolyte is of major importance concerning the time-resolution of the

spectroscopic system (dead-time, peak broadening) and affects the surface reactions by a

convective supply (see ORR on Pt, section 5.5.5) or removal of species. Similar to the chloride

concentration discussed in the previous section, a brief screening of the impact of this

parameter is required that justifies the selection of a standard value used when screening

additional parameters. For this purpose, an experimental series was performed similar to the

precious section with a zinc sample exposed to a constant flow of electrolyte (0.1 M NaCl)

while recording the OCP (2.5 ks). The resulting dissolution profiles are shown in Figure 6.4.

0 1 2 3 4

0.0

0.5

1.0

1.5

2.0

2.5

3.0 7.8 μl min-1

15.6 μl min-1

purging

31.2 μl min-1

[Zn2+

] / μ

mol

l-1

time / ks

OCP scan 2.5 ks

Figure 6.4: Dissolution

profiles of zinc recorded

during exposure to 0.1 M

NaCl solution at various

flow rates.


58

An immediate effect of the flow rate on both the dead time and the shape of the signal

decay during purging can be clearly observed. The measurement at 15.6 µl min-1 furthermore

agrees well with the data presented in Figure 6.2 for 0.1 M NaCl which supports the reliability

of the method employed. The measured concentration needs to be normalized to the flow rate

to draw conclusions about the effective corrosion current density, which is achieved by

transformation of the dissolution profile into a corrosion current density transient according to

equation (5-4):

0 1 2 3 4

0

20

40

60

80

100

15.6 μl min-1

31.2 μl min-1

7.8 μl min-1

i Dis

s / μA

cm

-2

time / ks

purging OCP scan 2.5 ks

Figure 6.5: Corrosion

current density transients

calculated from the

spectroscopic data for

different flow rates in

0.1 M NaCl solution.

The sequence of the profiles at different flow rates as compared to Figure 6.4 inverts,

showing that high flow rates apparently yield larger corrosion currents. This appears intuitive

given an increased supply of oxygen and accelerated removal of precipitates. However, the

difference in the observed signal decay (tailing) may lead to errors if only the plateau values and

not the integrals are considered. Numerical integration along the dataset yields the following

table.

Flow rate / µl min-1 Charge density / mC cm-2

7.8 142.4

15.6 169.1

31.2 196.3

Table 6.1: Numeric integrals

calculated from Figure 6.5 for

different flow rates.


59

The charge density confirms the aforementioned observation that the corrosion rate

increases with higher flow rates. Since the corrosion current densities (60-90 µA cm-2) all fall

significantly below the transport limit of oxygen in the electrolyte (1.53 mA cm-2, see page 53),

the pumping speed needs to primarily affect the surface film. It has been previously described

that precipitates and surface films on zinc originate from a limited uptake of zinc species by the

solution (see section 2.2.5) under these conditions. Therefore, a higher convection results in a

decreased diffusion layer, steepening the concentration gradient from the electrode into the

solution, and increasing the zinc dissolution rate and consequently icorr.

The increased time-resolution at higher flow rate is accompanied by a lower signal to noise

ratio as evident from Figure 6.5, because the intrinsic noise of the spectroscopic system is

multiplied with a larger factor (being the flow rate, see equation (5-4)). Considering both

aspects, an intermediate pumping speed of 15.6 µl min-1 was selected as a standard for further

experiments.

6.1.2 Galvanostatic experiments The comparison of spectroscopic data obtained downstream with current readings from the

potentiostat faces an obvious difficulty: While the potentiostat records only electrochemical

reactions induced externally (and therefore misses spontaneous corrosion), the spectroscopic

system is only sensitive to dissolved species (and therefore misses precipitates and absorbed

species). This apparent challenge constitutes one of the major advantages of the integrated

approach presented, but requires further investigation.

Since cathodic or anodic polarization from the OCP suppresses the respective counterpart

(e.g. anodic polarization reduces the cathodic reaction rate, see section 2.1.5), the deviation

between measured current and effective current (including the spontaneous reactions) vanishes

for sufficiently large overpotentials. Cathodic polarization can therefore be utilized to suppress

the anodic zinc dissolution, which can be directly monitored downstream. The current at which

the dissolution exactly vanishes therefore corresponds to the corrosion current in the absence

of an external polarization and can be directly compared to the values obtained

spectroscopically as previously shown in Figure 6.5. Negative differences between

electrochemical and dissolution current may originate from film formation or other processes

allowing species to escape detection. The following figure summarizes the data obtained during

a galvanostatic (chrono-potentiometric) series on pure zinc:


60

0 250 500 750 1000 1250

0.0

0.4

0.8

1.2

1.6

2.0

2.4

70 nA60 nA50 nA

20 nA

- 20 nA- 30 nA

- 40 nA

- 50 nA

80 nA

0 nA

time / s

[Zn2+

] / μ

mol

l-1

- 60 nA

(a)

0

20

40

60

80

100

120

IDiss / nA

Figure 6.6: Zinc

concentration (left axis)

and corresponding current

(right axis) during

galvanostatic experiments

(500 s) in 0.1 M NaCl

at different applied

currents. Flow rate:

15.6 µl min-1.

It is to note that the experiment time is comparably short so that the corrosion does not

reach steady state. The data therefore corresponds to the initial periods in the previously shown

dissolution profiles, with very good agreement between the values recorded at 0 nA and the

OCP dissolution presented earlier. The reason for the short experiment duration is the strong

tendency of the electrolyte to leak out of the cell upon cathodic polarization, effectively

penetrating the interface between working electrode (zinc) and silicone sealing. This effect is

most probably driven by a transport process similar to cathodic delamination and unfortunately

limits the applied cathodic potential [50, 173].

As evident from Figure 6.6, a cathodic polarization suppresses the zinc dissolution in

accordance with the previously mentioned principle. All curves recorded at negative applied

currents appear shifted downwards, with the curve at -60 nA (maximum cathodic current due

to leaking) completely inhibiting zinc dissolution except for the final 150 s of the experiment.

As indicated by dashed lines (a), the shift of the dissolution current at -60 nA applied current

equals -60 nA with remarkable accuracy, proving excellent correlation between potentiostat

readings and spectroscopic data with the setup.

The potentials recorded during the galvanostatic series are shown in the following graph:


61

0 100 200 300 400 500

-1000

-950

-900

-850

-800

-750

-700

-650

time / s

0 100 200 300 400 500

0.0

0.4

0.8

20 nA

- 30 nA- 40 nA

- 50 nA

80 nA

time - td / sc

[Zn2+

] / μ

mol

l-1

80 nA20 nA

-30 nA -50 nA-40 nA

Pote

ntia

l / m

VSH

E

Figure 6.7: Potential

transients during

galvanostatic experiments

(500 s) in 0.1 M NaCl

at different applied

currents. Flow rate:

15.6 µl min-1. The inset

shows the initial

dissolution from Figure

6.6, shifted by td.

The inset allows comparison of the prolonged anodic drift observed for negative applied

currents with the onset for dissolution as taken from Figure 6.6. The potentials recorded show

a strong decrease to values around -950 mVSHE to -1 VSHE within less than 2 seconds, followed

by an anodic drift up to approximately -650 mVSHE. The latter process is interpreted as a

change in the surface film due to the externally applied current because the effect of the ORR

on the pH at the electrode surface differs largely in the case of galvanostatic experiments as

compared to free corrosion. In the latter case, the simultaneous liberation of zinc ions provides

a local buffer [4, 174] according to

−+ + OHZn2 +)]([ OHZn (6-1)

and

−+ + OHOHZn )]([ 2)(OHZn (6-2)

continuously scavenging the hydroxyl ions produced. Equation (6-1) shows that the formation

of a mono-hydroxo complex (dominant around pH 8 [166]) already implies a buffer effect

without the formation of precipitates. At higher pH values, the follow up reaction (eq. (6-2)) is

furthermore determining the film formation and therefore zinc dissolution.

In case of an externally applied current, the ratio between anodic and cathodic reaction on

the zinc surface is shifted according to the magnitude of the current, resulting in severe changes

of the surface pH in the case of cathodic polarization. Hence, the corrosive character of the

electrolyte is changed as the pH increases, supporting the stability of surface films according to

the Pourbaix diagram (up to pH values around 11), with possibly minor effects on the chloride

concentration due to incorporation into the film (e.g. Simonkolleite [Zn5(OH)8Cl2·H2O]). The

change in potential from Figure 6.7 is therefore assumed to originate from a shift in the


62

corrosion potential, which has been demonstrated for the formation of surface films in borate

solutions [71]. The previously presented dependence of the corrosion potential on the NaCl

concentration also suggests that the corrosive behavior of the electrolyte changes the reversible

potential of the zinc electrode and therefore Ecorr significantly. The recorded potentials during

galvanostatic experiments then change appropriately to maintain a steady overpotential.

The data from Figure 6.7 further suggests that the passive behavior of the surface is of

limited duration, ultimately resulting in a strong cathodic shift of the potential in conjunction

with active zinc dissolution. Given the presence of a native oxide on zinc exposed the

atmosphere (usually in the range of several nm [43]), the dissolution or stabilization of this layer

is assumed to be the dominant process immediately after contact to the electrolyte is

established. Therefore, the anodic shift of the potentials at applied cathodic currents most likely

reflects the delay concerning the destabilization of the oxidic/hydroxidic layer which, even at

high applied currents, is not completely prevented. This breakdown most likely originates from

the presence of chloride as a corrosive anion that triggers pitting corrosion [175]. To clarify the

impact of the native oxide, an experiment with a corrosion period at the OCP (300 s) prior to

the chrono-potentiometry was performed and the results displayed in the following figure.

0 250 500 750 1000 1250

0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2PurgingGalvanostaticOCP

time / s

[Zn2+

] / μ

mol

l-1

td

0

20

40

60

80

100

120

140

160

80 nA

-20 nA

IDiss / nA

-40 nA

Figure 6.8: Zinc

concentration profiles during an

OCP - chrono potentiometry

couple in 0.1 M NaCl. The

dashed line indicates the

expected progression in the

absence of external

polarization estimated from the

OCP dissolution profiles

shown earlier.

A remarkable reproducibility of the zinc concentration during the initiation period of the

free corrosion was observed, followed by a shift of the further profile reflecting the applied

current. The potentials recorded during the galvanostatic experiment did not show significant

shifts and all fall between -730 and -750 mVSHE.


63

6.1.3 Potentiodynamic sweeps Potentiodynamic sweep experiments are a commonly employed technique in corrosion

science. The main characteristics investigated with this method are listed in the following:

I. Individual half cell reactions: Both cathodic and anodic reactions are effectively isolated in

the respective potential region of the sweep experiment if it covers a sufficient range.

Transport limitations can be immediately quantified [24] and Tafel constants derived,

even though the latter implies a complexity that is often underestimated (see section

2.1.3)

II. Corrosion rates: Both Tafel extrapolation [176] and linear polarization resistance (LPR)

measurements [177] are widely used to determine the corrosion current density

assuming the presence (or absolute dominance) of only one anodic and one cathodic

reaction.

III. Passivity: The presence of a passive region and the corresponding critical current density

for passivation and passive current density are accessible by potentiodynamic methods

[1] (pp. 119).

Usually, the sweep starts in the cathodic region with anodic progression at low scan rates

(e.g. 0.6 V h-1, being 0.167 mV s-1 according to the ASTM standard [178]). In the case of zinc

though, the applicability of potential sweeps for accurate corrosion monitoring or –prediction

has not been demonstrated [2, 179]. Especially the reproducibility appears challenging in mildly

corrosive environments (e.g. near neutral NaCl) where complex surface reactions occur [24].

Since the feasibility of monitoring the corrosion rate of zinc using the SFC has been

demonstrated in the previous chapter, the validity of potential sweep experiments can be

addressed easily within the experimental conditions present. A flow rate of 31.2 µl min-1 was

selected with an electrolyte composed of aerated 0.1 M NaCl solution. The potential was swept

anodically starting from -1.48 VSHE at various scan rates ranging from 1 to 60 mV s-1 and the

resulting graphs are combined in Figure 6.9.


64

-1.6 -1.4 -1.2 -1.0 -0.8 -0.6

10-7

10-6

10-5

10-4

10-3

10-2

-0.9 -0.8 -0.7

10-6

10-5

10-4

i /

Acm

-2

E / VSHE

Tafel fit

i / A

cm

-2

E / VSHE

Scanrate / mV s-1: 60 - 40 - 20 - 10 - 5 - 2 - 1

Figure 6.9:

Potentiodynamic sweeps at

different scan rates for pure

zinc in aerated 0.1 M NaCl.

Flow rate: 31.2 µl min-1. The

inset shows the magnified

region around the current

inversion with the

approximated region of the

crossing of linear Tafel fits

(grey circles).

It can be recognized from the figure that the potential of current inversion (being the

potential of zero current) shifts towards more anodic potentials as the scan rate is decreased.

This indicates that the onset for an anodic total current is shifted significantly, while this shift

covers more than 200 mV in range. This is surprising considering that this point is generally

assumed to reflect the corrosion potential and has been reproducibly measured to be around -

760 mVSHE in former experiments. Furthermore, a drastic change in the shape of the curve in

this particular region occurs along the scan rate variation that immediately affects a possible

Tafel fit. It is to note that the concept of Tafel constants is not fully applicable in this case

because it requires the system to be at steady state at each applied potential. However, the

approximated regions for this data evaluation technique are indicated in the inset of Figure 6.9

and show a slight increase in the corrosion current density (y-axis position of the intercept of

cathodic and anodic linear Tafel fit) with decreasing scan rate. These values ranging from

around 6 to 50 µA cm-2 all fall below the values determined spectroscopically for the flow rate

used (~ 90 µA cm-2, see Figure 6.5). Given the scan rate dependency of corrosion current

density and corrosion potential, it is impossible to adjust the scan rate in such an experiment to

yield values for both properties that are comparable to galvanostatic or spectroscopic

techniques.

The scan rate dependency observed for a theoretically faradaic process implies that the

system is not at steady state at all points, but undergoes changes during the measurement. Assaf

et al. made a similar observation under the same conditions (except for the absence of

convection) [180]. It was demonstrated that the anodic current density during potentiodynamic


65

scans increases with higher scan rates, which was explained on the basis of diffusion processes

limiting the dissolution. Plotting the potentials of current inversion from Figure 6.9 against the

square root of the scan rate, a linear relationship is obtained as shown in Figure 6.10.

0 1 2 3 4 5 6 7 8

-0.90

-0.85

-0.80

-0.75

-0.70

-0.65

E i=0 /

V SHE

(ν / mV s-1)1/2

R2=0.9954

Figure 6.10: Potentials of current inversion

from Figure 6.11 as a function of the square

root of the scan rate. The coefficient of

determination for a linear fit is given in the

figure.

This result suggests that the scan rate dependency originates from a diffusion process [18] (p.

281 ff.), i.e. uptake of zinc complexes by the solution. The diffusion zone generated during the

anodic sweep includes the formation of surface films, as an inhibition of the anodic reaction

only occurs if the generation of zinc ions exceeds the amount that can be taken up by the

solution. The fact that the surface film formation process dominates the potentiodynamic scans

results from a reduced zinc surface as the initial condition in all experimental runs. This can be

seen from the scan rate independent current densities for the transport limited oxygen

reduction, matching the transport limit measured on platinum (see section 5.5.5) very well.

However, an exact determination of the corrosion current and –potential in unbuffered

NaCl solution demonstrated for other techniques is not possible from the data shown in Figure

6.9. Especially the shape of the curve around the corrosion potential denies data evaluation by

linear polarization resistance and renders Tafel extrapolation highly questionable. While still

valuable for the investigation of passivity, the use of potential sweep experiments will further

be limited to this application within the scope of this study.

6.1.4 Summary of the results Even on a well characterized system as pure zinc, the complementary coupling of

electrochemical experiments and online spectroscopy was shown to provide novel and essential

information on corrosion processes. Within a comprehensive evaluation of the fundamental

parameters chloride concentration and flow rate, it was shown that higher chloride

concentrations cause a significant cathodic shift of the corrosion potential. This process was


66

primarily attributed to a change in the reversible potential of zinc since the Tafel slopes only

show minor deviations. The dissolution profiles showed an increase in dissolution rate, with

strong indication of complex film formation processes at NaCl concentration higher than

0.1 M. The flow rate was shown to increase the corrosion rate, suggesting a high impact of

diffusion processes on the formation and dissolution of surface films. The impact of zinc

transport from the electrode into the solution was further illustrated on potentiodynamic

sweeps, which showed a strong scan rate dependence as a result of diffusion processes during

the anodic zinc dissolution. As a consequence, potentiodynamic sweeps appear inappropriate

for an accurate determination of Ecorr and icorr in near neutral NaCl solutions. However, the

integrated system presented is capable of a measurement mode that overcomes this issue by

recording the corrosion potential and deriving the corresponding corrosion current density

from the dissolution profiles obtained by downstream analytics.

