corrosion evaluation and diagnostic: use of ... · corrosion scientists also make extensive use of...

Corrosion evaluation and diagnostic: use of electrochemical techniques

in metal conservation

Paola Paola LetardiLetardi

CNR- ISMAR, via De Marini 6, I-16149 Genovae-mail: [email protected]

AutumnSchool2011 Corrosion evaluation and diagnostic

Corrosion

Electrochemistry

Electrochemical techniques Met

allic

Ant

iqui

ties


Corrosion

The smelting of metals from ores (their separation from oxygen, sulfur and carbon) involves metallurgical processes requiring elevated temperatures (typically in the range of 800 -1400 °C) and a spec trum of redox conditions spanning from highly oxidizing to strongly reducing.

As a result of this thermodynamic uphill struggle, the obtained metal has a strong driving force to return to its native, low-energy oxidized state.

Such an inevitable comeback (corrosion) is governed by spontaneous reactions with the environment, involving the oxidation of a metal and the corresponding reduction of another material

+-- +

ee ee e

OxidationOxidation ((anodicanodic reactionreaction))

4Cu → 4Cu+ + 4e-

Reduction (Reduction (cathodiccathodic reaction)reaction)

O2 + 2HH22OO + 4e- → 4OH-

4Cu+ + 4OH- → 2CuCu22OO + 2HH22OO


Corrosion

W. von Baeckmann, W. Schwenk, W. Prinz (eds): Handbook of Cathodic Protection (Gulf, Houston 1997)

Corrosion = “physicochemical interaction between a metal and its environment that results in changes in the properties of the metal, and which may lead to significant impairment of the function of the metal, the environment, or the technical system, of which these form part”(ISO 8044)

In most cases the interaction between the metal and the environment is an electrochemical reaction where thermodynamic and kinetic considerations apply.

From a thermodynamic point of view the driving force as in any electrochemical reaction is a potential differencebetween anodes and cathodes in a short-circuited cell.

Depending on the characteristics of the corrosion system various types of corrosion occur.


Corrosion

The corrosion process generally produce effects which could be detrimental and may lead to:• loss of material

• contamination of the environment with corrosion products• damage of a technical system

Numerous concepts have been applied to classify corrosion into categories. Due to the complex interaction between a material and its environment different classification schemes have developed to classify corrosion by the type of attack, the rate of attack, the morphology of the attack, or the properties of the environment, which may change in the ongoing corrosion process.

Corrosion damage

Corrosion failure


The vast possibilities of interaction between distinct metals and types of environment give rise to unlimited combinations of corrosion forms, making gathering occurrences following traditional classifications useless for conservation purpouses. Instead of this, and because the exchange with its surroundings is so intensive, metallic heritage can be better understood and conserved when its context is considered.

buried

outdoors

Corrosion

Archaeological

Artifacts

Monuments

Historic

Artifacts

museums


Corrosion

Archaeological artifacts have been buried for very long periods of time (centuries or even millenaries): after such long periods, they are deeply modified, consisting of mostly a mixture of metallic remnants and mineral products, which sometimes hinders their identification.

When archaeological artifacts are recovered from the ground, theequilibrium state reached during burial may be broken, which is sometimes quite dangerous because the object can become subjected to rapid and irreversible changes unless certain precautions are taken.

Even though some of the classical corrosion forms can be also found in archaeological finds, they are frequently configured in a very complex form (either from a morphological, structural, or chemical viewpoint).

Corrosion products bear a large piece of information about the artifact’s “life” and should not be inadvertently removed, contrarily to modern metals, which would simply be etched. It is important to keep most of the mineralized layer because of the information it contains (it is sometimes all that is left from the original object). On the other hand, it is quite necessary to remove harmful contaminants, like chlorides, because they are often confined at the interface between the remnant metal core and the mineralized layers, threatening the stability of the artifact.

Archaeological

Artifacts


Corrosion

Monuments are characterised by a considerable range of metals and finishes, sizes, and functions. The outdoor location demands treatment that will withstand wind, weather, traffic vibrations, and atmospheric pollution. Accessibility to public touch and vandalism are a threaten too.Although weathering is an expected evolution corresponding to a natural aging, the increasing air and water pollution following modern industrialization, and the development of transportation in urban areas have had copious consequences on the durability of the objects exposed to the outdoors. In addition to the intrinsic properties of the metallic substrate, the kinetics of the weathering mechanisms strongly depend on the relative humidity and the aggressiveness of the environment. Conservation problems in this area usually fall into two basic categories:

• “skin” -> problems of surface deterioration Surface deterioration regarding copper alloys is by far the most reported case in the literature. Such alloys are generally covered with a more or less stable conversion layer (the patina) mostly originally made for aesthetic purposes. The progressive wearing down of such layers by weathering and pollution inexorably leads to the deterioration of the sculpture. The problem is even worse with gilded sculptures, because the development of local galvanic piles accelerates the formation of corrosion products, which disturb the cohesion between the metallic substrate and the gold foil.

• “bone” -> problems of structural deteriorationA complex structure consisting primarily of the armatures (made of ferrous alloys to give mechanical stability) but also in an assemblage of different materials (nonferrous joining parts, as well as sand from casting, and other types of fillings) is generally present. The formation of a microclimate inside the sculpture leads to accelerated corrosion of its ferrous parts, which is even more severe at the points of contact with the more noble cuprous skin (owing to their bigger volume) generate strains and create an oxidizing environment, which are both very harmful to the monument.