The validity of this method has been proven by galvanostatic techniques, which showed a

very good correlation between applied and spectroscopically determined currents. The

underlying dataset covers a large number of dissolution profiles at different applied current

densities and therefore provides an exceptionally detailed view on the correlation between

electrochemical techniques and downstream analytics on zinc.

6.2 Borate buffers of various pH 6.2.1 Open circuit potential and dissolution

As repeatedly pointed out in the previous section, the pH of the solution, especially in the

vicinity of the electrode, is of major importance. Since both anodic and cathodic reactions in

the case of zinc corrosion affect the pH (see page 61), an experimental series was designed

where this effect was countered by the use of a buffer system. Unfortunately, most buffer

systems in the neutral range can not be used in the case of zinc due to undesired interactions

between the buffer anions and zinc (e.g. precipitation in the case of phosphate or carbonate,

increased dissolution of ZnO in the presence of acetate [62]). The boric acid - sodium borate

system (pKs 9.25) is a suitable system with minimum interactions that is very frequently used in

electrochemical experiments on zinc samples [36, 50, 71, 100]. As the transport of zinc in the

SFC is highly sensitive to the pH in the carrier stream (see Figure 5.11), a proper coupling can

only be achieved in near neutral or acidic media, while the latter is additionally restricted due to

the very low buffer capacity in the acidic region.


67

In order to investigate the buffer effect and the impact of the solution pH in the near

neutral region, an experimental OCP (1000 s) series was performed in 0.1 M borate buffer of

various pH values. The recorded potentials are shown in the following figure [181].

0 200 400 600 800 1000 1200-800

-700

-600

-500

-400

-300

-200

-100

passive region

pH 9.0

pH 8.0pH 7.8pH 7.4

pH 7.1pH 6.6

Pote

ntia

l / m

VSH

E

time / s

active region

Figure 6.11:

Open circuit potentials

recorded for zinc in 0.1 M

borate buffer of various pH.

Flow rate: 15.6 µl min-1.

Two measurements are shown at each pH value to allow an estimation of reproducibility.

An immediate observation is that the OCP exhibits a shift covering around 500 mV within the

pH window between 6.6 and 9.0. This shift is clearly attributed to a surface change (e.g.

coverage with oxides) since the pH dependence of anodic and cathodic reaction can not

account for this magnitude [182]. Additionally, nearly the total shift is confined between pH 7.1

and 7.4 with only minor increases in the recorded potential at lower and higher values up to 9.0.

At this last value, the OCP did not stabilize within the duration of the experiment which is

attributed to the very low solubility of zinc hydroxide at this value, leading to a nearly

quantitative precipitation that remains nearly unaffected by the convection in the cell [166].

Since the presence of surface films on zinc was reported to start around pH 4 [14, 30], the

pH region between 7.1 and 7.4 can not be the onset for its formation. Nevertheless, the sole

presence of surface species does not necessarily imply a corrosion mechanism through the

surface film with the resulting anodic shift of the OCP [71]. It has been shown by several

authors that many surface species on zinc are not passivating [7, 70, 71, 183], resulting in the

presence of active regions despite a surface coverage composed of a variety of corrosion

products. In fact, as the open circuit potential is not area dependent, the presence of a single

active region is considered sufficient to dictate a cathodic OCP. As easily recognized from


68

Figure 6.11, the region between pH 6.6 and 8.0 consists of only two major states, which are

labeled “active” and “passive”. The dominating parameter would therefore be the integrity of

the surface film, which is apparently achieved at a pH value of 7.4 and above at the OCP under

electrolyte flow (15.6 µl min-1).

6.2.2 Potentiodynamic sweeps To further characterize the electrochemical behavior, potential sweep experiments were

performed subsequent to the OCP-scans from Figure 6.11 with a scan rate of 2 mV s-1. Anodic

or cathodic directions were swept separately, each after an individual OCP-period of 1000 s.

The following figure displays anodic and cathodic sweep combined. Note that the

measurement at pH 6.6 can not be evaluated since it is dominated by the non-ionic nature of

boric acid and the very low concentration of borate resulting in high electrolyte resistance.

-2 -1 0 1 2 3

-500

-400

-300

-200

-100

0

100

200

300

400

-1 0 1 2 310-5

10-4

10-3

i / A

cm

-2

E / VSHE

(C)

(B)

i / μ

A c

m-2

E / VSHE

pH 6.6 pH 7.1 pH 7.4

pH 7.8

pH 8.0

pH 9.0

(A)

Figure 6.12:


2 mV s-1 starting from the

previously recorded OCP in

both anodic and cathodic

direction on different locations.

Flow rate 15.6 µl min-1. The

inset shows the anodic

direction, excluding pH 6.6,

on a logarithmic scale.

A clear trend towards lower current densities is observed for increasing pH. Especially the

logarithmic presentation gives clear evidence that the passive current density is strongly pH

sensitive and shifts to lower values as the solution alkalinity increases. Another feature is the

presence of a current peak (A) in the case of pH 7.1 which shows similarity to a classical pre-

passive peak [1, 136], even though the following region of reduced current density is

comparably short. Nevertheless, it is an indication that the formation of a closed film is

induced by the anodic sweep which is absent under OCP conditions. The anodic progression

to high potentials around 2 VSHE indicates an additional oxidation process (B) which is

interpreted as the inhibition of a film breakdown by supersaturation of the interface [7, 72] and

subsequent zinc precipitation. The final increase in the current signal is associated with film


69

breakdown and/or oxygen evolution, but requires the spectroscopic data shown later to be

clarified.

The cathodic scans (C) combine well with the anodic counterparts, causing the combined

curves to appear as a single measurement. A slight increase in current density with increasing

pH is observed between pH 7.4 and 9.0, probably indicating different blocking properties of

the surface film concerning oxygen transport [30].

The clear and steady trend observed in the plateau current density along the pH series is in

contrast to the discrete behavior observed in the corresponding OCP values from Figure 6.11.

As previously pointed out, the open circuit potential is highly sensitive to the presence of at

least one active region, while potential sweeps induce surface changes on the whole surface

obviously able to change the surface film characteristics. To clarify whether a change in the

nature of the surface film (between pH 7.1 and 7.4) is present, additional dissolution data is

required.

0 1 2 3 4 5

0

2

4

6

8

10

0

200

400

600

800

i Dis

s / μA

cm

-2

(A)

(A)

pH 8.0 pH 7.8

pH 7.1 pH 6.6

pH 7.4

pH 9.0

[Zn2+

] / μ

mol

l-1

time / ks1000 s OCP - sweep anodic - 800 s flush - 1000 s OCP - sweep cathodic

(A)

Profiles shift at higher pH due to a prolonged anodic sweep duration

Figure 6.13: Zinc concentration profiles and corresponding dissolution current density for an experimental

series indicated on the bottom for pure zinc in 0.1 M borate buffer of various pH at a flow rate 15.6 µl min-1.

The shift of the profiles at higher pH values originates from a prolonged duration of the anodic sweep.

Figure 6.13 shows the concentration profiles recorded during the electrochemical

experiments discussed before with the sequence of measurements indicated on the bottom. All

curves show an initial rise to a plateau value (A), followed by a zinc peak as a consequence of


70

the subsequent anodic sweep experiment. After purging the cell, another OCP scan is

performed on a different location on the substrate that yields a very reproducible shape, while

the following cathodic scan direction leads to an immediate decline of the signal. Due to the

different experiment time for the anodic scan (with dynamic end points, see Figure 6.12), the

profiles are shifted along the time axis as the pH (and therefore the passive range) increases. All

events indicated on the bottom are therefore not confined to an absolute time with the

exception of the first 1000 s OCP measurement (starting at t = 0 s) and the anodic sweep

(t = 1000 s).

The initial rise of the zinc concentration is remarkably sharp compared to the profiles in

unbuffered NaCl solution (see Figure 6.5) reaching a high dissolution current density associated

with plateau of ~200 µA cm-2 for pH 7.1 as compared to around 80 µA cm-2 for unbuffered

NaCl solution. Especially the profile at pH 6.6 shows very high zinc dissolution, suggesting an

active etching of the sample by the electrolyte. Please note that the absolute zinc concentrations

in NaCl and borate buffer are not comparable because different capillary cells of different

diameter were used. The dissolution current density though corrects for that fact as it is

normalized to the wetted area.

This steady state corrosion rate is achieved very quickly in the absence of chloride and,

especially, in the presence of a pH buffer system. The impact of the buffer itself becomes clear

when comparing the measured zinc plateau and proton concentrations shown in Figure 6.14.

0.0 0.1 0.2 0.30

2

4

6

8

0100200300400500600

[Zn2+

] / μ

mol

l-1

[H3O+] / μmol l-1

i Dis

s / μA

cm

-2

[Zn2+] = 29.33[H3O+] - 0.0904

R2 = 0.9996

Figure 6.14: Steady state

concentrations and corresponding

dissolution current densities from

Figure 6.13 (A) as a function of

the proton concentration. A

linear regression is performed

along all data points.

The linear dependence observed suggests a chemical dissolution mechanism which is limited

by proton transport [181] according to


71

++ HZnO +)]([ OHZn (6-3)

or

++ HOHZn 2)( OHOHZn 2)]([ ++ (6-4)

The etching proceeds via ZnO or Zn(OH)2 because of the presence of a surface film under

these experimental conditions. It is further supported by investigations on ZnO:Al as shown

later in this study and the results presented by Guśpiel and Riesenkampf [184] demonstrating

ZnO dissolution to be controlled by proton diffusion in slightly acidic media (pH 4-7). The

same applies for the protonation of zinc hydroxide [166, 185].

The high slope of 29.33 of the linear regression from Figure 6.14 is a consequence of the

difference between proton activity (pH) and proton availability (buffer capacity) present in all

buffer systems. A local depletion of protons is therefore countered by the buffer species,

resulting in a significantly higher diffusion limit than estimated from the proton concentration

alone. The linearity between proton concentration and dissolution current density furthermore

excludes zinc transport to be rate determining since the current density would be proportional

to the zinc gradient in the diffusion layer. The zinc concentration in solution is however not

proportional to the proton concentration, but the solubility product Ksp divided by [OH-]²

according to

22

][][ −

+ =OH

KZn sp (6-5)

as experimentally shown by Reichele et al. in the pH region used in this study [166]. This

equation transforms to

22

2 ][][ ++ = HKK

Znw

sp (6-6)

with Kw as the dissociation constant of water. The resulting relationship can be clearly excluded

from the data.

Moreover, it is possible to compare the dissolution profiles from Figure 6.13 to the

measured current density from Figure 6.12:


72

0.0

1.2

2.4

3.6

4.8

0.0 0.5 1.0 1.5 2.0 2.50

100

200

300

400

time / ks

i Dis

s / μA

cm

-2

0

100

200

300

4000.0 0.8

i / μA cm

-2

pH 7.1- 0.7

tdPurgingE / VSHE

[Zn2+

] / μ

mol

l-1

OCP

0.0

1.2

2.4

3.6

4.8

0.0 0.5 1.0 1.5 2.0 2.5 3.00

100

200

300

400

time / ks

i Dis

s / μA

cm

-2

0

100

200

300

4001.7 2.40.3 1.0

i / μA cm

-2

pH 7.4

- 0.4

td PurgingE / VSHE

[Zn2+

] / μ

mol

l-1

OCP

Figure 6.15:

Zinc dissolution profiles and

corresponding dissolution current

density (left axis) compared to the

measured current density during

potentiodynamic sweeps (right axis)

for pure zinc in 0.1 M borate

buffer at pH 7.1 and 7.4 at a flow

rate of 15.6 µl min-1. The

measurement sequence including the

applied potentials is indicated on

the top.

The difference between dissolution current density and measured current is a consequence

of parallel spontaneous reactions i.e. free corrosion (oxygen reduction) in the case of a higher

iDiss, or film formation in the opposite case. In the latter case, zinc ions are incorporated into

surface films and therefore escape detection downstream. This effect is observed at the peak

current density at pH 7.1 and during most of the anodic sweep at pH 7.4. In the former case,

the sharp increase in the measured current is not quantitatively followed by the dissolution

current signal, being in full agreement with the necessity of film formation to be the origin of

the current decrease thereafter. Due to the strong background dissolution as a consequence of

the comparably low pH, quantification proves difficult.

This is different in the case of a higher pH with the background dissolution significantly

lowered. The second diagram in Figure 6.15 shows a measured current exceeding the

dissolution current throughout most of the anodic sweep duration, being evidence of

continuous film formation. Since the dissolution at the OCP still proceeds with significant rates,

it is not possible to immediately quantify the film formation rate as the reduction of the


73

spontaneous corrosion rate is unknown. Nevertheless, the resulting film thickness can roughly

be estimated according to

ZnO

ZnOanodicDissZnO F

Mtiidρ⋅

⋅⋅⋅−=

210)( 7

(6-7)

with (iDiss-i)·tanodic being the averaged charge consumed for film formation and 107 as the

conversion factor from cm to nm in the case of zinc oxide formation. For an experiment time

of 1000 s with an average difference between iDiss and i of 25 µA cm-2, this relation gives an

increase in oxide thickness of 18.8 nm. A complementary XPS surface analysis as shown in the

following chapter is required for clarification.

The strong mismatch between measured and dissolution current density at the final stage of the

experiment indicates that the steep increase in measured current is mainly attributed to oxygen

evolution because the measured current density is not quantitatively reflected by the released

zinc ions. However, it will be shown in chapter 7 that this reaction triggers chemical dissolution

due to a change of the surface pH (even in buffered solutions as present here) in addition to a

possible direct lattice attack (film breakdown). Consequently, an increase of zinc dissolution is

detected in the respective potential region.

6.2.3 XPS-Analysis In order to investigate the dependence of the surface film thickness on the electrochemical

treatment, small spot XPS-measurements (100 x 100 µm2) were performed on locations that

have been previously subject to different electrochemical treatment. For this purpose, a

quadratic 2 x 2 array spaced 600 µm was generated by executing 4 different electrochemical

experiments: 100 s OCP, 1000 s OCP, anodic sweep to 0.3 VSHE and anodic sweep to 2.0 VSHE.

Within this array, 5 XPS measurement spots were placed with one inside each electrochemically

addressed location and one on the native surface in the centre. For depth profiling, all spots

were sputtered simultaneously with a 2 x 2 mm² area and a sputter depth of 2 nm SiO2 per step

(64 s at 1 kV). This measurement arrangement is schematically shown in Figure 6.16.

Zinc sample

100s OCP

1000s OCP

100s OCPsweep to 0.5 V

100s OCPsweep to 2.0 V

XPS-locations

Argon sputter area

SHE

SHE

SFC-Locations

Figure 6.16: Illustration of the XPS investigation

of electrochemically treated locations on pure zinc in

borate buffer of pH 7.4. The blue area indicates that

all spots were simultaneously sputtered for depth

profiling.


74

The binding energies of the metallic peak was found to be 1022.1 eV for zinc (Zn2p3),

which is in good agreement with the literature [186]. The oxygen (O1s) peak showed two

components well separated at 531.7 eV (oxide) and 533.8 eV (hydroxide) [29]. Depth profiles

to an equivalent sputter depth of 40 nm SiO2 are shown in Figure 6.17.

0 10 20 30 400

20

40

60

80

100

native surface

Zinc

Oxygen

Carbon

Ato

mic

per

cent

sputter depth / nm SiO2

0 10 20 30 400

20

40

60

80

100 100 s OCP, Sweep 2.0 V

Zinc

Oxygen

Carbon

Ato

mic

per

cent


0 10 20 30 400

20

40

60

80

100 100 s OCP, Sweep 0.3 V Zinc

Oxygen

Carbon

Ato

mic

per

cent


0 10 20 30 400

20

40

60

80

100

Zinc

Oxygen

Carbon

Ato

mic

per

cent


1000 s OCP

0 10 20 30 400

20

40

60

80

100 100 s OCP

Oxygen

Carbon

Zinc

Ato

mic

per

cent


The reference to SiO2 as an equivalent sputter depth does not allow to directly derive the

effective surface layer thickness because the sputter rate of metallic or oxidic zinc as present on

the surface is not known. Nevertheless, a very recent article states that ZnO thin films obtained

by pulsed laser deposition (PLD) are sputter etched at a rate being almost similar to SiO2 [187]

which leads to the assumption that the depth profiles shown are at least roughly reflecting the

effective thickness. Carbon was only detected prior to the first sputter step, therefore

originating from surface contaminations rather than carbonate incorporations in the film.

Figure 6.17: XPS depth profiles on

zinc subject to different electrochemical

treatment (indicated in each graph) in

borate buffer of pH 7.4 at a flow rate of

15.6 µl min-1.