.

Monuments


Corrosion

Historic artifacts includes collections of a most different provenance (such as scientific instruments, fine arts, historic pieces, ethnographic specimens, etc.) which are usually kept in museums.

Contrary to the belief that an object is safe once it enters a museum, certain storage or display conditions may lead to corrosive reactions that are different from those found in the natural environment. Some of these dangers come from “off-gassing” from materials used to build display cases and rooms, as well as air pollution introduced by visitors.The most common gaseous pollutants released by construction materials, are NOx, SO2, H2S, O3, HCOOH, CH3COOH and HCHO, which potentially interact with metal alloys, either directly or as catalytic reagents. The presence of moisture also plays a key role, resulting in the formation of a thin condensation layer on the metallic surface that acts as an electrolyte. This provides a direct way for pollutants to reach the surface, as well as to metallic ions that may move far from it, thus accelerating the corrosion process. Moreover, particulate matter deposited on the object surfaces allows the entrapment of water vapor, which could enhance its deterioration.

For all the above reasons, the concern about environment characterization and monitoring is expanding and a new discipline (preventive conservation) has emerged. Its goal is to adapt the environment to materials constituting artefacts, thus minimizing further deterioration..

Historic

Artifacts


CorrosionCopper and copper alloys (Brass and Bronze)


CorrosionIron and Iron alloys (Wrought and Cast)


CorrosionLead and Lead alloys (Solder)

Tin-plate and Tin alloys (Pewter)


Object may be corroded but stable. Very few shiny metal artifacts will remain in that condition. Only metals such as gold and platinum will retain a fully metallic or polished surface for a long time. Inactive corrosion occurs as a stable oxide layer (a tarnish or colour change that slowly forms on metal artifacts and protects the underlying surface). The oxide layer is often considered to be a desirable patina, particularly if it has a pleasing appearance. Artificial patinas are often applied to the surface of a metal object to protect it and change its appearance. Artificially patinatedsurfaces on artifacts are found mostly on sculptures, medals, weapons, and tools.

Corrosion

Metal Corrosion

inactive

Object may be actively corroding. An important part of preventive maintenance of metals is to recognize the early stages of destructive corrosion. Active corrosion causes a continuing loss of material from the object. Action must be taken to slow down or prevent further deterioration. Examining a collection to identify corroding metal artifacts can reveal problems with environmental conditions: high relative humidity (RH) or pollutants can initiate many of the corrosion reactions.

active


According to the International Standards Organization (ISO) standard ISO 8044 a corrosion investigation includes corrosion tests and their evaluation and is directed towards

the following objectives:• the explanation of corrosion reactions,• obtaining knowledge on corrosion behavior of materials under corrosion load,• selecting measures for corrosion protection.

Although the bulk properties of the solid and the solution may play an important role for the degradation of the materials, the properties of the metal surface and the reactions at the solid/electrolyte interface are most important. Therefore corrosion science and corrosion engineering apply the concepts of electrochemistry and electrode kinetics.

Corrosion involves electrode processes which are ruled by: • the electrode potential and thermodynamic • kinetic factors.

The equilibria and the related thermodynamic properties of the metal/electrolyte interface and their electrode kinetics are the driving forces.

Since electrochemistry was recognized many years ago as the basis for corrosion, a number of electrochemical techniques have been developed specifically for corrosion measurement. These are generally referred to as "DC Techniques". Among these techniques are Polarization Resistance, Tafel Plots, Potentiodynamic Plots, Cyclic Polarization... they are all very similar.

Corrosion scientists also make extensive use of Electrochemical Impedance Spectroscopy (EIS). Indeed, corrosion science was the first major scientific discipline to embrace EIS in the early 80s and is largely responsible for the popularity of this powerful technique.

Electrochemistry


Whether artifacts are ancient or modern, the principle underlying the corrosion of metallic artifacts is basically the same: an electrochemical reaction occurring between the metal and its environment. Indeed, the leading factor is the presence of a thin film of moisture on the metallic surface that may arise from rain, mist, or condensation owing to a high relative humidity.

Depending on the environmental conditions, such an invisible film may remain on the surface for a while, providing an ideal way for air pollutants to dissolve and reach the metallic surface.

The rate and severity of the attack is usually determined by the conductivity of the electrolyte, which depends upon the level of dissolved contaminants.

Furthermore, in the case of high relative humidity, dust or grit particles easily adhere to metal surfaces, generating local variations of oxygen partial pressure and hence a potential difference.

- - - -

++ + +

Charge flow (Charge flow (CurrentCurrent ))

Ene

rgy

leve

l ( P

oten

tial

Pot

entia

l)

Electrochemistry


It is the kind of process which determines whether an electrode acts as an anode or a cathode. Depending on the applied

electrode potential and further electrochemical conditions an electrode may be either an anode or a cathode.

If anodic and cathodic currents of a redox or a metal/metal-ion electrode compensate each other the electrochemical

equilibrium is established at the equilibrium potential E0.

If two different electrode processes compensate each other, such as an anodic Fe dissolution and cathodic hydrogen evolution with vanishing current in the external circuit, the

electrode is at its rest potential ER.

Electrochemistry

+

--+

ee

ee e

involves the transfer of positive charge from the electrode to the electrolyte (e.g. Fe dissolution to Fe2+)

Orthe transfer of negative charge in the opposite direction (e.g. the oxidation of Fe2+ to Fe3+)

positive current

involves the transfer of positive charge from the electrolyte to the electrode (e.g. the deposision as Fe metal from the Fe2+ in the solution)

Orthe transfer of negative charge in the opposite direction (e.g. the reduction of Fe3+ to Fe2+)

negative current

Anodicprocess

Catodicprocess


Electrochemistry

An electrochemical cell usually consists of two electrodes.

Potential drops at the electrode/electrolyte interface cannot be measured separately in principle: the contact of a voltmeter to the

electrolyte introduces automatically a second interface so that the measured

voltage necessarily includes the potential drop of two electrodes.

The potentials of any electrode is therefore given relative to the SHE. The electrode

potential of the standard hydrogen electrode is thus set to 0V by definition.


Electrochemistry

Corrosion normally occurs at a rate determined by an equilibrium between

opposing electrochemical reactions. The first is the anodic reaction, in which a metal is

oxidized, releasing electrons into the metal. The other is the cathodic reaction, in which a solution species (often O2 or H+) is reduced,

removing electrons from the metal. When these two reactions are in equilibrium, the

flow of electrons from each reaction is balanced, and no net electron flow (electrical current) occurs. The two reactions can take

place on one metal or on two dissimilar metals (or metal sites) that are electrically

connected.

The equilibrium potential assumed by the metal in the absence of electrical

connections to the metal is called the Open Circuit Potential, Eoc. In most

electrochemical corrosion experiments, the first step is the measurement of Eoc. The

terms Eoc (Open Circuit Potential) and Ecorr(Corrosion Potential) are usually

interchangeable, but Eoc is preferred.


Electrochemistry

Icorr cannot be measured directly. One may estimate it from current versus voltage data. The voltage scan is centered on Eoc. One then fit the measured data to a theoretical model of the corrosion process.

If one assumes that the rates of both the anodic and cathodic processes are controlled by the kinetics of the electron transfer reaction at the metal surface the Tafel Equation is obtained :

Which describes the current I resulting from one isolated reaction as a function of a reaction dependent constant called the Exchange Current I0, the electrode potential E, the equilibrium potential E0 (constant for a given reaction) and the reaction's Tafel Constant β (constant for a given reaction, in volts/decade)

I = I0 exp(2.3(E-E0)/β)


Electrochemistry

The Tafel equations for both the anodic and cathodic reactions in a corrosion system can be combined to generate the Butler-Volmer Equation :

I = Ia + Ic = Icorr (exp(2.3(E-E0C)/βA)-exp(2.3(E-E0C)/βC))

Which describes the measured cell current I [ A] as a function of the corrosion current Current Icorr [A], the electrode potential E, the corrosion potential E0C [V], the anodic and cathodic Tafel Constant βA and βA

respectively [V/decade]

Near E0C, the current versus voltage curve approximates a straight line. The slope of this line has the units of resistance (ohms). The slope is, therefore, called the Polarization Resistance, Rp. If we approximate the exponential terms in Butler-Volmer Equation with the first two terms of a power series expansion and simplify, we get one form of the Stern-Geary Equation :

Icorr = βAβC/ (2.3RP(βA +βC))


Electrochemistry

There can be complications, such as:

1. Concentration polarization or "diffusion controlled" (the rate of a reaction is controlled by the rate at which reactants arrive at the metal surface). Often cathodic reactions show concentration polarization at higher currents, when diffusion ofoxygen or hydrogen ion is not fast enough to sustain the kinetically controlled rate.

2. Oxide formation, which may or may not lead to passivation , can alter the surface of the sample being tested. The original surface and the altered surface may have different values for the constants βa , βc.

3. Effects that alter the surface, such as preferential dissolution of one alloy component,

4. A mixed control process where more than one cathodic, or anodic, reaction occurs simultaneously (i.e. the simultaneous reduction of oxygen and hydrogen ion).

5. Finally, potential drop as a result of cell current flowing through the resistance of your cell solution causes errors in the kinetic model. This last effect, if it is not too severe, may becorrectable via IR compensation in the potentiostat.


Electrochemical techniques

Electrochemical measurements are performed in an electrochemical cell, which contains a three-electrode arrangement• working electrode (WE) • counter-electrode (CE) • reference electrode(RE)

For most experiments the electrode potential is fixed by an electronic potentiostat and the current is measured.

Electrochemical cell

Electrolytes• Diluite Harrison Solution: 0.35wt% (NH4)2SO4 + 0.05 wt% NaCl in H2O (in distilled water)• Sodium Cloride: NaCl 0.1M• Sintetic Rain• Mineral water



Potential Measurements

DC techniques

AC techniques


Despite similar general rules regulating events occurring at the metal-environment interface, some particularities are evident when approaching the corrosion of metallic antiquities. One of them is the specificity of the object itself, which is essentially chemically and structurally heterogeneous due to ancient production techniques and treatments, as described from old texts that are not always very precise. On the other side, there is the context of where the object is kept, and the environment with which it exchanges continuously. Here there are also some singularities, arising from both the location (burial, museum, outdoors) and the duration of exposure (few days for an exhibition, to centuries and millenaries for archeological finds).