75

The results demonstrate a different thickness of the oxidic surface film as evident from the

decay of the oxygen content with increasing sputter depth. Already, the OCP scans did increase

the thickness of the oxidic layer compared to the native state, while the experiment time is

apparently without effect when comparing 100 and 1000 s hold time at the OCP. This

observation suggests the presence of a steady state thickness of the oxide as a function of the

film formation- and dissolution rate, as previously concluded from the dissolution data (Figure

6.13). It further indicates that this state is reached within less than 100 s since the oxide

thickness did not increase from 100 to 1000 s.

Moreover, during anodic polarization, the oxidic layer on the surface is drastically increased

with an oxygen signal reaching deep into the substrate. It is unlikely that this effect originates

from an increased roughness, since then a similar effect would have been expected between

both OCP profiles where the total amount of dissolved zinc differs by a factor of 10.

The anodic growth of the surface film is further supported by the current density decrease

during anodic sweeps previously demonstrated in pH 7.1. The apparently lower thickness in the

case of high anodic progression reflects film breakdown that has been shown to manifests in a

zinc dissolution peak detected downstream.

The oxygen peak (O1s) is well separated into hydroxide and oxide species by approximately

2 eV which allows quantification of the individual components. The detailed spectra before

sputtering for the native surface and electrochemically treated locations (100 s OCP and 100 s

OCP with subsequent sweep to 0.3 VSHE) are shown in Figure 6.18.

540 538 536 534 532 530 528 5263456789

10111213 oxide

Sweep to 0.3 V

100s OCP

native oxide

Cou

nts /

10-3

Binding energy / eV

hydroxide

Figure 6.18:

O1s detail spectra of the native

surface and locations addressed

with the SFC as indicated in

the figure. Dots represent

measurement points and lines

are showing a component fit

performed with Casa XPS.


76

The spectrum obtained on the native surface clearly indicates the coexistence of hydroxidic

and oxidic species, even though the latter covers about 72% of the total oxygen present. In

contrast, all spectra taken from electrochemically treated surfaces lack this dualism and show

oxide species exclusively. The oxide film is dynamic at all times and therefore subject to

dissolution and re-formation being in a steady state in the OCP case and thickening during

anodic sweeps. As a consequence, it is not possible to assume a selective removal of hydroxidic

species from the native film to be the origin of the oxidic peak. Rather than that, the formation

of the surface film needs to proceed via oxide species, which is not surprising given the high

solubility of zinc hydroxide at the respective pH values (several hundred µmol l-1, see section

5.5.4). These findings are in good agreement with the results found by Powers and Breitner [72]

concerning the formation of different types of surface films and following works by different

authors [70, 188] using the same terminology: Type I surface films originating from local super

saturation, thus being composed of hydroxidic species primarily, and Type II films being

compact oxide layers being the dominant source of passivity (Mokaddem et al. even introduce a

type III oxide, but this distinction is not relevant for the further discussion [70]). All these

studies (as well as Conway and Kannangara [7]) conclude that Type II oxide film grow beneath

Type I films, which themselves are very sensitive to convection in the system [72]. As an

immediate consequence, a direct formation of ZnO is possible being in full agreement with the

data presented. The mentioned studies focus on alkaline solutions, and it is rather surprising

that the conclusions apparently apply for near neutral solutions as well. The origin of

passivation, even though characterized by comparably large current densities, is therefore of

kinetic nature because the rate of oxide dissolution in the electrolyte is lower than the rate of

formation.

6.2.4 The effect of Sulfate anions Sulfates are a very common electrolyte constituent in both electrochemical experiments [6,

189] and environmental corrosion processes [40, 183, 190]. However, even though widely used,

the aggressive nature of sulfate in the zinc corrosion process appears underestimated [35, 191].

This may be a direct result from the fact that most studies focus on the film breakdown by

sulfate ions and the resulting decreased passivity range, which is only relevant for zinc-steel

couples if the passivity range is reduced to less than the mixed potential, which causes an

anodic polarization of zinc by coupled steel, and not of immediate importance concerning the

OCP case. A direct monitoring of the real zinc dissolution rate parallel to a classical

electrochemical characterization, especially when held at the OCP, therefore appears valuable

to estimate the impact of sulfate on zinc corrosion processes.


77

The experimental series was designed according to the previous sections with OCP-sweep

couples in borate buffer of pH 8.0 and 9.0 with a sulfate content ranging from 0 to 100 mM.

Potentials recorded during 1000 s are shown in Figure 6.19.

0.0 0.2 0.4 0.6 0.8 1.0 1.2-800

-700

-600

-500

-400

-300

-200

-100

100 mM

10 mM

1 mM

Pote

ntia

l / m

VSH

E

time / ks

0 mM

pH 8.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2-800

-700

-600

-500

-400

-300

-200

-100

100 mM

10 mM 1 mM

Pote

ntia

l / m

VSH

E

time / ks

0 mMpH 9.0

Figure 6.19:

Open circuit potentials recorded for

zinc in 0.1 M borate buffer of pH

8.0 and 9.0 with varying sulfate

content as indicated in each graph.

These results demonstrate a clear tendency of sulfate to interfere with the formation of a

closed surface film. Both the active and passive potential are in very good agreement with the

results obtained as a function of pH (see Figure 6.11). Especially interesting are the

combinations of pH 8.0 and 10 mM sulfate or pH 9.0 and 100 mM where the potential

apparently oscillates between both states, coming to a rest at the active value after

approximately 800 s in both cases. Two possible effects can be attributed to the presence of

sulfate. First, the integrity of the surface film may be reduced by incorporation of bulky sulfate

ions, demanding a higher thickness to achieve passive corrosion potentials. Secondly, the

dissolution rate may be affected resulting in a decreased steady state thickness. The latter effect


78

can be clarified with the downstream analytics presented in the following figure. Due to the

very low solubility of zinc at pH 9.0 (resulting in low signals and massive peak broadening),

only the less alkaline medium is used for quantification of zinc dissolution.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0

50

100

150

200

250

(A)

(A)

(A)

i Dis

s / μA

cm

-2

0 20 40 60 80 1000

1

2

3

c [Z

n2+] /

μm

ol l-1

c [SO42-] / mmol l-1

(A)

100 mM

10 mM

1 mM

0 mM

[Zn2+

] / μ

mol

l-1

time / ks

Plateau values (A) vs c [SO4]2-pH 8.0

[SO42-]

Figure 6.20: Zinc concentration profiles and corresponding dissolution current density for an OCP-anodic

sweep couple on pure zinc in 0.1 M borate buffer (pH 8.0) with varying sulfate content. Flow rate

15.6 µl min-1. The inset shows the plateau concentration during the OCP scan (A) plotted over the sulfate

concentration.

The dissolution rate of zinc increases significantly with increasing sulfate content. This

dependency strongly appears like a squareroot law with respect to the sulfate concentration

(R2=0.9977), but uncertainties exist concerning the determination of the steady state

dissolution rate as evident from the lack of a plateau region for 10 and especially 100 mM

sulfate. A similar relationship was reported by Brasher for the impact of sulfate ions on the

corrosion rate of iron [192], even though a fundamental reasoning for the proportionality

between weight loss and [SO42-]1/2 was not given. Mechanistic considerations for anion effects

found in the literature include an increase in the oxygen reduction kinetics [193], absorption

processes [194], catalytic effects on hydrogen evolution [195] and increased zinc solubility by

complex formation [196]. While a direct effect on the oxygen reduction kinetics appears

unlikely given the fact that the corrosion potential lies well within the transport limit of this

reaction, an effect on the hydrogen evolution can be excluded under near neutral or alkaline

conditions. The increased solubility of zinc species with an inhibiting effect on the film


79

formation appears most likely regarding the high complex formation constant of sulfate [2] (p.

25).

However, the sole magnitude of sulfate induced dissolution demonstrated in Figure 6.20 for

the OCP case underlines the initial statement of an underestimation of sulfate as a corroding

agent for zinc. The dissolution current density (iDiss) rises by a factor of 12.5 by the addition of

100 mM sulfate compared to dissolution in the absence of such species, leading to a complete

transition from a passive to an active surface. In the case of 10 mM, a dissolution current

density around 80 µA cm-2 was obtained, which falls into the active-passive transition measured

in the pH series without sulfates (between ~60 and ~200 µA cm-2, see Figure 6.13). The

corresponding OCP value oscillates between the active and passive state, which shows that the

critical dissolution rate for passivity in the presence and absence of sulfate is comparable. This

observation does not disprove the possibility of sulfate incorporation into the film (by a

dissolution-precipitation mechanisms where sulfate takes part in precipitate formation [40]),

even though it appears unlikely considering the direct oxide growth as described in the former

section.

From the increased dissolution rate induced by the sulfate addition, it is expected that the

film breakdown by anodic polarization is affected as well [197]. A cathodic shift of the

breakdown potential with increasing sulfate content can be observed in anodic sweep

experiments in Figure 6.21.

-0.5 0.0 0.5 1.0 1.5 2.0 2.5

10-5

10-4

10-3 pH 8.0 10 mM 1 mM 0 mM

pH 9.0

100 10 mM 1mM 0 mM mM

i / A

cm

-2

E / VSHE

Figure 6.21:


2 mV s-1 starting from the

previously recorded OCP at a

flow rate 15.6 µl min-1. Both

solutions pH (8.0 = black,

9.0 = red) and sulfate content

were varied.


80

The mechanism of film breakdown on zinc is not fully clarified, even though some

fundamental dependencies have been reported by different authors. Keitelman et al. [198]

postulate local OH- depletion to cause breakdown and ultimately pitting, demonstrated by a

variation of the buffer capacity in borate solution. A fundamental similarity exists to parallel

works by Augustynski et al. [196] who investigated the effect of different anions and concluded

local accumulation to increase the solubility of zinc and cause film breakdown. The results

presented in this study are in full agreement, as an increase in the solubility by lowering the pH

or additions of sulfates cause the film breakdown potential to shift cathodically. The

observation that the passive current densities in Figure 6.21 appear nearly unaffected results

from the fact that the existence of a passive region requires very low dissolution rates. A

massive sulfate induced dissolution (e.g. 10 and 100 mM in Figure 6.20) leads to an active

behavior during potentiodynamic measurements and prevents passivity within the current

density window investigated.

6.2.5 Summary of the results The dissolution mechanism of zinc in borate buffer (0.1 M) was clearly demonstrated to

proceed through a surface oxide in the pH region from 6.6 to 9.0. XPS surface analysis

revealed the purely oxidic nature of the surface species, indicating a direct oxide growth as

postulated by several authors [20, 70]. Surprisingly, a sudden increase in the corrosion

potentials covering approximately 300 mV was observed between pH 7.1 and 7.4, which was

attributed to the formation of a closed oxide layer at higher pH values. At pH values of pH 7.1

and 6.6, active corrosion potentials around -700 mVSHE were measured and correlated to the

existence of active sites within the oxide film. This case evolves at high film dissolution rates

(approx. 200 µA cm-2 dissolution current density), while the overall dissolution behavior is

independent of the corrosion potential and governed by proton transport as shown by

downstream zinc detection. Consequently, the passive current density during potentiodynamic

sweeps correlates to the dissolution rate at the oxide-electrolyte interface as a function of the

electrolyte composition. However, dissolution current density and measured current density do

not match exactly, as film growth and superimposed corrosion processes during anodic sweeps

constitute non-equilibrium conditions with either positive or negative deviation. These findings

demonstrate that the passivity model by Heusler [68] is fully applicable for zinc under the

conditions presented. It has been furthermore shown that the oxide film under steady state

conditions (OCP) is quickly established, as the dissolution profiles rapidly converge the final

plateau values with the oxide depth profiles (XPS) independent of the immersion time. The

dissolution rates of zinc in 0.1 M borate buffer are however surprisingly high, being


81

approximately doubled when using 0.1 M borate buffer of pH 7.1 instead of 0.1 M NaCl

solution. The inhibition of precipitation and the high proton supply by buffer species therefore

constitute a major cause for metal dissolution which needs to be carefully considered.

A thorough investigation of the effect of sulfate additions at different pH values revealed a

strongly increased film dissolution rate besides the well known effect of an earlier film

breakdown during anodic sweeps. This increased dissolution rate was furthermore able to

trigger active corrosion potentials in pH regions where passive potentials are obtained in the

absence of sulfate. In these cases, fluctuations in the corrosion potential were observed for

certain combinations of sulfate concentration and pH, indicating pitting and passivation

processes in competition [68]. The results presented provide new insights in order to

understand the effect of electrolyte constituents on zinc corrosion, especially because sulfate is

very commonly used and not well investigated beyond its effect of reducing the pitting

potential. Interestingly, there is strong indication that sulfate increases the dissolution rate and

icorr proportional to the square root of its concentration from 1 to 100 mM, a relationship that

has been previously demonstrated for iron corrosion by Brasher [192]. The dissolution profiles

available in the integrated setup presented therefore provide exceptional information beyond

the electrochemical data and contribute significantly to the investigation of corrosion

phenomena.

Chapter 7: Stability of ZnO

82

7 Stability of ZnO The aim of this chapter is to investigate the chemical and electrochemical dissolution of

ZnO in borate buffer and NaCl solutions by a combination of electrochemical techniques and

dissolution monitoring to provide mechanistic insights and allow comparison to the results on

pure zinc. To enable electrochemical investigations on ZnO, all samples were doped with

Al2O3 (carrier concentration around 1020 cm-3) [199]. Alumina itself shows neither chemical nor

electrochemical dissolution and has been considered inert for the investigations.

Quantification of the chemical dissolution of ZnO is of high importance to evaluate its role

as a corrosion product in the corrosion process of zinc. Especially the corrosion mechanism

through an oxidic surface layer present in borate buffer immediately profits from an increased

understanding of the decisive factors for the stability of ZnO in aqeous solutions.

Electrochemical investigations furthermore allow insights into anodic degradation with an

impact on film breakdown processes of passive layers as well as desired texturing efforts to

improve the properties of ZnO:Al for technical applications.

7.1 Chemical dissolution 7.1.1 Unbuffered NaCl solution

As aerated 0.1 M NaCl solution is a frequently used corrosive medium for studies on zinc,

an initial dissolution experiment (1000 s OCP) was carried out in this medium with a flow rate

of 15.6 µl min-1.

0 500 1000 1500 2000-200

-100

0

100

200

300

120

140

160

180

200

220

240

260

280

300

Pote

ntia

l / m

VSH

E

[Zn2+

] / n

mol

l-1

time / s

ZnO dissolution in 0.1 M NaCl

Figure 7.1:

ZnO:Al dissolution profile (left

axis) during a 1000 s OCP

scan in aerated 0.1 M NaCl

solution at a flow rate of

15.6 µl min-1. The recorded

potential is displayed on the

right axis.


83

The detected concentrations were remarkably low, essentially ranging around the detection

limit of 10-7 mol l-1. This value is well comparable with the estimated H+/OH- concentration in

neutral solutions, but an exact correlation proves difficult due to the low signal to noise ratio.

However, a dissolution process can be qualitatively stated. In comparison to zinc corrosion as

shown in section 6.1 observed under identical conditions, the dissolution rates differ by a factor

of around 17. This suggests that the corrosion of metallic zinc in aerated NaCl solution does

not proceed through surface oxides as observed in borate buffer, but rather liberates zinc from

active sites parallel to the (significantly lower) oxide dissolution.

Another obvious feature of Figure 7.1 is the cathodic drift of the recorded potentials. In

general, the rest potential in the dark (Erd) depends on the flat band potential (EFB) and the

band bending in the surface charge (ΔESC) as shown in equation (7-1).

SCFBrd EEE Δ+= (7-1)

It has been shown by Matsumoto et al. [200] that both the flat band potential and the

surface charge region for ZnO are pH dependent, but inversely affected, thus resulting in a

(nearly) constant rest potential around 230 mVSHE along the pH scale. A change in the surface

pH as a consequence of the local pH effect of zinc dissolution (see page 61) as the origin of the

potential drift is therefore unlikely.

A decay of the flat band potential during etching in several solutions (including HCl) was

reported by Dewald [201] and ascribed to possible changes in the surface dipole due to ion

interactions. A surface etching needs to proceed as the time constant for equilibration appears

too large for absorption/desorption phenomena. Due to the large number of assumptions and

variables, a sound and conclusive model for the transient behavior of Erd is still missing and will

not be approached within this study.

To clarify the effect of the experiment on the surface morphology, SEM images were taken

before and after the experiment from Figure 7.1 and the results are shown in the following

figure.


84

Figure 7.2: SEM images of the ZnO:Al surface in the native state (a) and after 1000 s OCP

measurement in aerated 0.1 M NaCl solution under electrolyte flow (15.6 µl min-1).

Examination of the surface structure reveals pronounced grain boundaries, being a

consequence of the removal of material previously proved by downstream analysis. It gives

clear indication that the etching proceeds dominantly at the grain boundary areas, while the top

surfaces of the grains appear unaffected. It is well known that ZnO exhibits columnar growth

during sputter deposition with the c-axis parallel to the surface normal [57, 202]. The top

surface exhibits a (0001) orientation while the sides (therefore the grain boundary areas) are

composed of a variety of crystal orientations. It was reported by Jo et al. [203] that the etch pit

in HCl solution grows preferably along the c-axis, essentially being a negative crystal.

Furthermore, it was reported that this process originates from dislocations and defect areas.

Lattice mismatches at grain boundaries therefore provide potential etch sites as a result of

compression and tension of Zn-O bonds [204] or step sites due to a variation in grain height. It

is therefore reasonable that etch pits originate preferably at boundary areas and proceed along

these, parallel to the lattice c-axis. It is still surprising that this etching proceeds with such a

degree of homogeneity as all grain boundaries appear to be removed with comparable rates.

That is in contrast to the texturing of ZnO:Al thin films in KOH (~30 %) or HCl (~0.5 %)

where the surface topography is dominated by large craters [46, 47] (several µm compared to

grains in the 100 nm range), which are assumed to originate from grain boundaries, especially

triple points [205], but lack the confinement to these regions. These harsh conditions therefore

eliminate the aforementioned selectivity. As the described solutions of HCl and KOH are of

a) Native surface b) After experiment


85

minimal relevance for corrosion applications, a buffer system was used to follow the effect of

acidification between pH 7.0 and 6.0 in an attempt to correlate etch rate and surface

morphology.

7.1.2 Acetate buffer pH 6.0 – 7.0 The acetic acid / sodium acetate buffer system was selected because of the large buffer

capacity against protons in the pH region between 6.0 and 7.0 due to the dominance of the

deprotonated acetate since pH > pKs. This selection was made because the electrochemical

experiments presented in a later section include oxygen evolution, potentially causing massive

acidification in the vicinity of the electrode. The borate buffer as used in the case of zinc

corrosion exhibits the reversed case (dominance of the protonated species), and was selected

due to the oxygen reduction reaction on the electrode surface.

To increase the comparability to previously presented data, a chloride content of 0.1 M

(NaCl) was added to all acetate buffer systems. A series of locations on ZnO:Al were addressed

by using the SFC with an OCP measurement of 1000 s followed by a purging step under

continued flow (15.6 µl min-1). All measurements were performed twice and exhibit a high

degree of reproducibility as presented in Figure 7.3. The inset shows the plateau concentration

(mean value) as a function of the proton concentration with a linear least square fit (R2=0.9998).

The slight inclination of the plateau region is assigned to non-quantitative mass transport in the

flow system that is evident from the peaks (A) which originate from lowering the solution pH

and a subsequent purging of residual zinc accumulated in the system.


86

0 2 4 6 8 10 12 14 160

1

2

3

4

5

12

y = 4.08x + 0.42

c [Z

n2+] /

μm

ol l-1

c [H+] / μmol l-10 0.4 0.8 1.2

345

(A)(A)(A)

pH 6.5

pH 6.0

pH 6.75[Zn2+

] / μ

mol

l-1

time / ks

pH 7.0

Figure 7.3: Zinc dissolution profiles of ZnO:Al in acetate buffer (0.1 M + 0.1 M NaCl) of different pH

under constant electrolyte flow (15.6 µl min-1). The inset shows the mean plateau value plotted against the

proton concentration. The spikes (A) originate from changing the solution pH.

The data demonstrates a linear dependence between zinc and proton concentration as

previously observed for pure zinc in borate buffer (see chapter 6.2, in particular Figure 6.13). A

noticeable feature is that the linear regression on the data points does not cross the origin,

therefore indicating an etching process independent of the proton concentration. Gerischer

and Song [62] have shown that acetic acid/acetate acts as an etching agent by formation of

soluble complexes, by far exceeding the destabilizing effect of chloride in the near neutral pH

region. The pH independent etching is therefore attributed to the buffer system. Furthermore

the buffer causes a mismatch between zinc and proton concentration evident from the slope of

4.08. This effect has been previously discussed in section 6.2.2 and is a consequence of high

proton availability due to protonated buffer species. Please note that the effect is less

pronounced for the acetate buffer compared to borate, as the latter is dominated by its

corresponding acid (pH < pKs) and therefore possesses a high potential regarding proton

supply.

It is highly probable that the dissolution reaction is transport limited as previously shown

for pure zinc, and confirmation is easily possible because the present case exhibits only one

reaction, being zinc oxide dissolution. Accordingly, an increase in buffer concentration should

directly raise the transport limit in direct proportion, which has been clearly confirmed by an


87

experimental series similar to the pH variation but with the buffer concentration as parameter.

The results are shown in the following figure.

0 2 4 6 8 10 12 14 16 18 20 220.0

0.5

1.0

1.5

2.0

2.5

3.0

12 pH 6.5

c [Z

n2+] /

μm

ol l-1

c [buffer] / mol l-10 0.1 0.2

345 pH 6.0

0.025 M

0.1 M

0.05 M[Zn2+

] / μ

mol

l-1

time / ks

0.2 M

Figure 7.4: Dissolution profile of ZnO:Al in acetate buffer (containing 0.1 M NaCl) of pH 6.5 under

variation of the buffer concentration. The inset shows the mean plateau values plotted against buffer

concentration for the experimental series shown and another run at pH 6.0.

A variation of the buffer concentration (at constant chloride content) between 0.2 and

0.025 mol l-1 is shown for a pH value of 6.5, while the inset additionally contains a similar series

at pH 6.0. The linear relationship between dissolution and buffer concentration as shown in the

inset of Figure 7.4 undoubtedly proves transport control of the reaction and agrees well with

results obtained by Guśpiel and Riesenkampf who showed ZnO dissolution in H2SO4 to be

transport limited to a concentration around 0.1 M at 25°C [184].

Using Ficks law for the flux of species according to

dxdc

DJ iii −= (7-2)

it is possible to calculate the expected zinc concentrations by

Nf

ii

f

i

VAcD

VAJZn

δ⋅⋅⋅

=⋅

=+0

2 ][ (7-3)

using the electrode area A, the volume flow Vf, the diffusion coefficient and bulk

concentration of species i, and the diffusion layer thickness δN for a case where the dissolution


88

rate is governed by transport of i. It is to note that the exact diffusion layer thickness is

unknown, but will be roughly estimated with 50 µm. The following table displays the results for

protons at pH 6.0 and acetic acid (0.1 M acetate buffer, pH 6.0, conc. acetic

acid = 5.3 mmol l-1) as transport limited species.

Species Diffusion coefficient Expected zinc concentration at δN = 50 µm

Protons 9.3·10-9 m2 s-1 7.7·10-8 mol l-1

Acetic acid 1.2·10-9 m2 s-1 4.7·10-5 mol l-1

Table 7.1: Expected zinc concentrations for diffusion control of protons and acetic acid.

The results indicate that the observed dissolution rate of 4.5 µmol l-1 can not be explained on

the basis of proton transport through the diffusion layer. This was expected since a variation of

the buffer concentration alters the dissolution rate without affecting the proton concentration.

However, a calculation based on acetic acid yields expected dissolution rates that exceed the

measured ones by approximately one order of magnitude, most probably being a consequence

of the number of assumptions made concerning the diffusion layer thickness and the flow

profile at the capillary tip. Especially the fact that the velocity profile on the substrate differs

significantly in lateral direction may cause significant errors in the calculations. The experiments

shown further indicate that the destabilization of ZnO by chloride is negligible, because the

fixed chloride content would result in a “background” etching in the inset of Figure 7.4

(intersect with y-axis). This illustrates that the increase of the corrosion rate of zinc with

increasing chloride concentration shown previously (Figure 6.2, page 61) does not originate

from increased rates of ZnO dissolution.

Since the etch rates in buffered solutions are significantly higher than in unbuffered

solutions, a different surface morphology is expected and investigated by SEM imaging shown

in Figure 7.5.


89

Figure 7.5:

SEM images of the ZnO:Al surface in the native state

(a) and after 1000 s OCP measurement in aerated

0.1 M acetate buffer of various pH (b-e) under

electrolyte flow (15.6 µl min-1).

The images demonstrate a significantly lower selectivity of the etching process towards the

grain boundaries compared to the results previously shown for 0.1 M NaCl (see Figure 7.2).

However, there is still indication that the material removal originates from these areas, as seen

from pH values from 7 to 6.5. The image at pH 6.0 (e), representing the highest dissolution

rate (increase by a factor of 2.57 compared to pH 6.5), in contrast, shows massive surface

roughening with no indication of an influence of the crystallinity of the substrate. Considering

that the etching agents only differ in pH value with identical chloride content, it can be

concluded that the dissolution rate is the main determinant for the surface morphology.

7.1.3 Summary of the results Al doped ZnO is thermodynamically unstable in neutral media. Especially in the presence of

convection and buffered solutions, a local saturation of the electrolyte with zinc species is

hindered and a strong difference in the chemical potential between oxide and solution prevails.

In these cases, the equilibrium condition is not met and the oxide dissolves continuously. The

transfer rate of metal ions from the oxide into the solution is concentration and potential

dependent according to Heusler [206] and Wagner [65]:


90

)/)((exp)/exp( /´

),(,/),(, RTFzaakRTFaaki eloxidrxelMelceloxid

rxoxidMoxidcc zz φγφ −−⋅⋅−⋅⋅= ++

−+

(7-4)

with a forward and backward reaction each dependent on the respective rate constant k, the

activity of the metal ions and complexing agents (ax), the potential difference in the Helmholtz

layer φ, and the effective transfer coefficient γ. The overall reaction rate can therefore be under

either activation control, depending on the potential difference in the Helmholtz layer, or

transport controlled by a depletion of species involved. In case of ZnO, both cases have been

reported depending on the electrolyte composition and convection imposed [184]. For ZnO in

acetate buffer of near neutral pH, the results presented give clear evidence that the dissolution

of is limited by proton/proton-carrier transport up to buffer concentrations of 0.2 M, as an

increase in the buffer capacity linearly increases the dissolution rate. These findings are of

immediate importance for the corrosion of metallic zinc in comparable media, since icorr is a

function of proton transport under these circumstances. The corrosion model presented earlier

(chapter 6.2) has therefore been confirmed. Another interesting finding is that the chloride

concentration, in contrast to acetate species, does not induce a significant dissolution of ZnO.

This allows concluding that the corrosive effect of chloride ions on metallic zinc mainly

originates from a destabilization of the precipitation layer or requires high field strength within

passive layers.

For sputter deposited ZnO:Al, the polycrystalline nature of the surface is very important for

the etching behavior. It has been shown that the dissolution is mainly ascribed to the grain

boundary areas, which dissolve preferably at low etch rates. This appears surprising given the

fact that the reaction is under transport control, but it needs to considered that the diffusion

layer thickness is very large compared to the grain size and therefore allows different

dissolution kinetics to preferably etch active sites within the proton depletion zone. These

results indicate that the crystallographic characteristics of ZnO films are an important

parameter for the film stability, especially relevant for passive films which are mostly

polycrystalline with a high number of boundary areas in consequence. This aspect has been

emphasized comparably little in the literature so far.

7.2 Electrochemical dissolution 7.2.1 Unbuffered NaCl solution

Following the systematic of the previous section, an aerated, unbuffered 0.1 M NaCl

solution was used in the primary experimental series to determine the anodic behavior of


91

ZnO:Al under typical corrosive conditions. An anodic sweep experiment starting from

0.27 VSHE was performed at a sweep rate of 1 mV s-1 with dissolution monitoring in parallel.

The results are shown in the following figure.

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0

100

200

300

400

0

100

200

300

400

1.0 1.5 2.0

02468

10

0246810

i / μ

A c

m-2

i Dis

s / μA

cm

-2

time / ks

purgingE / VSHE

i / μ

Α c

m-2

i Dis

s / μA

cm

-2

time / ks

magnified region

0.5 1 1.5 2 2.3

Figure 7.6: Dissolution profile of ZnO:Al in aerated 0.1 M NaCl solution during a potential sweep

experiment (potential given on top, shifted by td) at 1mV s-1 including a subsequent purging period. The

current density is given on the right axis (red curve). The inset magnifies the region between 1 and 2 ks.

The current measured during the potential sweep displays three significant regions: The

blocking region up to approximately 1.4 VSHE where neither current nor dissolution occurs (0 -

1.3 ks), a region of low current density between 1.4 and 2.1 VSHE (1.3 – 2 ks) and an exponential

increase beyond 2.1 VSHE. The correlation between dissolution and current density measured is

very good in the intermediate region, while the final exponential increase displays a slight delay.

This effect is attributed to zinc transport processes in the spectroscopic system as discussed

previously (section 5.5.4, page 46), also evident from the peak broadening and signal decay

during the purging period.

For data interpretation, three electrochemical reactions need to be considered:


92

ZnO −+ ++ eOZn 221

22 (7-5)

OH 2 −+ ++ eOH 2212 2 (7-6)

)2( yZn − −+ + yeZn2 (7-7)

Equation (7-7) describes the oxidation of overstochiometric zinc in the oxide film [62], even

though the currents associated with this process are very small [60]. The oxidation of oxygen

therefore constitutes the primary anodic reaction, either by lattice decomposition (7-5) or water

splitting (7-6). It is highly probable that both reactions exhibit a different onset potential, which

is supported by the presence of two anodic regions in Figure 7.6. The congruency between zinc

dissolution and current density in the intermediate region between 1.4 and 2.1 VSHE suggests

that lattice decomposition occurs, most probably at defect sites and grain boundary areas.

Further anodic progression of the potential ultimately leads to a strong oxidation process

with an onset at around 2 VSHE, quickly reaching the stop condition of 400 µA cm-2. Zinc is

liberated at very high rates as seen from the concentrations detected downstream. This is

attributed to an electron exchange process with the valence band, with both lattice

decomposition or water splitting as possible oxidation processes. These two reaction pathways

were mentioned by Pettinger et al. [52] who detected electrochemically produced oxygen in the

system during high anodic current densities. They could however not clarify the origin of the

evolving oxygen. It therefore appears more specific to include zinc detection since zinc

liberation only proceeds directly during lattice decomposition, while the OER causes this effect

indirectly by subsequent chemical etching with the protons generated. The use of a buffer

system to influence the latter reaction, while leaving lattice decomposition unaffected, will be

shown in a later section.

It has been previously demonstrated on zinc that the spectroscopic detection is in good

correlation to galvanostatic measurements (see section 6.1.2), and an experimental series was

designed under variation of the applied current density in NaCl solution. The duration of each

measurement was set to 300 s and the results are shown in Figure 7.7.


93

0 50 100 150 200 250 3001.7

1.8

1.9

2.0

2.1

2.2

2.3

2.1 2.2 2.310

100

i / μ

A c

m-2

E / VSHE

600

500 300

100150200

50E

/ VSH

E

time / s

20

Applied current density / μA cm-2

i vs. Et=300

Figure 7.7: Potential transients of ZnO:Al in 0.1 M NaCl solution subject to different applied currents

densities. The inset shows the final potential values at t=300 s plotted on the x-axis and the corresponding

applied current density on the logarithmic y-axis. Two measurement curves are shown for each current density,

being nearly indistinguishable due to a very high reproducibility.

Initially, the potential transients rise sharply, passing a maximum in case of higher applied

current densities. After approximately 200 s, all potentials level to comparably stable potential

values, that are in direct correlation to the applied current. The inset shows a reversed i vs. E

plot, which allows comparison to the potential sweeps shown previously (see page 91). It is

evident that an exponential relationship between potential and applied current density exists

(dashed line in the inset of Figure 7.7) for values above 50 µA cm-2, similar to the behavior

during potentiodynamic scans. The deviation from this linearity in the logarithmic scale in case

of 20 and 50 µA cm-2 is attributed to the lattice decomposition reaction with an earlier onset,

limited to comparably low anodic currents.

The shape of each individual potential transient, and especially the existence of a potential

maximum in certain cases, originates most probably from complex surface reactions that can

not be clarified within the scope of this study. Nevertheless, the reproducibility of this behavior

is remarkable.

Parallel to the galvanostatic series, zinc detection was performed as shown in the following

dissolution profiles.


94

0 2 4 6 8 10 120

2

4

6

8

10

0

100

200

300

400

500

i Dis

s / μA

cm

-2

2.4 2.7

0

1

2

3

2.9c

[Zn2+

] / μ

mol

l-1

time / ks

[Zn2+

] / μ

mol

l-1

time / ks

500

300

100 150

200

50 20

Applied current density / μA cm-2

Figure 7.8: Zinc dissolution profiles during the previously described galvanostatic series on ZnO:Al in

0.1 M NaCl solution. The inset shows the magnified 150 µA cm-2 measurement of the first run.

The similarity to the chemical etching previously shown in Figure 7.3 is strong; the slight

inclination of the plateau value and the tailing effect prove to be characteristics of the flow

system. Besides that, very sharp signals were obtained which increase along increasing applied

current densities.