Perhaps the most peculiar factor to be considered when approaching a cultural artifact, however, is its uniqueness. This actuality makes its conservation a challenge—an attractive and interesting case raising numerous difficulties—because the artifact is often the only witness of the past and cannot be sampled to perform all the adequate characterization that would be necessary to understand traditional materials and disclose ancient technologies. Furthermore the alteration compounds (corrosion products) that are considered part of the artifact because they are a testimony of its past, should not be taken away but instead should be studied and conserved in place whenever possible.





At the Open Circuit Potential Eoc, the system is at steady state:

Total current I = Anodic current Ia + Catodic current Ic = 0

Several oxidation and reduction processes can occur simultaneously, and Eoc reflects all the half reactions (anodic (Ia>0) and cathodic (Ic<0)) taking place at the same time on the metal without an external current.Eoc is measured with a multimeter vs a reference electrode (RE).


The corrosion potential is determined by: • the chemically active compounds (chemical potential

and concentration)• the number of electrons involved in the redox

reaction• the temperature.

In the case of steel without incoherent corrosion productssteel without incoherent corrosion productssteel without incoherent corrosion productssteel without incoherent corrosion products,

Pourbaix showed the free corrosion potential EEEEoc (in mVSCE) is

proportional to the instantaneous corrosion rate vvvvc (in µm/year) :



log vc = 1.183 – 0.004416 Eoc


Cicileo and co-workers had tested the Eoc measurement on some

bronze statues in “La Recoleta” cemetery in Buenos Aires .

The portable arrangement used for in situ potential measurementsconsists of:

• a glass tube with aaaa reference saturated calomel electrodereference saturated calomel electrodereference saturated calomel electrodereference saturated calomel electrode SCE (Esce = 242 mV/H) in one of its entrances;

• the other entrance is obtruded with a cotton piecea cotton piecea cotton piecea cotton piece, which becomes damp with distilled waterdamp with distilled waterdamp with distilled waterdamp with distilled water and is applied to the zone whose potential is to be measured.

•A testerA testerA testerA tester connected the reference electrode to a small bare area of the structure for potential measurements.




Table 1. Three series of in situ potential measurements (Esce (mV)) with 2 years'intervals on the El Karma sculpture.

Zone Color Year E (mV) 30 s E (mV) 60 s E (mV) 90 s2000 208 213 2152002 160 162 162

Ear interior Black

2004 165 164 164

2000 175 174 1722002 163 162 162

Forehead Grayishgreen

2004 77 83 912000 165 160 1562002 145 143 141

Chin Green

2004 159 155 1532000 153 150 1472002 165 164 163

Left hand palm Black

2004 182 179 179

Right hand (Alrivet)

White 2000 020 009 005

2000 164 158 1562002 107 100 90

Left thigh Yellowishgreen

2004 24 24 37

Crespo M., Cicileo G., Rosales B., in: J. Ashton, D. Hallam (Eds.), Proceedings of Metal04, National Museum of Australia, 2004, pp. 185-194

An increase in Eoc or a stable potential with time is suggested to characterise protective patinas. Unstable potentials or its decrease with time is ascribed to patinas with

no protecting ability. A maximum30mV variation in Eoc on a two years time is indicated for the bronze patina to be considered as stable

Caution is suggested in the evaluation of patina protectiveness by electrochemical methods because of heterogeneous alloying elements

distribution (Corr.Sci. 46 (2004), 929-953)

Cicileo et al, Corr.Sci 46 (2004), p. 931






Degrigny has proposed the use of Ecorr.vs.time plots for the qualitative analysis of copper-based elements from scientific and technical objects

http://www.lorentzcenter.nl/lc/web/2010/364/presentations/Degrigny_SPAMT-Test.pdf



DC techniques

In a Polarization Resistance experiment, you record a current versus voltage curve as the cell voltage is swept over a small range of potential that is very near to Eoc (generally ± 10 mV). A numerical fit of the curve yields a value for the Polarization Resistance, Rp. In order to evaluate the corrosion rate from Polarization Resistance data values for the Beta coefficients are needed.

Rp has been applied by ICR and others in some campaigns on bronzes in outdoor exposure, as the Marcus Aurelius monument in Rome and Perseo and Medusa in Florence, the monument “Ai Mille” near the sea side in Genoa, as well as on archaeological finds from the sea, as the Warriors of Riace and the Dancing Satyr from Mazara del Vallo.This procedure has been adapted to the study and control of ancient metals from the industrial and engineering fields, in which this methodology is largely applied.



DC techniques

Polarization Resistance measurements on bronze statues


The dying horse

Rp measurements before the restoration treatment showed a very high corrosion rate. After the restoration, the sculpture has been treated with a double layer protective system: the inner one based on synthetic resins while the outer one is a microcrystalline wax used as a “sacrificial layer”, which should be controlled and renewed at regular intervals. Rpmeasurements enlightened a sharp decrease in the corrosion rate after that treatment.