As especially evident from the last 4 measurements, a deviation between applied and

spectroscopically determined current density is observed. This might indicate that the anodic

reaction does not liberate zinc at 100 % faradaic efficiency, even though a possible impact of

tailing needs to be considered. Therefore, rather than evaluating the peak height, numeric

integration of the peaks was performed to correct for peak broadening. The calculation of the

total amount of zinc released during the time interval t2-t1 was performed according to

f

t

tZn VdtZnN ⋅= ∫ +

2

1

2 ][ (7-8)

using a flow rate (Vf) of 260 nl s-1 (15.6 µl min-1). The theoretically possible release of species at

100 % faradaic efficiency equals the charge (applied current multiplied with the experiment

time) divided by 2F (charge number of zinc times Faraday constant). Data examination by this

technique yields the following graph.


95

0.0 2.0x10-10 4.0x10-10 6.0x10-10 8.0x10-100.0

2.0x10-10

4.0x10-10

6.0x10-10

8.0x10-10

1.0x10-9

0.0 1.0x10-10 2.0x10-100.0

1.0x10-10

2.0x10-10

Fit 2:slope: 0.98Int.: 1.82x10-11

Fit 1:slope: 1.01Int.: 1.84x10-11

Intersect: 2.21x10-11

peak

are

a [Z

n2+] /

mol

q (2F)-1 / mol

Slope: 0.886

Figure 7.9: Total amount of detected zinc as a function of the consumed charge divided by 2F. Two data

sets are displayed and the slope and intersect of a mean linear regression are given in the graph. The magnified

region shows the data points corresponding to 20, 50 and 100 µA cm-2 with a linear regression along the

lower two for each dataset.

A mean slope of 0.886 was calculated along the whole dataset, indicating that zinc

dissolution proceeds with 88.6 % faradaic efficiency. As the linear regression does not cross the

origin, a “background” etching exists even when interpolating the applied charge to zero (OCP

case). The magnitude of this process is given by the intersect of the linear fit, and an OCP etch

concentration of 280 nmol l-1 proceeding 300 s is estimated for the background etching. The

measured dissolution rates under OCP conditions (see Figure 7.1) were approximated with

100 nmol l-1, which appears as a strong deviation. Two things need to be considered though.

Firstly, a quantification of species around the detection limit may contain significant errors.

Secondly, a slight shift in the baseline of Figure 7.8 causes all peak areas, as they are determined

numerically, to increase by the baseline shift multiplied with the time integral used for

integration. As the concentrations in question are very low, even the slightest baseline shift will

take effect, which can be magnified by the longer integration time (around 500 s to cover the

whole peak) compared to the contact time (300 s). In any case, the rate of OCP dissolution is

not the primary aim of Figure 7.9. The major outcome is the constant faradaic efficiency of

88.6 %.


96

For exclusive lattice decomposition, a faradaic efficiency close to 100 % would be expected

because the oxidation of lattice oxygen is direcly linked to the liberation of zinc ions (see

equation (2-21), page 16). Consequently, a side reaction takes place that is therefore assumed to

be the OER, which has been assigned to the oxidation process with an onset above 2 VSHE.

According to the afore mentioned lattice decomposition reaction with an earlier onset, it would

be logical to assume a significantly higher current efficiency for dissolution at very low applied

current densities. The inset of Figure 7.9 shows this region and provides a linear regression

along the first two points (20 and 50 µA cm-2) of each set, which already deviate from the

exponential dependence between current density and potential (Figure 7.7). Even though a

linear regression along two data points is a questionable procedure, it appears that the slope

increases at lower applied current densities. The slopes of both individual measurements are

very reproducible and close to 100 % faradaic efficiency. These results support the hypothesis

of lattice decomposition at lower potentials being superimposed by OER at higher applied

current densities.

The surface topographies as determined by SEM are shown in the following figures. Please

note that the removal of material is directly proportional to the applied current density

indicated in each image, due to the linearity observed in Figure 7.9. The first figure shows low

magnification images of the locations addressed in the galvanostatic series, while the following

shows a small section inside the measurement location to allow determination of the surface

structure.

Figure 7.10: SEM images of the measurement locations in a galvanostatic series in 0.1 M NaCl solution

demonstrating a very high degree of reproducibility regarding the area and geometry of the area addressed with

the SFC.


97

Figure 7.11:

High magnification SEM images of selected

measurement locations from Figure 7.10,

showing a very selective removal of material

confined to the grain boundary areas.

The very high selectivity towards the removal of grain boundaries observed in all cases is

remarkable, given the fact that the total material loss is comparable to the chemical etching as

seen in Figure 7.5 (e.g. the material dissolution at 300 µA cm-2 is slightly above pH 6.5, and

500 µA cm-2 equals approximately pH 6.0). This implies that the grooves formed exhibit a high

aspect ratio, reaching deep into the substrate.

Lattice decomposition and water oxidation are in direct competition and these reactions

most probably take place at the same active sites. While the SEM images clearly indicate that

the lattice attack proceeds at the grain boundaries, it still needs to be proved that water

oxidation triggers chemical dissolution, even though strong evidence already exists.

7.2.2 Acetate buffered solution It has been previously pointed out that the generation of protons by OER may trigger

subsequent chemical etching of ZnO:Al. As lattice decomposition proceeds with direct

liberation of zinc, a buffer system might allow distinguishing between this direct and indirect

mechanism by removal of protons and suppression of OER induced chemical dissolution.


98

For this purpose, the acetate buffers introduced in section 7.1.2 were used as electrolytes

during galvanostatic experiments. The applied current density was varied between 25 and 450

µA cm-2 and the numerical integrals of the dissolved zinc were plotted as a function of the

applied current density. Please note that the applied charge as shown in Figure 7.12 is

significantly compared to Figure 7.9 since a different cell size (4 times higher) was used. This

required adjusting the current to yield comparable current densities.

0.0 1.0x10-9 2.0x10-9 3.0x10-90.0

1.0x10-9

2.0x10-9

3.0x10-9

unbuffered NaCl Slope 0.881

peak

are

a [Z

n2+] /

mol

q (2F)-1 / mol

acetate bufferpH 6.5pH 7.0

Slope 0.616

Figure 7.12: Total amount of detected zinc as a function of the consumed charge divided by 2F for a

galvanostatic series in unbuffered 0.1 M NaCl and 0.1 M acetate buffer of different pH.

The comparability of data obtained with microcells of different diameter is high as the slope

of 0.881 for NaCl is well comparable to the 0.886 from the former section. As expected, a shift

of the y-axis intersect of the linear regression occurs when using acetate buffer, an effect, that

has been thoroughly discussed before as parallel chemical etching. Of course, this intercept

offset is additionally increased as the solution pH decreases.

A particularly interesting observation is the significant decrease in the linear slope, indicating

a reduction of the faradaic efficiency for zinc dissolution from 88.1 to 61.6 % by the buffer.

This can be explained only by an influence of the buffer on the proton induced etching

subsequent to the OER, as the electrochemical lattice decomposition liberates zinc without

contribution of H+ or OH-. Still the change in surface pH [32] as a consequence of water

oxidation and the buffer effect are considered as major reasons for ZnO dissolution. Both

processes are apparently confined to the grain boundary areas, while the etching by protons is


99

probably the reason for a widening of the etch pits as observed in Figure 7.11. The high

efficiency of lattice attack by generated protons in the unbuffered case is furthermore

remarkable. Despite the comparably high buffer concentration of 0.1 M, it is not probable that

protons generated at the surface are quantitatively scavenged by acetate, which does not allow

isolating the direct lattice decomposition. Further experimental and theoretical work will be

necessary in order to identify the exact relation between lattice decomposition and OER.

7.2.3 Surface profilometry Surface profilometry was performed to finally clarify whether the OER and lattice

decomposition occurs selectively or exclusively at the grain boundaries. The stylus of 12.5 µm

radius of the profilometer is not able to resolve the etch pits, and therefore only displays the

integral surface level. The following figure shows the surface profiles of an electrochemically

and chemically etched location addressed by a 400 µm microcell.

0 100 200 300 400 500

-120-100-80-60-40-20

020406080

original surface level

(b) 500 μA cm-2

(a) pH 6.0

heig

ht z

/ nm

coordinate x / µm

-140

Figure 7.13:

Surface profiles of a ZnO:Al surface (a)

chemically etched in 0.1 M acetate

buffer of pH 6.0 for 1000 s and (b)

galvanostatically etched at 500 µA cm-2

for 300 s.

In case of electrochemical etching, the surface level appears nearly unchanged and shows

spikes at the sealing areas, most likely due to the formation of precipitates. A slight deviation

can be stated that may well originate from chemical etching in 0.1 M NaCl (see Figure 7.1). In

contrast, the chemically etched profile clearly generates a large pit that qualitatively reflects the

assumed flow profile at the capillary tip [133]. It is to mention that the total removal of material

is larger in case of anodic dissolution (1.04 nM during chemical, 1.85 nM during

electrochemical etching), which implies that the removed grain boundary areas exhibit a large

aspect ratio, therefore reaching deep into the substrate. This fact is further illustrated by the

loss of lateral conductivity upon extended galvanostatic treatment [205], as the etch pits will at

some point reach down to the glass carrier substrate. Figure 7.13 therefore confirms a very high

selectivity of electrochemical dissolution towards the grain boundaries.


100

7.2.4 Summary of the results The focus of this chapter was set on the electrochemical decomposition processes taking

place on ZnO:Al upon anodic polarization. The electrochemical data agrees well with the

results on ZnO presented by Pettinger et al. [52, 54] with a coexistence of oxidation of lattice

oxygen and water. In contrast to the former studies, it has been possible with the integrated

system presented to distinguish between both possible pathways by means of zinc monitoring.

In anodic sweep experiments, it has been shown that an oxidation reaction occurs with an

onset potential significantly lower than water splitting. This process is accompanied by a zinc

dissolution current density congruent to the potentiostat readings. Further progression leads to

an exponential increase of both current and zinc dissolution. At these potentials however, the

dissolution current densities deviate from the imposed current density by a constant factor

depending on the electrolyte composition. For unbuffered NaCl solution, the integral amount

of zinc released constitutes 88.1 % of the applied charge. For buffered electrolytes, this

Faradaic efficiency decreases to values around 61 %. These findings strongly suggest that

oxygen evolution on the surface triggers subsequent chemical dissolution by the protons

generated, while buffers inhibit this process by scavenging of H+ ions in the vicinity of the

surface. These findings contribute to the general understanding of relevant processes as a

distinction and comprehensive comparison of different decomposition processes on ZnO is

not found in the literature. The data furthermore emphasizes the impact of the surface pH, as

the use of a buffer system reduces the faradaic efficiency for dissolution by nearly 30 %. These

insights need to be considered in all film breakdown processes where water decomposition

contributes to the oxidation process, as well as the opposite case where OH- species are

generated and act as a complexing agent for the electrode material.

With respect to the crystallographic orientation of the surface, an extraordinary behavior of

the grain boundary areas was observed upon anodic polarization. It has been previously shown

that chemical etching preferably initiates at these sites, but the electrochemical decomposition

was found to be strictly confined to these areas even at high dissolution current densities. This

surprising degree of selectivity underlines the importance of the crystallographic orientation

and especially the grain boundary areas [207] for electrochemical film breakdown processes. It

furthermore constitutes a powerful and well controllable tool for surface texturing processes,

e.g. tuning of the optical properties of ZnO:Al as a TCO in photovoltaic applications. The

impact of the electrochemical texturing process described on the performance of thin film solar

cells is currently investigated [205, 208].

Chapter 8: Corrosion of Zn-Mg alloys

101

8 Corrosion of Zn-Mg alloys The focus of this chapter is the combinatorial screening of laterally graded Zn-Mg material

libraries to clarify the influence of alloying magnesium on the electrochemical behavior with an

exceptionally high resolution along the composition axis. The experiments are based on the

fundamental insights presented in the previous chapters. The aim is to demonstrate

comparability between bulk and thin film samples, and the feasibility to execute extended

experimental series in a fully automated setup. Furthermore, the structural aspects of co-

deposited Zn-Mg films as an alternative production method to classical coating techniques [93]

(e.g. hot dip galvanizing) are investigated and correlated to the electrochemical behavior

observed.

8.1 Surface characterization 8.1.1 Optical appearance

As described in the experimental section (4.2.3), Zn and Mg was co-deposited by thermal

PVD onto steel substrates of approximately 10 x 2 cm2. The typical average thickness ranges

around 400 nm, while the exact thickness varies due to locally different deposition rates. As an

example, the thickness in the middle of the sample is approximately 30 % less than right above

the sources if Zn and Mg are deposited at equal rates assuming an average sharpness parameter

of 2.4.

The optical appearance of the samples is strongly non linear despite the continuous

(sigmoidal) increase of magnesium along the x-axis as shown in Figure 8.1. The image

additionally contains a schematic line scan illustrating a typical sequence of measurement

locations subsequently addressed by the SFC. It can be observed from the optical image that

the zinc rich area up to approximately 14 at. % Mg appears grey and matt due to a large surface

roughness. However, the optical roughness vanishes abruptly as the magnesium content is

further increased, accompanied by a dark appearance between around 18 and 45 at. % Mg. The

latter feature is not attributed to the surface roughness since the reflectivity remains. Moreover,

the surface structure or the oxides formed (dark ZnO, see [72]) may affect the optical

appearance. It will be shown later that this region indeed exhibits a pronounced growth of

native oxides.


102

0 20 40 60 80 1000

20406080

100

Magnesium

cont

ent /

at.%

position / mm

Zinc

Figure 8.1: Optical image of a Mg11Zn89-Mg95Zn5 material library with schematic insets illustrating

the locations addressed with the SFC. The composition along the x-axis is shown by an EDX line scan.

While the roughness is certainly a very important parameter for electrochemical studies, a

variation of the native oxide thickness needs to be considered as well because polishing

techniques to equalize the surface topography are not feasible on this kind of samples. All

observation therefore suggest a strong non-linearity that encourages further structural

investigations.

8.1.2 SEM imaging High resolution SEM images were taken at specific locations to clarify the structure of the

thin film formed during deposition. It is well known that zinc exhibits a high surface mobility

[209] (pp. 447) and grows voluminous in the hexagonal crystal system. Baker et al. showed that

the roughness of the film can be reduced by cooling of the substrate [74], which is not available

in the PVD system used. Therefore, a sponge-like film was obtained for pure zinc that

dominates the overall film structure up to approximately 14 at. % Mg, after which the co-

deposition significantly alters the film growth. Figure 8.2 shows a series of SEM images

following increasing Mg content from 0 to 91.2 at. %.


103

Figure 8.2: SEM images of thermally evaporated Zn-Mg alloys at 100 k magnification (acc. Voltage

12 kV). The images were taken on 3 different samples to cover the composition range presented.

A particularly interesting feature is the lamellar growth of upright standing plates, all being

horizontally oriented. The zinc source was located on top of each image while the magnesium

source was located on the bottom. Therefore, the orientation of the lamellas is in all cases

perpendicular to the connecting vector between the sources.

The rough morphology observed at high Zn contents dominates the structure up to around

13- 15 at. %, which is in exact agreement with the optical images. The surface is smoothed by

this structural change towards lamellar growth, probably being a consequence of the formation

of Zn-Mg intermetallics.

Further increase of the Mg content (χMg) causes the lamellas to tilt with respect to the

surface normal, ultimately leading to a smooth layer of plates lying flat on the substrate. This

change of lamellar orientation is again reflected by the optical appearance as the dark coloration

vanishes in this region, resulting in a mirror like appearance at all Mg contents exceeding

~44 at. %. The strong decrease of the number and size of the inclusions evident from the last 3

SEM images indicates that these are composed of a zinc rich phase, while the matrix


104

morphology is comparable to pure magnesium films obtained by sputter techniques reported

by Blawert [210].

8.1.3 XRD analysis To determine the crystallographic composition of the material libraries, X-ray diffraction

(see section 4.1) was performed along the gradient. The incidence angle was kept at 5° and the

sample was positioned in a way to ensure that the long side of the rectangular shaped

illuminated area (slit aperture) was perpendicular to the composition gradient (therefore,

parallel to the y-axis on the substrate). Due to the low incidence angle, the illuminated area is

comparably large and covers around 6.88 mm on the substrate in x-direction [122], causing the

diffraction patterns to correspond to a composition range. The following figures show an XRD

survey (step size 0.1°, integration 5 s) including a large number of compositions und three

detailed diffraction patterns recorded with high resolution and long integration times (step size

0.05°, integration 18 s).

30 40 50 60 70 80123456789

1011121314

ZnZn

Fe

Zn

Fe

Zn

log

(inte

nsity

/ a.

u.)

2 θ / degree

Mg

Fe

374453627178848992

Zn content / at. %

(a)

Figure 8.3:

Grazing incidence XRD along

the Zn-Mg material library with

a logarithmic intensity scale. The

individual patterns are shifted

along the y-axis for clarity. The

graph shows a survey covering a

large number of compositions

(mean value given, deviation

± 4 at. %).


105

30 40 50 60 70 801

2

3

4

5

6

7

8(b)

61.4 - 66.2 at. % Zn

77.0 - 81.1 at. % Zn

93.8 - 95.7 at. % Zn

MgZn2

ZnFe

Zn

FeZn

log

(inte

nsity

/ a.

u.)

2 θ / degree

FeZn

Figure 8.4:

Grazing incidence XRD along

the Zn-Mg material library with

a logarithmic intensity scale. The

individual patterns are shifted

along the y-axis for clarity. The

graph shows a detailed pattern

with high integration time to

resolve the low intensity MgZn2

peaks with the exact composition

ranges covered during the

experiment.

The survey shown in Figure 8.3 demonstrates a decrease of the metallic zinc pattern [211]

that completely vanishes for a zinc content of 53 ± 4 at. %. Further decrease of χZn results in

the emergence of a metallic Mg peak while the substrate material (Fe) is present in all cases. An

interesting observation is that the transition region shows neither Zn nor Mg features, but

instead a broad region of increased intensity that originates from several overlapping, poorly

resolved peaks. Figure 8.4 provides clarity for a composition range from 61.4 to 66.2 at. %

where a large number of low intensity peaks were detected that correspond to the intermetallic

MgZn2 [212]. The absence of metallic magnesium over a large composition range proves that

Mg is incorporated into the film as intermetallic. Given the stochiometry of MgZn2, it would be

expected that excess magnesium occurs at 53 ± 4 at. % in Figure 8.3 which is most probably

reflected by the slight increase of the XRD intensity at 34.5° in the respective dataset. The

presence of other intermetallics like MgZn and Mg2Zn11 [79] can be excluded within the

detection limit. The results presented are in full agreement to the literature stating MgZn2 to be

the dominant intermetallic formed under typical solidification conditions [213]. The presence

of amorphous material (especially Mg) can be excluded according to the low crystallization

temperature and ease of crystal formation during PVD deposition [214].

8.1.4 AES maps The formation of lamellas during the PVD process is the most noticeable feature of the Zn-

Mg material libraries and was shown to be roughly confined to a region between 90 and

50 at. %. zinc. This composition range is also characterized by the emergence of the

intermetallic MgZn2, which might be the origin of the structure observed through local


106

differences in composition, as long as these regions match the small size of the structural

features. To investigate this possibility, AES maps of high resolution (~15-20 nm) were

performed using a take off angle of 30°, an acceleration voltage of 25 kV and a current of

10 nA. Increasing intensity in the corresponding images is indicated by an increase in color

brightness.

Figure 8.5:

SEM image and corresponding AES maps of a

Zn79Mg21 alloy part of a Zn-Mg material library

deposited by thermal PVD. The image beside shows

the intensity difference between Zn and Mg indicating

regions of relative dominance of one particular element.

Please note that this measurement procedure only

allows for qualitative comparison.

Figure 8.6 shows the SEM image and corresponding AES maps on a Zn-Mg material library

at a magnesium content of 21 at. %. These maps only allow for qualitative comparison and the

peak/valley intensities of zinc and magnesium were matched to allow the illustration of the

local composition distribution in the difference map. While the Zn and Mg images themselves

strongly reflect the surface topography, the comparison in the last image corrects for that fact

and reveals regions of different composition that match the size of the lamellas considerably

well. It is therefore concluded from the XRD and AES data that phase separation between Zn

and MgZn2 is the major reason for the surface structure observed.


107

8.1.5 Native oxide thickness The thickness of the native oxide was estimated by means of X-ray photoelectron

spectroscopy (XPS) and sputter depth profiling. Peak locations and measurement procedures

were described in section 6.2.3.

The depth profiles on a Zn-Mg material library at three different compositional domains are

shown in Figure 8.6. The first observation is that the oxygen signal does not decay to zero

despite a leveling of the intensities with progressing sputter depth. Complementary EDX

analysis reveals an oxygen content in the complete film below 10 at. % as the sum of surface

oxides and oxides within the film formed during the deposition procedure. Therefore, the high

level of oxygen at large sputter depth is not assumed to reflect a high intrinsic level of oxygen

in the film itself, but is rather due to a topographic effect or an oxidation process during the

XPS measurement. It is to note that the exact determination of the oxide thickness is limited

because of the uncertainty regarding the exact sputter rate in comparison to SiO2, especially

because the etch rates of both metals and their oxides may all be different [29]. Therefore, the

data will be used for a qualitative comparison between different compositions only.

A well known effect during oxide formation on Zn-Mg alloys under environmental

conditions is the enrichment of magnesium at the surface due to its high affinity for oxygen

[79]. The data presented clearly reflects this fact by very low zinc intensities measured at the

surface (almost zero except for 6 at. % Mg). Furthermore, magnesium has the tendency to

oxidize deeper than zinc as shown by the emergence of metallic zinc prior to metallic

magnesium in sputter profiles presented by Hausbrand [29, 50]. The estimation of the oxide

thickness by e.g. the crossing point between zinc and oxygen therefore proves questionable.

However, the total intensity of oxygen and the slope of the signal decay during depth profiling

can be used for comparison and indicate that the native oxide thickness in the series presented

follows the order Mg6 < Mg37 < Mg19. This is surprising to some extend as a correlation

between Magnesium and oxygen content can not be concluded. The strong non-linearity

observed along the gradient previously demonstrated by different characteristics apparently

applies for the formation of native oxides as well.


108

0 5 10 15 20 25 30 35 400

20

40

60

80

Zn63Mg37

Mg

O

cont

ent (

at. %

)

Sputter depth / nmSiO

Zn

2

0 5 10 15 20 25 30 35 400

20

40

60

80

2

Zn81

Mg19

cont

ent (

at. %

)


Mg

O

Zn

0 5 10 15 20 25 30 35 400

20

40

60

80

100

2

cont

ent (

at. %

)


Zn94Mg6

Mg

OZn

Figure 8.6:

XPS depth profiles of the native oxide grown

on the material library at different

compositions. Carbon signals were only

observed prior to the first sputter step and are

not included.

8.1.6 Summary of the results Zn-Mg thin film obtained by thermal co-deposition exhibit a surface structure highly

dependent on the film composition. The rough morphology observed for very zinc rich

coatings vanishes quickly as the Mg content is increased above ~13 at. %, leading to the

formation of highly ordered lamellas. High resolution AES maps in combination with XRD

suggest the formation of MgZn2 intermetallics to be the origin of the structure observed. The

thickness of the native oxides on the material library estimated from XPS depth profiles was

shown to behave non linear along increasing Mg content.


109

8.2 Electrochemistry and dissolution 8.2.1 Unbuffered NaCl solution

Similar to the investigations on pure zinc presented in a former chapter (6.1, p. 54), aerated

0.1 M NaCl solution was used as a corrosive medium under steady electrolyte flow. It has been

presented in the respective chapter that open circuit potential measurements in combination

with downstream analytics constitute a very reliable way to measure ECorr and icorr, and the same

methodology is applied on Zn-Mg coatings in order to investigate the effect of magnesium on

the electrochemical behavior and dissolution rate of Zn-Mg material libraries.

8.2.1.1 Open circuit potentials A 1000 s OCP measurement was performed on a Zn-Mg material library ranging from

approximately 96 to 61 at. % zinc. The dominance of zinc along the whole library was chosen

because of the active nature of Mg in chloride containing environment, causing e.g. gas

evolution in the cell at very high Mg contents.

The measurement procedure was executed in a fully automated mode, and a linear array of

measurement locations (spacing 1 or 2 mm depending on the experimental run) was

programmed along the gradient vector (x-axis on the substrate, see Figure 8.1). Each location

was subject to 1000 s of OCP measurement, followed by a lifting of the cell and subsequent

purging for 1200 s. A steady electrolyte flow of 15.6 µl min-1 was maintained at all times. The

following figure shows a 3D graph of the recorded OCP in two independent experimental runs

after automated data processing, i.e. addition of the reference potential (212 mVSHE) and

conversion from position to composition (see page 33).

Figure 8.7: 3D OCP maps (1000 s) on a Zn-Mg material library displayed as a function of Zn content

(at. %) in 0.1 M NaCl solution under constant electrolyte flow. Two independent datasets are shown.


110

The corrosion potentials at high Zn content were stable within the duration of the

experiment and range around 780 mV. This value is well comparable to the literature [79] and

the data presented earlier on bulk samples of pure zinc (~760 mV, see p. 55).

As expected, a strong cathodic shift is observed at increasing Mg content within the first,

approximately 50 s of the experiment, which is attributed to the very active redox potential of

magnesium leading to rapid dissolution under these conditions [50]. Remarkably, this cathodic

shift is prolonged over a composition range between ~90 to ~70 at. % zinc, causing the system

to reach a stable potential value significantly later than 50 s. This delay shows a maximum

around 80 at. % Zn. It is probable that this behavior originates from an inhibition of the initial,

preferential dissolution of Mg by either compensatory dissolution of zinc or a generally

decreased corrosion current density. To further illustrate this effect, two-dimensional cuts at

fixed times were extracted from the datasets and are shown in the following figure:

50 60 70 80 90 100-900

-875

-850

-825

-800

-775

-750

OCP at 200 s series 1 OCP at 200 s series 2

OCP at 1000 s series 1 OCP at 1000 s series 2

Pote

ntia

l / m

VSH

E

Zn content / at. %

Figure 8.8:

Corrosion potentials of a

Zn-Mg material library as a

function of the composition

(at. % Zn given, rest Mg) at

different contact times

extracted from Figure 8.7.

It is clearly evident that the corrosion potential after 200 s of electrolyte contact exhibits a

minimum around 82 at. % in both replicates. This effective difference to the final potentials

measured at 1000 s vanishes for both high (> 90 at. %) and low (< 70 at. %) contents of zinc.

Local maxima are observed in the measurement at 200 s, reflecting peaks in the potential

transient as seen in Figure 8.7. The origin of this “overshoot” of the potential is purely

speculative and may originate from a temporary blocking effect of precipitates due to strong

magnesium dissolution expected during the anodic shift of the corrosion potential during the


111

initial seconds of the experiments. This effect can unfortunately not be clarified due to the

absence of magnesium detection.

8.2.1.2 Zinc dissolution monitoring Complementary zinc analysis, however, does provide information on the origin of the

prolonged cathodic potential region around 82 at. %. Because the preferential dissolution of

Mg in the respective region is assumed to proceed at lower rates, a decreased zinc signal would

immediately indicate lower total material dissolution. The measured zinc concentrations are

shown in Figure 8.9.

Figure 8.9: 3D illustration of the zinc concentrations detected downstream during an automated 1000 s

OCP scan on a Zn-Mg material library in aerated 0.1 M NaCl under electrolyte flow. Two different

perspectives are shown for clarification.

The shape of the dissolution profiles and the concentration range is well comparable to the

results obtained on bulk zinc under identical conditions (see section 6.1.1, pp. 57 ). However, at

very high zinc contents (> 90 at. %), the consistency of the data is relatively low compared to

the results in the following regions of increasing Mg. The partially large deviations observed

between neighboring dissolution profiles in the high-Zn region are not taken as reliable

differences in the dissolution behavior. Instead, the surface roughness, previously shown to be

very large in this region (Figure 8.2, p. 103), is assumed to cause irreproducible wetting that

immediately affects the dissolution rate, while the area independent corrosion potential remains

stable.

The most remarkable feature of the graphs shown in Figure 8.9 is the existence of a

minimum in dissolution rate that coincides with the maximal prolongation of the cathodic

corrosion potential shown in the two former figures. This is taken as a strong indication of a

reduced total dissolution rate of the material causing both reduced zinc liberation and a delay of


112

the preferential dissolution of magnesium. Even though the overall material loss covering zinc

and magnesium is not experimentally accessible with the setup presented, it can be roughly

approximated by including the film stochiometry into the dissolution profiles. The assumption

is that the homogeneity of the film causes the dissolution process to liberate zinc and

magnesium equal to the composition of the alloy if the experiment duration exceeds the region

of preferential dissolution of one particular component. The measured zinc concentration

therefore transform into a total dissolution rate through a division by the molar fraction of zinc

as shown in Figure 8.10 to compensate for an imposed linear decrease in zinc concentrations

that originates solely from reduced zinc content.

60 70 80 90 1000.0

0.5

1.0

1.5

0

10

20

30

40

50

60

[Zn2+

] t=10

00s Χ

-1 Zn /

μmol

l-1

Zn content / at. %

i Dis

s / μA

cm

-2

Figure 8.10: Dissolution transients from Figure 8.9 normalized to the molar fraction of zinc (left) and 2-D

cut at 1000 s with corresponding dissolution current densities (right).

As seen from the figure, decreasing zinc contents in the film lead to an up scaling of the

dissolution profiles especially evident at high magnesium fractions when comparing the graph

to the original dataset from Figure 8.9. The minimum in dissolution though is unaffected and

still falls in the region around 80 at. % zinc. The increasing trend towards the Mg-rich side

indicates dissolution rates that will most probably exceed the values observed at pure zinc.

In order to compare immersive and climate tests, it is important to consider the

observations made to bulk samples subject to climate chamber conditions (80 % RH, 20 °C, 28

days exposure after contamination with chloride) as reported by Prosek et al. [79]. The

following figure shows the weight loss as a function of the magnesium content after climate

corrosion tests:


113

Figure 8.11:

Weight loss of different cast model alloys (Mg

content in wt. %, rest Zn) subject to 28 days of

climate test after contamination with NaCl.

From [79].

Surprisingly, the weight loss minimum at 8 wt. % (19 at. %) Mg is in very good agreement to

the data presented in this study despite the large differences in sample preparation and

corrosion test methodology. The magnitude of the difference in weight loss between Zn and

ZnMg8 as shown in Figure 8.11 is large compared to the SFC based screening experiments.

This is most likely the consequence of the different testing procedures, which magnifies the

beneficial aspects of magnesium in case of climate tests (due to carbonate and hydroxide

buffering, see section 2.3.4, pp. 21).

Nevertheless, it is remarkable that short screening experiments with the SFC (1000 s) on

thermally evaporated Zn-Mg material libraries match long term climate tests (28 days) on cast

alloys to that extend. Furthermore, these results encourage investigations on technical

Zn80Mg20 (at. %) coatings to evaluate the performance as corrosion protection material.

8.2.2 Borate buffer pH 7.4 For pure zinc, it has been shown in a former chapter (6.2, pp. 66) that the existence of a

buffer system alters the corrosion mechanism significantly. Investigations at a steady pH value

are of high interest for Zn-Mg alloys as well, since Mg exhibits a different stability window

(stable at higher pH values than zinc, [4]) and is assumed to affect the surface pH during

corrosion processes [77, 96]. On the basis of the pH series presented earlier for zinc (Figure

6.12, p. 68), a borate buffer (0.1 M) of pH 7.4 was selected for experiments on Zn-Mg. This

specific pH value was chosen because of the comparably aggressive nature of the solution (high

dissolution rates) that nevertheless allows the characterization of surface films over a large

potential window due to kinetic passivity and the absence of pitting.

The experimental series performed consists of an OCP measurement for 600 s (the OCP

stabilizes quickly in this medium, p. 67) and an anodic sweep (5 mV s-1) starting at the OCP

with a current density of 650 µA cm-2 as stop condition. Figure 8.12 shows the recorded

corrosion potentials as a 3D illustration in a concentration range from 96.2 to 38.3 at. % Zn.


114

The extended range towards higher magnesium contents as compared to the experiments in

0.1 M NaCl solution is possible by the less active nature of Mg in near neutral, chloride free

environments.

8.2.2.1 Open circuit potentials

Figure 8.12:

3 D plot of the open circuit

potential measurements

(600 s) in borate buffer

(0.1 M) of pH 7.4 under

continuous electrolyte flow

(15.6 µl min-1) as a function

of zinc content.

It can be seen that the corrosion potentials are roughly 100 mV higher than in the

complementary experiment in NaCl solution. Furthermore, a strong cathodic shift of the initial

potentials is observed similarly, which is assumed to originate from preferential Mg dissolution.

However, a prolonged region of cathodic potentials as previously seen for NaCl solution in the

region between 70 and 90 at. % zinc is not found. Instead, the potentials recorded are very

stable and exhibit a high consistency along the composition axis.