G.D’Ercoli et al, ICR - http://www.icr.arti.beniculturali.it/Restauri/CavalloRai/Rai13122000.htm


DC techniques


Monumento ai Mille

A monitoring by Rp measurements has been performed by ICR on 6 areas of the monument. For each area, measurements have been performed on untreated/cleaned/cleaned and coated with 6 different protective systems zones. After 2.5 years the overall best performance in the aggressive urban-marine environments is provided by the double layer system Incralac+Soter.

D’Ercoli G., Marabelli M., in “Monumenti in bronzo all'aperto. Esperienze di conservazione a confronto”, Nardini, Firenze, 2004, pp. 113-117


DC techniques


Guggenheim collection in Venice

Angelucci S., Bartuli C., in “Monumenti in bronzo all'aperto. Esperienze di conservazione a confronto”, Nardini, Firenze, 2004, pp. 195-200

Marino Marini, The Angel of the City (1948). A natural compact patina is present, with the exception of the sections more exposed to the continuous touching of visitors (horse muzzle and tail, man’s penis). The average corrosion rate is not very high, apart in the area

more exposed to the rain washing and the touching.

0.23

0.14

0.05

0.08

0.03

0.13

0.07

0.03

0.35

2.78

1.02

2.99

0.11

0.49

0.08

0.35

0.18

1.22

52

57

62

89

48

63

60

65

25

59

59

56

144

41

46

58

70

25

muzzleheadback

buttockstail

man's abdomenright leg

left legpenis

muzzleheadback

buttockstail

man's abdomenright leg

left legpenis

01234 0 50 100 150 200

Marino Marini, The Angel of the City, 1948

Corrosion rate (µm/y) Thickness (µm)

(a)

(b)

(a) new protective coating (b) 2 years old protective coating

The alloy composition has a leading influence on the corrosion rate, even though the conservation history my play even a more relevant role. Different part of the same statue can be characterised by very different corrosion rate, according to the exposition and the patina present.The statues coated with the double layer acrylic+wax show very low corrosion rates just after treatment. The coating barrier effect persist on the surfaces less exposed to aggressive weathering effect, while it is completely removed after two years on the other surfaces.


DC techniques


Voltammetry is based on the record of the current I flowing across an electrochemical cell under the application of a given time-depending potential E. A voltage is applied between a working electrode WE and an auxiliary electrode CE, and the potential of the working electrode is controlled versus the reference electrode REF. The current flowing through. the working and auxiliary electrodes is the response to the potential of the working electrode—i.e., the excitation signal.

A.Doménech-Carbò, M.T.Doménech-Carbò, V.Costa (2009) Electrochemical Methods in Archaeometry, Conservation and Restoration, Springer-Verlag, Berlin Heidelberg, Chapter 2 Identification of Species by Electrochemical Methods, pp 33-24


DC techniques

http://www.lorentzcenter.nl/lc/web/2010/364/presentations/Domenech.pdf


The fundamental approach of all impedance methods is to apply a small amplitude sinusoidal excitation signal to the system under investigation and measure the response.

Theoretical Electrochemical systems are usually characterised by non-linear I-E curve .

A low amplitude sine wave ∆Esin(ωt), of a particular frequency, is superimposed on the dc polarization voltage E0. This results in a current response of a sine wave ∆Isin(ωt+φ) superimposed on the dc current I0. The current response is shifted with respect to the applied potential.


AC techniques


o Perturbation: E(t)=E0 cos(ωωωω t)

o System response: I(t)=I0 cos(ωωωω t-φ φ φ φ )

o Response function: Z (ω ω ω ω ) = E/I

Zre

Zim

ω→0ω→∞

|Z|

Φ

Nyquist Plot

|Z|

101

102

103

104

105

frequency [Hz]10-2 10-1 100 101 102 103 104 105

Φ

-90

-60

-30

0

Bode Plot


AC techniques

Basic of Electrochemical Impedance Spectroscopy EIS


Zre

Zim

ω→0ω→∞

|Z|

Φ

Nyquist Plot

|Z|

101

102

103

104

105

frequency [Hz]10-2 10-1 100 101 102 103 104 105

Φ

-90

-60

-30

0

Bode Plot


AC techniques

Basic of Electrochemical Impedance Spectroscopy EIS

The absolute value of impedance (|Z|=Z0… and the phase shifts are plotted on the Y-axis with log frequency on the X-axis in two different plots giving a Bode plotBode plotBode plotBode plot. This is the more complete way of presenting the data.

Z(Z(Z(Z(ωωωω………… is a complex quantitycomplex quantitycomplex quantitycomplex quantity with a magnitude and a phase shift which depends on the frequency of the signal. Therefore by varying the frequency of the applied signal one can get the impedance of the system as a function of frequency. Typically in electrochemistry, a frequency range of 100kHz 0.1Hz is used.

The plot of the real part of impedance on the X-axis and the imaginary part on the Y-axis gives a Nyquist PlotNyquist PlotNyquist PlotNyquist Plot. While plotting data in the Nyquist format the real axis must be equal to the imaginary axis so as not to distort the shape of the curve. The advantage of Nyquist representation is that it gives a quick overview of the data and by the shape of the curve one can make some qualitative interpretations of the data. The disadvantage is that one loses the frequency dimension of the data.