In order to clarify the difference between the intermediate (30 s) and final (600 s) corrosion

potentials apparent from Figure 8.12, a two dimensional cut along the time axis was performed

at the respective times as shown in the following.


115

30 40 50 60 70 80 90 100-750

-735

-720

-705

-690

-675

-660

70 80

-705

-700

-695

-690

OCP at 30 s OCP at 600 s

Pote

ntia

l / m

VSH

E

Zn content / at. %

Magnified region(t=600 s) 65 - 86 at.%

Figure 8.13:

Two dimensional cut through

Figure 8.12 at t=30 s and

t=600 s. The inset magnifies

the local minimum observed

after 600 s in a region between

65 and 85 at. % zinc.

While the initial potentials at 30 s decrease with increasing Mg content until reaching a

stable value around -740 mV, the final values show a strongly non-linear behavior with a local

minimum around 80 at. % zinc. The extraordinary behavior of this composition was previously

observed in NaCl solution, even though the potential differences in borate buffer are of small

magnitude (several mV). This observation can therefore not be taken as direct evidence for a

specific surface process, but it is nevertheless remarkable that the region around 80 at. % zinc

behaves extraordinary in all experiments presented so far on Zn-Mg material libraries. It is to

note that the XPS results shown previously (Figure 8.6, p. 108) demonstrate an increased

thickness of the native oxides formed at the respective composition, which allows assuming

that the local potential minimum indicates a more active potential preserved over a longer time,

possibly due to a decreased barrier effect of the surface film formed.

8.2.2.2 Potential sweep experiments This question can be addressed by the potential sweep experiments performed subsequent

to the OCP measurement. The following graph summarizes the results in a 3D illustration:


116

Figure 8.14: Anodic sweep experiments (5 mV s-1) starting from the previously recorded OCP (600 s) with

dynamic end points at 650 µA cm-2 in borate buffer (0.1 M) of pH 7.4 under constant electrolyte flow

(15.6 µl min-1). The dataset is shown from two perspectives for clarification.

The passive current density increases significantly around 80 at. %, being further indication

of a decreased barrier effect of the oxides formed during anodization. Besides that, a striking

consistency along the composition gradient is observed that allows separating the dataset into

composition regions of continuous trends.

-0.5 0.0 0.5 1.0 1.5 2.00

100

200

300

400

500

600

700

i / μ

A c

m-2

E / VSHE

43.9

54.1

-0.5 0.0 0.5 1.0 1.5 2.00

100

200

300

400

500

600

700

0.0 0.5 1.0 1.5200

300

400

i / μ

A cm

-2

E / VSHE

i / μ

A c

m-2

E / VSHE

55.4

78.4

-0.5 0.0 0.5 1.0 1.5 2.00

100

200

300

400

500

600

700

-0.5 0.0 0.5 1.0 1.5

250

300

350

i / μ

A c

m-2

E / VSHE

i / μ

A cm

-2

E / VSHE

87.6

79.3

-0.5 0.0 0.5 1.0 1.5 2.0 2.50

100

200

300

400

500

600

700

-0.5 0.0 0.5

200

300

i / μ

A cm

-2

E / VSHE

88.7

96.3

96.3

i / μ

A c

m-2

E / VSHE

88.7

Figure 8.15: Regions of continuous trends extracted from Figure 8.14. The numbers indicate the zinc

content in atomic %.


117

The first composition range between 96.3 and 88.7 at. % is characterized by a strong increase of the

initial peak in current density. An initial barrier formation in the highly dynamic equilibrium

between oxide formation and dissolution (section 6.2.2, pp. 68, especially pH 7.1) is assumed to

cause the observed behavior. Because of the strong change in surface topography in the

respective composition region, it is not possible to correlate the behavior observed to the effect

of roughness or increasing magnesium exclusively. However, it is to note that the passive

current density is not lowered with increasing magnesium content even though the roughness

(and therefore the real area of the electrode) decreases strongly. This is not unexpected because

the possibility to use the SFC on the sample surface implies that the rough, sponge-like

structure is not completely wetted and soaked with electrolyte, which would instantly drain

massive electrolyte volumes from the cell.

Another noticeable feature in the first composition range is the transition from an

exponential increase in current density starting around 2.5 VSHE at 96.3 at. % towards a steeper

increase with earlier onset with increasing magnesium content. While in case of high zinc

contents the increase in current density is assumed to reflect film breakdown, the steep increase

around 1.7 VSHE is attributed to oxygen evolution most probably enabled by the existence of

intermetallics as shown during anodization of Al-Cu material libraries in another study [122].

This oxygen evolution reaction remains steady for all further measurements of increased Mg

content as evident from all other graphs included in Figure 8.15. This separation between film

breakdown and oxygen evolution will be later clarified by the results of downstream zinc

detection.

The second composition range from 87.6 to 79.3 at. % zinc is dominated by a steady increase in

passive current density most probably originating from a decreased barrier effect of the surface

film formed. As shown on pure zinc (Figure 6.18, p. 75), it has been confirmed by XPS that the

surface film is of oxidic nature with no hydroxide signal within the detection limit. This

observation in conjunction with the active OCP (local minimum) in this particular region is

surprising given the superior corrosion resistance in unbuffered NaCl solution. It suggests that

the active nature of magnesium is pronounced at these compositions which is apparently

beneficial in neutral, unbuffered solutions, but of inverse effect in borate buffer of pH 7.4.

The third composition range from 78.4 to 55.4 at. % zinc shows a decrease of the passive current

density while the current density during the initial approximately 800 mV of anodic polarization

remains constant. The latter appears to consist of two broad peaks, similar to the peak shape

observed on Zn-Mg intermetallics in NaOH by Hausbrand et. al [29] (also see [50], p. 46). The

exact electrochemical processes at this stage are not clarified. However, a major difference in


118

current density during anodic progression of the potential is observed decreasing from

~320 µA cm-2 at 78.4 at. % Zn to ~210 µA cm-2 at 55.4 at. % Zn. This decrease may reflect an

increased barrier effect of MgO in the film as suspected by Prosek et. al [79] and supports a

direct contribution of magnesium to the film properties rather than an indirect stabilization of

zinc precipitates.

The final region starting from 54 at. % zinc shows an increase of both the peak and plateau

current density with a strong additional peak evolving around 0 VSHE. Since the oxide layer is

dynamic at all times due to a continuous dissolution by the electrolyte, a selective oxidation of

either zinc or magnesium can not be confined to a defined potential because both metals exist

in an oxidized state at the outer layer. Experiments performed on 45 at. % zinc with varying

OCP times prior to the sweep showed the peak in question to be more pronounced with

shorter periods of free corrosion. Since the peak integral appears to grow with increasing

magnesium content in the alloy and magnesium tends to leech from the surface (therefore

explaining higher peak integrals with shorter OCP periods), the origin of the peak is assumed in

a process where metallic magnesium is involved, even though an immediate association with a

distinct electrochemical process can not be given.


119

8.2.2.3 Zinc dissolution monitoring

The dataset recorded parallel to the OCP-anodic sweep couple is shown in Figure 8.16. A

highly consistent trend towards lower zinc concentrations is observed with increasing

magnesium content for both the OCP dissolution as well as the dissolution during the anodic

sweep. Close examination of the final region of the zinc dissolution profiles around 96 at. %

Zn reveals a sharp peak at the end of the anodic sweep that originates from film breakdown.

The transition to the OER with earlier onset around 92 at. % (Figure 8.15) causes this peak to

vanish.

The initial region around 160 s, that is immediately after electrolyte contact when

subtracting the delay time (156 s), is characterized by a steady increase in zinc concentration for

zinc contents above approximately 85 at. %, but exhibits a small plateau as the magnesium

content is further increased. To illustrate this effect, the inset of Figure 8.16 magnifies the first

500 s of the dissolution profile for 91 and 72 at. % Zn. It can be seen that the initial rise of the

zinc concentration is comparable in both cases, but followed by a strong decrease of the slope

Figure 8.16:

3D plots of the zinc concentrations detected

during the OCP-anodic sweep couple in borate

buffer (0.1 M) of pH 7.4 under steady electrolyte

flow (15.6 µl min-1) as a function of zinc content

on a Zn-Mg material library. Two different

perspectives are shown with indication of the

measurement sequence on the time scale. The inset

shows the initial dissolution profile Zn91Mg9 and

Zn72Mg28 for the first 500 s of electrolyte contact.


120

in case of the higher magnesium content. This dissolution peak most probably originates from

surface film formation during the first seconds of electrolyte contact that inhibits further

dissolution. Selective removal of magnesium, therefore cathodic protection of zinc by

magnesium, is taken as another probable process, even though this would not account for the

existence of a peak.

While the generally decreasing trend of the dissolution profile with increasing magnesium

content is clearly demonstrated, the film stochiometry also needs to be taken into account

because Mg detection is not available. Two dimensional cuts at fixed times were taken from

Figure 8.16 showing the zinc concentration divided by the molar fraction of zinc as a function

of the zinc content. The first set corresponds to the OCP region (50-600 s) while the second

displays the increase in concentration during the potential sweep (750 and 1050 s) including the

final OCP values (600 s) for comparison. Please note that all times were corrected by the dead

time (156 s).

40 50 60 70 80 90 1000.0

0.4

0.8

1.2

1.6

0

40

80

120

i Diss

/ μA

cm

-2

[Zn2+

] χZn

-1/ μ

mol

l-1

Zn content (at. %)

600 s

200 s

50 s

linear fit 40-80 at. %

linear fit 80-97 at. %

40 50 60 70 80 90 100

1.2

1.6

2.0

2.4

2.8

3.2

80

120

160

200

240

i Dis

s / μA

cm

-2

[Zn2+

] χZn

-1/ μ

mol

l-1

Zn content (at. %)

1050 s

750 s

600 s

Figure 8.17: Zinc concentrations at given times corrected by td as a function of Zn content in the film

extracted from Figure 8.16. The dataset is separated into times during the OCP (50-600 s) and the anodic

sweep (750 and 1050 s). The right axis shows the corresponding dissolution current densites at a flow rate of

15.6 µl min-1.

An interesting observation is that the normalized dissolution rate quickly after electrolyte

contact (50 s) rises with increasing Mg content and levels around 80 at. % Zn, reflecting the

emergence of an initial peak. However, an inverse trend is observed for longer contact times,

while again reaching a steady dissolution rate. These results support the hypothesis of a barrier

formation (between 50 and 200 s) that reduces the increase of the dissolution rates as the

experiment proceeds. 80 at. % again constitutes an exceptional composition and appears to be

the onset composition (towards higher Mg contents) for the behavior observed.


121

The normalized concentrations measured during the potential sweeps indicate a higher

dissolution rate with increasing magnesium content at 750 s (approximately 0 VSHE), which

correlates to the generally increasing current density at the respective potential (Figure 8.15). At

1050 s, the anodic sweep progressed to approximately 1.55 VSHE, being the final region of the

passive range shortly before either film breakdown or the onset of the OER. Major differences

were observed in this area along the composition gradient, the most significant being the

maximum around 80 at. %. This behavior appears to be reflected by the dissolution rates since

the measurement curve at 1050 s in Figure 8.17 exhibits a local maximum around this

composition. The final increase of the normalized concentrations starting around 55 at % for

both 750 and 1050 s appears very steep and coincides with a high peak current density

recorded during potential sweeps. Remarkably, the dissolution rate in this region is independent

of the potential, evident from the identical dissolution current densities at 750 and 1050 s in

Figure 8.17. It is possible that the mixed oxides at the surface exhibit a composition at which

the dissolution rate at the interface is unaffected by the applied potential, a case reported by

Wagner on iron-oxide with the composition Fe2.67O4 [65]. However, a deeper investigation of

this effect would require exact knowledge about the electronic structure and crystal

composition of the respective oxides formed.

A composition of 80 at. % zinc is characterized by a low dissolution and corrosion potential

at the OCP, which is consistent with the data recorded in 0.1 M NaCl where Zn~80Mg~20

was found to exhibit prolonged cathodic corrosion potentials and low dissolution rates.

However, a high dissolution and passive current density during anodic sweeps was recorded for

this composition. It appears that the character of the oxide film as estimated from the passive

current density is not decisive for the dissolution under open circuit conditions. Moreover, a

difference in the oxides formed on the surface need to be assumed since precipitation is absent

in borate buffer of pH 7.4 as concluded from its purely oxidic nature (determined by XPS, see

p. 75) and the high solubility of both zinc- and magnesium hydroxide. To clarify the oxide

composition, XPS depth profiling after electrochemical treatment is performed in the following

section.

8.2.2.4 XPS Analysis XPS-depth profiling was performed on three different compositions each subject to 1000 s

OCP measurement and 100 s OCP with subsequent potential sweep to 500 mV anodic of the

corrosion potential at a scan rate of 5 mV s-1. The results are shown in Figure 8.18.


122

0 5 10 15 20 25 30 35 400

20

40

60

80

100

cont

ent (

at. %

)

Sputter depth / nmSiO2

Zn94Mg6 - 1000s OCP

Mg

O

Zn

0 5 10 15 20 25 30 35 400

20

40

60

80

100

cont

ent (

at. %

)


Zn94Mg6 - Sweep

Mg

O

Zn

0 5 10 15 20 25 30 35 400

20

40

60

80

100

cont

ent (

at. %

)


Zn81Mg19 - 1000s OCP

Mg

O

Zn

0 5 10 15 20 25 30 35 400

20

40

60

80

100

cont

ent (

at. %

)Sputter depth / nmSiO2

Zn81Mg19 - Sweep

Mg

O

Zn

0 5 10 15 20 25 30 35 400

20

40

60

80

100

cont

ent (

at. %

)


Zn63Mg37 - 1000s OCP

Mg

O

Zn

0 5 10 15 20 25 30 35 400

20

40

60

80

100

cont

ent (

at. %

)


Zn63Mg37 - Sweep

Mg

O

Zn

Figure 8.18: XPS depth profiles of different compositions on the material library after electrochemical

treatment as indicated in each graph. Carbon signals were only observed prior to the first sputter step and are not

included.

The comparison between 1000 s OCP measurement and potential sweeps yields a slight

increase in oxide thickness as a consequence of applied anodic potentials. In contrast, an

increase in magnesium content significantly increase the oxygen signal in the depth profiles for

both OCP and potential sweep experiments, demonstrating that magnesium enhances the

formation of mixed oxides on the surface. Of particular interest is that a selective leeching of a

single metal can not be observed. This effect is attributed to the instability of both oxides at a

pH value of 7.4, dissolving at equal and diffusion limited rates. The electrochemical behavior

described in the former section is therefore mainly determined by the electrical properties of

the oxidic material on the surface and the total extends of oxide formation. It is to note that the


123

thickness of the oxides formed during electrochemical treatment can not be exactly determined,

because comparably high amounts of oxygen are found in the native state of these materials as

shown previously (Figure 8.6, p. 108). However, it is apparent that thick layers of oxides form

on the surface with considerably constant stochiometry. Proton diffusion as rate determining

process for material dissolution therefore applies for Zn-Mg mixed oxides as well under the

conditions presented. This is supported by the considerably stable normalized dissolution

current density shown in Figure 8.17 at the OCP, with the increased rates at high Zn content

being a consequence of a significant surface roughness.

8.2.3 Summary of the results The corrosion behavior of thermally evaporated Zn-Mg material libraries can be effectively

investigated with the microelectrochemical system presented. The corrosion potentials and

dissolution rates in both NaCl solution and borate buffer for zinc rich compositions are

comparable to pure zinc samples presented in chapter 6. However, the effect of magnesium on

the corrosion behavior differs largely between unbuffered NaCl solution and borate buffer of

pH 7.4.

In 0.1 M NaCl solution, a strong decrease of the dissolution current density at the OCP was

observed up to magnesium additions of 20 at. %, that coincides with a prolonged cathodic

corrosion potential after electrolyte contact. Further increase of χMg increases the corrosion rate

and reduces the duration of the initial cathodic corrosion potential, with only minor influence

on the potentials established after 1000 s of electrolyte contact. These findings are in good

agreement with climate test reported by other authors, which is surprising given the large

differences in methodology, sample preparation and time consumption. The results presented

demonstrate that material optimization procedures can be strongly supported by the integrated

microelectrochemical system presented and encourage the use material libraries for

combinatorial investigations. The most beneficial composition for the corrosion in aerated

NaCl solution was shown to range around 20 at. % magnesium.

In borate buffered solution however, fundamentally different results were obtained. The

dissolution rates at the OCP in borate buffer of pH 7.4 are well comparable to the bulk Zn

counterpart, with deviations only in the zinc rich region that exhibits an intrinsically high

surface roughness. The oxides formed are several tens of nm thick and do not show large

alteration in composition during depth profiling. However, the electrochemical behavior during

anodic potential sweeps shows an impact of the magnesium content, being strongly non linear

along the composition axis. The electrical properties of the mixed oxides formed on the surface

are taken as the most plausible cause for the observed differences.