Commercially available instrumentsWE Working Electrode: the electrode under investigation

CE Counter Electrode:the electrode necessary to close the electrical circuit

REF Reference Electrode:the electrode used to determine the potential of the working electrode precisely

Electrolyte:It ensures electrical conductivity between WE and CE

Potentiostat: it is used to fix the DC value and to apply a sinusoidal perturbation over it

Frequency Response Analyzer (FRA…: controls the EIS scan and impedance evaluation

Acquisition Software: controls Potentiostat and FRA, and store data file

E ∼I

CEWE

Electrolyte

Potentiostat

FRA

REF

PersonalComputer

Electrochemical cell and electrolyte


AC techniques


EIS spectrum can provide EIS spectrum can provide

information about information about

electrochemical processes...electrochemical processes...

……and interface and interface

propertiesproperties

C.Gabrielli “Identification of Electrochemical Processes by Frequency Response Analysis”, (1998)

http://accessimpedance.iusi.bas.bg/vlab2/e-school/tech04.pdf

A.Amirudin, D.Thierry “Application of electrochemical impedance spectroscopy to study the degradation of polymer-coated metals”,

Prog. Org. Coat. 26 (1995) 1-28


AC techniques


Equivalent circuits: a good fitting does not mean you choosed a good model!


AC techniques

Gamry Instruments (2007) “Basics of electrochemical impedance spectroscopy”http://www.gamry.com/App_Notes/EIS_Primer/EIS_Primer_2007.pdf


Intact coating

10-3 10-2 10-1 100 101 102 103 104 105

-90-80-70-60-50-40-30-20-10

0

100101102103104105106107108109

1010

(b)

Pha

se A

ngle

φ

Frequency / Hz

(a)

Impe

dand

e M

odul

us |Z

| / Ω

cm

2

20 40 60 80 100

-20

-40

-60

-80

-100

-Z"

/ MΩ

cm

2

Z' / MΩ cm2


AC techniques


a contact probe setup measurement method has been developed to make

measurements on art object also

CE

REF

Cloth soaked with mineral water

Contact probe

WE


AC techniques


Cast BronzeCast Bronze(Cu 90 -Sn 8 -Pb 2)

Coupon 3x3 cm and 6x6

Polished with metallographic papers Polished with metallographic papers (up to grit 1200)Degreased in methanol Degreased in methanol (ultrasonic bath)

Rinsed with deRinsed with de--ionised waterionised waterDried in airDried in air

Comparative study of Protective Coating Systems for OUTDOOR bronze Sculpture

Bronze MonumentBronze MonumentThe more abundant corrosion products identified are

Cuprite, Atacamite, Paratacamite and Mushistonite, with Nantokite, Gypsum and Quartz in some places

zones 6x4.5 cm

Cleaned by rotating brushCleaned by rotating brushWashed with water and acetone Washed with water and acetone

Dried in airDried in air

[C][C]

[B][B] [D][D]

[A][A] [F][F]

[E][E]

[A] [A] -- SoterSoter201LC201LC

[B] [B] -- R21R21

[C][C] -- TrommTrommTeCeTeCe3534F 3534F

[D][D] -- IncralacIncralac

[E] [E] -- Incralac+SoterIncralac+Soter

[F] [F] -- Incralac+R21Incralac+R21


AC techniques


Comparative study of Protective Coating Systems for OUTDOOR bronze Sculpture

[A]

texposure [months]

0 3 6 9 12 15 18 21 24

105

106

107

108

109

1010CM - Area III

[B]

texposure [months]

|Z| ν

=10m

Hz [Ω

cm2 ]

105

106

107

108

109

1010CM - Area III

[C]

texposure [months]

0 3 6 9 12 15 18 21 24

105

106

107

108

109

1010CM - Area III

[D]0 3 6 9 12 15 18 21 24

105

106

107

108

109

1010C

M - Area III

[E] 105

106

107

108

109

1010CM - Area III

[F] 105

106

107

108

109

1010CM - Area III

[F]

frequency [Hz]10-2 10-1 100 101 102 103 104 105

|Z| [

Ωcm

2 ]

103

104

105

106

107

108

Coupons 11m Coupons 23m Monument Area III - 11m Monument Area III - 23m

Letardi P., in: J. Ashton, D. Hallam (Eds.), Proceedings of Metal04, National Museum of Australia, 2004, pp. 379-387

The results obtained show that the coating [D] offer no advantage, after only 1 year, with respect to the waxy coatings [A], [B] and [C], which have the advantage of a better accepted aesthetic appearance and are known to give less problem concerning reversibility and re-coating. The wax [B], both alone and in the double layer system [F], is the product showing the more different performances on coupons and on the monument area III. The different trends observed for the selected coatings on the polished bronze coupons and on the monument area III should be considered in more detail. The patina, which is present between the metal and the coatings applied on the monument, is supposed to be the main source for the differences observed on coating behavior with respect to the polished bronze coupons


AC techniques


Bode Plot

|Z| [

Ω]

101

102

103

104

105

frequency [Hz]10 -2 10 -1 100 101 102 103 104 105

ΦΦ ΦΦ [d

egre

e]

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

C2

C1

A2

A1

B2

B1

Nyquist Plot

Z re [Ω ]0 5000 10000 15000 20000

Zim

[Ω]

-10000

-5000

0

Letardi P., in: F. Simonetti (Ed.), Il Cristo degli Abissi – 50 anni di storia, Tormena 2004, pp. 109-113

Cristo degli AbissiCristo degli Abissi ((G.GallettiG.Galletti , 1954), 1954)BaiaBaia didi S.FruttuosoS.Fruttuoso (GE)(GE)


AC techniques


Monumento aMonumento a BartolomeoBartolomeo ColleoniColleoni ((A.VerrocchioA.Verrocchio , 1496), 1496)VeneziaVenezia , Campo , Campo SantiSanti Giovanni e PaoloGiovanni e Paolo


AC techniques

P.LetardiL'elettrochimica al servizio della conservazione: l'applicazione sul monumento a Bartolomeo Colleoni del Verrocchio

In Il CNR e le strategie per la conservazione del Patrimonio Culturale, M.Matteini (ed.), CNR, Roma, pp 161-166, (2007)


EIS on the Monument to Bartolomeo Colleoni (Andrea del Verrocchio, 1496)

AArreeaa PPhhoottoo DDeessccrriipptt iioonn 11

Horse; neck right side. Rain washed area. Grey-green patina

22

Horse; back. Area hit by rain. Green patina.

33

Horse; left thigh. Almost sheltered area. Black patina.

44

Horseman; left shoulder. Rain washed area. Green patina

55

Horseman; left leg. Sheltered area. Black patina.

During the pre restoration work, five homogeneous Areas have been identified.

In each Area three 5x5 cm zones have been delimited:

-one (A) has been left untouched;

-one (B) has been washed;

-one (C) has been washed and coated with a double layer

system Incralac+Wax.


AC techniques


C1C1C1C1

|Z| [

Ω]

102

103

104

105

106

107

108

A

B

C

frequency [Hz]10-2 10-1 100 101 102 103 104 105

ΦΦ ΦΦ [

deg

ree]

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

C2C2C2C2

A

B

C

frequency [Hz]10-2 10-1 100 101 102 103 104 105

C3C3C3C3

A

B

C

frequency [Hz]10-2 10-1 100 101 102 103 104 105

C4C4C4C4A

B

C

frequency [Hz]10-2 10-1 100 101 102 103 104 105

C5C5C5C5

|Z| [

Ω]

102

103

104

105

106

107

108

A

B

C

frequency [Hz]10-2 10-1 100 101 102 103 104 105

ΦΦ ΦΦ [

deg

ree]

-90

-80

-70

-60

-50

-40

-30

-20

-10

0


For zones (A)

5A>3A>2A>4A>1AFor zones (B)

5B>(3B≈ 2B)>(4B≈ 1B)For zones (C)

3C>4C>(2C≈ 1C≈ 5C)

untreated

washed

washed + Incralac + wax

Area

1 2 3 4 5

|Z| ν

=10mHz [

Ω]

104

105

106

107

108

untreatedwashedwashed + coated

Area 1 Area 2 Area 3 Area 4 Area 5


AC techniques


Area 1Area 1Area 1Area 1

|Z| [

Ω]

102

103

104

105

1B (0.5 h)1B II (1.5 h)1B III (11 h)

frequency [Hz]10-2 10-1 100 101 102 103 104 105

ΦΦ ΦΦ [d

egr

ee]

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Zre [Ω]0,0 2,5x103 5,0x103 7,5x103

Zim

[ Ω]

-2,5x103

0,0

0 250x100 500x100 750x100Z

im [ Ω

]

-250x100

0


|Z| [

Ω]

102

103

104

105

3B (0.5 h)3B II (1.5 h)3B III (10.5 h)

frequency [Hz]10-2 10-1 100 101 102 103 104 105

ΦΦ ΦΦ [

deg

ree]

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Zre [Ω]0 10x103 20x103

Zim

[ Ω]

-10x103

0

0 1x103 2x103

Zim

[ Ω]

-1x103

0



|Z| [

Ω]

102

103

104

105

5B (0.5 h)5B II (1.6 h)5B III (11 h)

frequency [Hz]10-2 10-1 100 101 102 103 104 105

ΦΦ ΦΦ [

deg

ree]

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Zre [Ω]0 25x103 50x103 75x103

Zim

[ Ω]-25x103

0

0,0 2,5x103 5,0x103 7,5x103

Zim

[ Ω]

-2,5x103

0,0

Black patina

Black

patina

Green-greypatina

Evolution with time on selected patinas


AC techniques


Coating

none T9 T6 T5 T3b T3c T3d T1 T2 T4

none T9 T6 T5 T3b T3c T3d T1 T2 T4

|Z|10

mH

z [M

Ω]

10-3

10-2

10-1

100

101

102

103

Sceptre

Coupons

Vcorr increase

Vcorr decrease

IncralacIncralac

Tromm TeCeTromm TeCe3534F3534F

Negligible effect

Better protection

Coatings Comparison on natural patinas

EUEU--ARTECH / JR1ARTECH / JR1 -- evaluation of new treatments for outdoor bronze monumentsevaluation of new treatments for outdoor bronze monuments


AC techniques


[C][C]

[B][B] [D][D]

[A][A] [F][F]

[E][E]

[A] [A] -- SoterSoter201LC201LC

[B] [B] -- R21R21

[C][C] -- TrommTrommTeCeTeCe3534F 3534F

[D][D] -- IncralacIncralac

[E] [E] -- Incralac+SoterIncralac+Soter

[F] [F] -- Incralac+R21Incralac+R21

Test on monuments could be prone to several limitations:accessibilityoutdoor measurementsvariables out of controlauthorisations