Chapter 9: Comprehensive discussion

124

9 Comprehensive discussion The majority of the experiments presented in this work are based on a novel

microelectrochemical setup that has been developed within the scope of this study. The

primary experimental challenge was the integration of a steady electrolyte flow into a capillary

microcell with a tip diameter around 200 µm and the implementation of downstream UV-VIS

analytics. Due to the novelty of the measurement procedure, a comprehensive system

characterization has been the foremost aim to prove the validity of the data obtained. During

these efforts, it has been shown on the example of oxide formation on valve metals and

platinum that the validity and reproducibility of microelectrochemical data is high and well

comparable to literature values. A comparison to a classical channel electrode has been

achieved by correlating the transport limited current density of the oxygen reduction reaction

on platinum with the volume flow rate. This investigation clearly demonstrates that the

transport limit is a function of the cube root of the volume flow, a dependency well known for

classical channel electrodes.

Downstream analytics have been successfully integrated using Zincon as a complexing agent

and a UV-VIS flow cell. The detection limit was shown to range around 10-7 mol l-1 and the

dead time between substrate and detector was about 156 s. The successful coupling of micro

electrochemistry and downstream detection was demonstrated on copper, where the

electrochemically released amount of metal ions was quantitatively detected in the

spectroscopic system. This correlation proved the calibration procedure and measurement

sequence to be valid. The three initially formulated aims of (i) a high reproducibility and

comparability of data, (ii) a miniaturized and fully automated setup and (iii) a high sensitivity

and reliability of downstream detection were achieved.

The fundamental investigation of pure zinc was performed subsequent to the system

characterization with the aim to investigate the impact of different parameters on zinc

corrosion with both electrochemical and spectroscopic data. It was shown that an increase of

the chloride content from 0.01 to 1 M gradually increases the corrosion current density under

constant electrolyte flow, even though high amounts of chloride increase the equilibration time

of the system. It was further shown that the corrosion potentials shift cathodically, which was

attributed to a shift in the reversible potential of zinc dissolution because of nearly identical

Tafel slopes measured. Furthermore, a variation of the volume flow rate of the electrolyte

revealed an increased zinc dissolution profile with increasing electrolyte flow, originating from

an enhanced removal of precipitates from the surface. This was supported by the fact that the


125

measured corrosion current densities lie well below the theoretically possible oxygen reduction

rates, indicating a strong hindrance of oxygen transport by surface films formed.

One of the key questions (I, see p. 24) on pure zinc was to evaluate the possibility to

accurately determine the corrosion current density from the dissolution profile. This was

confirmed by galvanostatic experiments, showing a very good correlation between dissolution

current as calculated from downstream zinc concentrations and imposed current. This finding

is of particular importance since it provides a method of measuring corrosion current densities

without driving the corroding system from its steady state. Potentiodynamic sweep experiments

(II) in contrast did not yield comparable corrosion current densities, but instead revealed a

linear variation of the results depending on the square root of the scan rate. This indication of a

diffusion controlled processes was interpreted on the basis of a local saturation of the

electrolyte with zinc as a consequence of the anodic dissolution, causing film formation and a

deviation from the steady state due to the time dependence of the corresponding processes.

The electrochemical and spectroscopic data presented provide a comprehensive picture of the

corrosion process of zinc in NaCl solution, being mainly determined by precipitate and surface

film formation and therefore susceptible to parameters with an immediate effect thereon.

Because the surface pH is of uttermost importance in this mechanism and affected by both

anodic and cathodic reactions, the effect of a buffer system was thoroughly studied.

There it was found that the corrosion mechanism changes fundamentally in borate buffered

solutions. The dissolution process in this medium is governed by proton diffusion and the

resulting decomposition of ZnO formed on the surface. The dissolution with hindrance by

corrosion products observed in NaCl solution therefore changes to dissolution through an

oxidic film, which was confirmed by surface analysis techniques. The corrosion current

densities are linearly dependent on the concentration of protons and proton carriers, and

comparably high in neutral borate buffer (0.1 M) under electrolyte flow. In contrast, the

corrosion potentials behave strongly non linear with passive values at pH 7.4 and above and

active values at lower pH. The strong cathodic shift of the corrosion potential at higher pH

values is taken as an indication that a closed oxide layer is formed, with a thickness depending

on the relation between film formation and dissolution. The presence of active sites and active

corrosion potentials accordingly, can be stimulated by the addition of sulfate anions, leading to

oscillations in the OCP at certain combinations of pH and sulfate concentration. While sulfate

is primarily regarded as a pitting anion, is has been shown that the steady state dissolution rate

is significantly increased by addition of these anions. The earlier onset for film breakdown is

therefore not the main effect, but the consequence of zinc complexation and solubility increase

caused by these species. It appears that the severe influence of sulfate ions and buffered


126

electrolytes on zinc corrosion is often underestimated in the literature. The integrated zinc

detection presented however provides a very fundamental property that largely aids the

evaluation of electrochemical data.

In order to verify the dissolution mechanism of ZnO concluded from the results in buffered

solutions, polycrystalline, RF-sputtered ZnO:Al thin film were investigated with respect to the

chemical and electrochemical dissolution mechanism. It was accurately confirmed (I) that

proton transport is the rate determining process since the dissolution rate of ZnO is a linear

function of the buffer capacity at constant pH. Furthermore, this proton induced etching

initiates preferably at grain boundaries, causing a texturing of the surface along the grain

boundaries that vanishes at high dissolution rates. This chemically induced etching of ZnO was

shown to be very low in unbuffered NaCl solutions, clearly demonstrating that this process is

not decisive for the corrosion current densities of zinc in these electrolytes (II).

Due to the high conductivity of ZnO:Al, it was possible to investigate the electrochemical

decomposition processes taking place at high anodic potentials. These efforts were strongly

supported by the dissolution data of zinc, which allowed distinguishing between the two

competing oxidation processes of either water or lattice oxygen. It was shown (III) that pH

buffers significantly decrease the faradaic efficiency for dissolution from around 88 % in

unbuffered NaCl solutions to 61 % in 0.1 M acetate buffers of pH 6.5 and 7.0. This difference

illustrates that both water splitting and lattice decomposition trigger ZnO dissolution, while the

former can be significantly reduced by stabilizing the surface pH. It was concluded that protons

generated on the surface attack the lattice surprisingly effective, causing the stability window of

water to indirectly determine the stability of surface oxides prone to proton etching. The

surface texture obtained after electrochemical decomposition of polycrystalline ZnO:Al thin

films showed a unique degree of selectivity towards the grain boundaries, clearly demonstrating

that both water splitting and lattice decomposition proceeds at these sites exclusively. The

importance of the crystalline structure and phase composition of oxidic films is strongly

emphasized. Furthermore, these findings offer a novel tool for the modification of the optical

properties of ZnO:Al for solar cell applications.

The final aim of this study was to apply the new methodology on laterally graded Zn-Mg

material libraries and perform high throughput screening experiments with an exceptionally

high information depth, covering both electrochemical and dissolution behavior (I). This

application takes advantage of all features of the system presented as it combines high

throughput experimentation, local confinement and parallel electrochemical and spectroscopic

data acquisition. For sample preparation, a thermal PVD unit was modified to allow co-

deposition of a variety of metals onto large substrates (~10 cm length), and a mathematical


127

deposition model was developed. An element mapping procedure was presented based on this

model to allow a precise and automated transformation of the measurement position into an

alloy composition. The Zn-Mg material libraries produced showed a complex crystal growth on

the surface and MgZn2 as dominant intermetallic, as shown by XRD, SEM and scanning AES

measurements. The electrochemical screening experiments revealed a strong impact of the

magnesium content on the dissolution behavior and corrosion potentials on 0.1 M NaCl

solution. The initial cathodic shift of the corrosion potential as a consequence of the active

redox potential of magnesium was prolonged at compositions from 90 to 70 at. % Zn, with

maximum duration around 80 at. %. The alloy dissolution was found to be significantly

lowered in this region, suggesting an increased corrosion resistance (II). These results were

found to be in surprising accordance to a recent study on bulk materials in climate tests by

other authors, and demonstrate the feasibility to utilize the complex setup presented for

material screening processes. The use of a borate buffer (III) however altered the results

significantly. The data is in good agreement with the previous investigations on Zn and ZnO,

demonstrating a change in the corrosion mechanism to chemically controlled dissolution of

thick oxidic surface layers. While XPS surface analysis revealed an oxide composition reflecting

the Zn/Mg ratio in the alloy, the electrochemical properties of the respective layers differed

strongly non linear along the composition axis. It was found that 4 at. % Mg is sufficient to

trigger oxygen evolution at approximately 1.5 VSHE independent of a further increase of the

magnesium content, while compositions with lower additions of Mg showed film breakdown at

more anodic potentials. Interestingly, a maximum in the plateau current density during

potential sweeps was observed at a composition around 20 at. % Mg, indicating low insulating

properties of the surface layer formed. The beneficial effect of magnesium is therefore strongly

dependent on the electrolyte system used, which further emphasizes the need to include the

effect of different media into corrosion testing procedures.

The scanning flow cell system presented may significantly contribute to the large

experimental demands resulting from this approach, as the high throughput capabilities and the

validity of the obtained data has been repeatedly confirmed within this study. It further allows

monitoring the zinc dissolution as a fundamental parameter and complementary analysis

technique parallel to electrochemical investigations, which has been shown to strongly aid the

interpretation of electrochemical data. The applicability of microelectrochemical systems with

online dissolution monitoring has been proven for Zn, ZnO and Zn-Mg alloys, and the

comparative data analysis conducted provided valuable information on the corrosion

mechanism of these materials in various electrolyte systems.

Chapter 10: Outlook

128

10 Outlook The technical developments achieved within this work constitute a solid basis for further

investigations. It would be valuable to thoroughly study the flow profile in the tip and optimize

the transport characteristics, possibly allowing to study electrochemical reactions involving

dissolved gasses. A detailed comparison to the rotating disc electrode would therefore be of

high relevance.

Furthermore, an extensive parameter screening on zinc with focus on the electrolyte

composition could largely contribute to the understanding of environmental corrosion

processes, as online dissolution monitoring and surface analysis techniques can be easily

conducted. It is to be expected that a combination of electrolyte constituents causes a different

corrosion behavior than estimated from combining the singular impact of each substance. An

example would be the coexistence of aggressive (e.g. Cl-, SO42-) and passivating anions (e.g.

CO32-) very commonly encountered in environmental conditions. Furthermore, a detailed

characterization of precipitates formed under various conditions would improve the

understanding of this highly complex process which is of uttermost importance for zinc

corrosion as repeatedly demonstrated in this study.

As a result of the combinatorial corrosion testing performed on Zn-Mg alloys, the most

promising composition for corrosion protection ranged around 80 at. % Mg. It would be

consequent to expose model alloys of the respective composition to a variety of technical

testing conditions, and to perform a comprehensive investigation of the effect of the electrolyte

composition on the corrosion behavior. This possibly enables to translate the scientific results

presented into technical coatings with broad profit in a variety of applications. These

investigations would furthermore significantly profit from additional analysis techniques of the

electrolyte downstream, covering both zinc and magnesium for alloy testing or multi element

analysis (e.g. ICE-OES) for a much broader range of possible investigations.

Chapter 11: Bibliography

129

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140

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Chapter 0: Appendix

141

Appendix Publications

S.O. Klemm, A.G. Martin, J. Lengsfeld, J.C. Schauer, B. Schuhmacher, A.W. Hassel,

Theoretical simulation and preparation of binary and ternary combinatorial libraries by thermal PVD,

Physica Status Solidi A-Applications and Materials Science, 207 (2010) 801-806.

S.O. Klemm, J.P. Kollender, A.W. Hassel, Combinatorial corrosion study of the passivation of

aluminium copper alloys, Corrosion Science, 53 (2011) 1-6.

S.O. Klemm, S.E. Pust, A.W. Hassel, J. Hüpkes, K.J.J. Mayrhofer, Electrochemical texturing of

Al-doped ZnO thin films for photovoltaic applications, Journal of Solid State Electrochemistry, (2011)

doi:10.1007/s10008-011-1313-z.

S.O. Klemm, J.-C. Schauer, B. Schuhmacher, A.W. Hassel, A Microelectrochemical Scanning Flow

Cell with Downstream Analytics, Electrochimica Acta, 56 (2011) 4315-4321.

S.O. Klemm, J.-C. Schauer, B. Schuhmacher, A.W. Hassel, High throughput electrochemical

screening and dissolution monitoring of Mg-Zn material libraries, Electrochimica Acta, (2011),

doi:10.1016/j.electacta.2011.05.065

S.E. Pust, J. Worbs, J. Hüpkes, S.O. Klemm, K.J.J. Mayrhofer, Electrochemical Etching of Zinc

Oxide for Silicon Thin Film Solar Cell Applications, ECS Transactions, 33 (2011) 41-55.

S.E. Pust, J.-P. Becker, J. Worbs, S.O. Klemm, K.J.J. Mayrhofer, J. Hüpkes, Electrochemical

Etching of Zinc Oxide for Silicon Thin Film Solar Cell Applications, Journal of the Electrochemical

Society, 158 (2011) D413-D419.

I. Katsounaros, J.C. Meier, S.O. Klemm, A.A. Topalov, P.U. Biedermann, M. Auinger, K.J.J.

Mayrhofer, The Effective Surface pH during Reactions at the Solid-Liquid Interface, Electrochemistry

Communications, 19 (2011) 634-637.

Chapter 0: Appendix

142

Oral presentations

S.O. Klemm, A. W. Hassel, J.-C. Schauer, B. Schuhmacher

”A Novel Flow-Type Scanning Droplet Cell for Miniaturized Electrochemical Characterization of Sub-mm

Spots on Metallic Substrates”, Electrochem 09, Manchester, UK, 16-17 September 2009

M. Stratmann, S. O. Klemm, M. Rohwerder, A. W. Hassel,

“Electrochemical Design of Novel Zinc Alloys for the Corrosion Protection of Steel”, 216th Meeting of the

Electrochemical Society, Vienna, Austria, 4-9 October 2009


“A high throughput approach towards understanding of zinc corrosion and its alloys”, Electrochemistry

2010, Bochum, Germany, 13-15 September 2010


“A Microelectrochemical Flow System with In Situ UV-VIS Spectrometric Analysis Capable of High

Throughput Experimentation”, Eight International Symposium on Electrochemical Micro &

Nanosystems Technologies (EMNT) 2010, Nice, France, 21-24 September 2010

S. E. Pust, J. Worbs, J. Hüpkes, S. O. Klemm, K. J. J. Mayrhofer

“Electrochemical Etching of Zinc Oxide for Silicon Thin Film Solar Cell Applications“, 218th Meeting of

the Electrochemical Society, Las Vegas, USA, 10-15 October 2010

S.O. Klemm, K. J. J. Mayrhofer

“Elektrochemische Hochdurchsatzuntersuchungen mit gekoppelter online Analytik”, 4. Korrosionsschutz-

Symposium, Trent, Germany, 25-27 May 2010

Poster presentations


„High Throughput Screening of Combinatorial Zn-Mg Libraries for Application Dependent Corrosion

Protection”, Engineering of Functional Interfaces (ENFI), Hasselt, Belgium, 18-19 June 2009


„Optimization of Materials by Using a Microelectrochemical High Throughput Approach: Anodization of Al-

Cu Alloys”, Engineering of Functional Interfaces (ENFI), Marburg, Germany, 15-16 July 2010

Chapter 0: Appendix

143

Curriculum Vitae

Name: Sebastian Oliver Klemm

Date of birth: 30.6.1982

Nationality German

University

April 2008 – June 2010 PhD student, Max-Planck Institut für Eisenforschung,

GmbH, Faculty for Chemistry and Biochemistry, Ruhr-

Universität Bochum

October 2002 –April 2008 Studies in “Wirtschaftschemie”, Diploma Degree,

Heinrich-Heine Universität Düsseldorf

Civil service

September 2001 –Juli 2002 SV Bayer Wuppertal

School

August 1992 – Juli 2001 Secondary school, Gymnasium Vohwinkel, Wuppertal

August 1988 – Juli 1992 Primary School, Yorkstraße, Wuppertal

Research experience

September 2005 – March 2006 Research stay, Kyoto university, Kyoto, Japan

Supervisor: Prof. Dr. Shunsaku Kimura

Focus: Organic and macromolecular chemistry

August 2007 – February 2008 Diploma thesis, University of Utah, Salt Lake City,

USA

Supervisor: Prof. Dr. Florian Solzbacher

Focus: Coatings and semiconductors

microelectrochemical characterization of zn, zno and … · microelectrochemical characterization...

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