AC techniques


Test on monuments could be prone to several limitations:

previous restoration treatments are also to be considered carefully


AC techniques


UN MN UA

◊ Natural ◊ 80 years urban◊ 20 µm◊ brochantite|Z|10mHz = 69 ± 18 KΩΩΩΩ

◊ Natural ◊ 1 year marine ◊ 30 µm◊ Atacamite |Z|10mHz = 5 ± 2 KΩΩΩΩ

◊ Artificial ◊ Pichler process◊ 80 µm◊ brochantite|Z|10mHz = 29 ± 4 KΩΩΩΩUrban artificial UA Substrate

Treatment

none Tref T1S T2S T3S

|Z| 10

mH

z [K

Ω]

100

101

102

103

104

105

UA substrate

Urban Natural UN Substrate

Treatment

none Tref T1S T2S T3S I1S Iref

|Z| 10

mH

z [K

Ω]

100

101

102

103

104

105

UN substrate

UN substate (1)

Marine natural MN Substrate

Treatment

none Tref T1S T2S T3S I1S Iref

|Z| 10

mH

z [K

Ω]

100

101

102

103

104

105

MN substrate

Test of coatings for

cultural heritage:

Patina plays a relevant role on

coating behaviour

• composition• thickness• texture• porosity• …


AC techniques


Test of coatings for Test of coatings for

cultural heritagecultural heritage

Because of the large number of variables involved, one need several coupons in order

to check the behaviourStatistical methods such as Experimental Design could be of great help in order to

reduce the number of experiments required

http://www.itl.nist.gov/div898/handbook/pri/section3/pri3.htmhttp://www.chemometrics.se/images/stories/pdf/aug2002.pdf

J. A. Bishopp, M. J. Parker and T. A. O Reilly, The use of statistical experimental design procedures in the development and robust manufacture of a water-based, corrosion-inhibiting adhesive primer, International Journal of Adhesion and Adhesives 21212121,(2001…, 473-480

M.J. Anderson, P.J. Whitcomb, Design of experiments for coatings, in A.A. Tracton, Coatings Technology Handbook, Third Edition, 3rd ed., CRC Press, 2005


References

Cano E, Lafuente D, Bastidas DM (2009) Use of EIS for the evaluation of the protective properties of coatings for metallic cultural heritage: a review. Journal of Solid State Electrochemistry, 1-11.

Loveday D, Peterson P, Rodgers B (2004) Evaluation of organic coatings with electrochemical impedance spectroscopy: Part 1: Fundamentals of electrochemical impedance spectroscopy. JCT CoatingsTech 1, 46-52.

Loveday D, Peterson P, Rodgers B (2005) Evaluation of organic coatings with electrochemical impedance spectroscopy. JCT CoatingsTech2, 22-27.

Loveday D, Peterspm P, Rodgers B (2004) Evalution of organic coatings with electrochemical impedance spectroscopy part 2: Applicationof EIS to coatings. JCT CoatingsTech 1, 88-93.

Letardi P (2004) Laboratory and field test on patinas and protective coating systems for outdoor bronze monuments, in: Ashton J., HallamD.(Eds.), Proceedings of Metal2004, National Museum of Australia, Canberra, pp. 379-387

Christian Degrigny (2010) Use of electrochemical techniques for the conservation of metal artefacts: a review, Journal of Solid State Electrochemistry 14, 353-361

Antonio Doménech-Carbò, Marıa Teresa Doménech-Carbò, Virginia Costa (2009) Electrochemical Methods in Archaeometry, Conservation and Restoration, Springer-Verlag, Berlin Heidelberg

D. Watkinson (2010) Preservation of Metallic Cultural Heritage , Shreir's Corrosion Volume 4, Pages 3307-3340, Elsevier

Lyndsie Selwyn, Metals and Corrosion: A Handbook for the Conservation Professional (2004) Canadian Conservation Institute, Ottawa

Bart Ankersmit, Martina Griesser-Stermscheg, Lyndsie Selwyn, and Susanne Sutherland (2008) Rust never sleep – Recognizing metals and their corrosion products, Canadian Conservation Institute, http://www.cci-icc.gc.ca/crc/articles/metals/corrosion-eng.pdf

David A. Scott (2002) Copper and Bronze in Art - Corrosion, Colorants, Conservation, Getty Publications

P.Letardi,I.Trentin, G.Cutugno (a cura di) (2004) Monumenti in bronzo all’aperto – esperienze di conservazione, Ed. Nardini Firenze

Judy Logan, Lyndsie Selwyn (2007) Recognizing Active Corrosion, Canadian Conservation Institute CCI Notes N9/1 http://www.cci-icc.gc.ca/publications/ccinotes/enotes-pdf/9-1_e.pdf

P.Letardi (2005) Metodi Elettrochimici per la caratterizzazione e il monitoraggio dei Monumenti Bronzei, In Patrimonio Monumentale –Monitoraggio e conservazione programmata, (P.Croveri, O.Chiantore ed.), 36-42, Ed. Nardini Firenze,

corrosion evaluation and diagnostic: use of ... · corrosion scientists also make extensive use of...

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