siandaii corrosion protection lecture

8/22/2019 SIandAII Corrosion Protection Lecture

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Corrosion protection:short overview and summary

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Materials Aggressive medium

2) Extrinsic protection: Separation with the medium

1b) Modification of the medium

Materials Aggressive medium

Aggressive medium

1) Intrinsic active protection1a) Modification of the material

Materials

Corrosion protection: possible strategies

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Active corrosion protection: surface passivation !!!

Addition of Cr and Ni changes completely the passivation behavior of steeland allows to obtain a good corrosion protection.

Passivation is a good active corrosion protection method. Active protectionin this context refers to a protection present from the alloy side and availableat any time.

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Active corrosion protection of steel II

Currentdens

ity

Potential

Alloying elements are the best way of performing activecorrosion protection.

They have an influence on:

- Critical current density icrit(Cr,Mo)

- Passive current (Cr)

- Stability domain (Mo)

Alloys Icrit [A/cm2]

Ecorr

icrit

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Active corrosion protection of steel III

- Nickel is essentially playing a role and obtaining the FCC austeniticstructure which is much less sensitive to hydrogen embrittlement(hydrogen diffusion is hindered).

- The BCC structure of Fe-Cr ferritic stainless is as corrosionresistant as the austenitic one for the other type of corrosionmechanisms. It is only extremely susceptible to SCC.

- Nitrogen is also an element allowing to maintain the austeniticstructure and can allow to reduce the Ni content of an alloy. Nickelis very expensive, so it is an important factor.

- Nitrogen containing steel are prone to the formation of unstablenitrides when exposed to higher temperatures. These ones act aslocalized corrosion attack sites and decrease the corrosionresistance drastically (alloys difficult to weld)

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Limit of the active corrosion protection with alloying elements:

Aluminum alloy

Example of 2024T3:

Large intermetallics (useless) Dispersoids (strengthening)

Micrometer domain nanometer domain

SEM/ EDS:Particles 1 - 4: Al-Cu-(Fe, Mn)Particles A - C: Al-Cu-Mg

TEM/EDSAl2Cu 6


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Goal: modification of the environment in order to decrease

the corrosion reaction rates. This strategy is usedwhen nothing can be done to increase the intrinsiccorrosion resistance of the material:

1) Cathodic protection

2) Anodic protection

3) Inhibitors

Active corrosion protection: modification of the media

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i

V

Cathodic protection Corrosion Anodic protectionOnly passive metals

Protection with electrical current/ voltage generators:Rebars, Pipelines, water tubes, aso

Protection with sacrificial anodes: Zinc, Magnesium, Aluminum

Corrosion protection with external applied currents

Anodic protection only in chemical reactors for example. Therisk of localized corrosion is too high if the environment is not

very well know8


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Galvanic coupling can be used to protect structure

- For example: Mg, Zn anodes contacted to Steel

- Be careful to the electrolyte (soil) conductivity- Do not use cathodic protection on materials unstable in

the alkaline domain (For example Al alloys)

Cathodic protection with sacrificial anodes

Steeltank

Sacrificialanode

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Cathodic protection with current / potential generators

Pipe ortank

Power generator

Cable

Inert electrode

C, Pt, Pb

Pipe ortank

Soil

Surface

A controlled well defined current is applied between thestructure (pipe, tank) to protect and an inert electrodelocated in the ground near to the structure

- Used when the

efficiency of thesacrificial anodesare not guaranteed

- Main disadvantage of themethod: it is extremely expensivefor large structure

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Costs for thecorrosion

protection

Costs for thecathodic

protection

% unprotected metal

Costs for thecoating

Minimalcosts for

corrosionprotection

Cheap version: cathodic protection of coated surface

Two processes are extremely expensive:

- Provide large amount of current for cathodic protection

- A perfect, defect-free coating

95% coveragefor a coatingis easilyrealizable

The currentnecessaryis reducedby 20

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Use of corrosion inhibitors

Definition: a species that reduces the corrosion rate of amaterial when added to an aggressive electrolyte.They are classified in three categories:

1) Adsorption inhibitors:Substances that reduces the reaction rates through physical orchemical adsorption on the surface

1) Passivating substances:Substances that favors passivation by influencing the anodic partialreaction

1) Passivators:Substances that influence passivation by modification of the cathodicreaction

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Adsorption inhibitors A)

B)

C)

Used mainly against uniform corrosion inacidic media

In any case Icorr is reduced

A) Anodic inhibitorCorrosion potential increase

Ex: Dimethylchinoline

B) Cathodic inhibitorCorrosion potential decrease

Ex: Aethylamine

C) Mixed inhibitorCorrosion potential stays constant

Ex: amines13


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O2 reduction, passivator

Currentdensity

Passivating agents

Mechanism: decrease the anodic reaction to lower valuesthan the cathodic reaction rate

Example:

Species increasingthe pH, OH

-

Risk:

Avoid for materialsunstable in thealkaline domain

Ecorr,2 Ecorr, 1 Ecorr, 3

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Passivators

Mechanism: increase the cathodic reaction to a point thatstable passivation is reached

Example:

Cr2O72-, NO2-

Risk:

If the concentration isinsufficient, corrosionacceleration occurs

instead of inhibition

Current

density

Potential

Acceleration due to insufficientinhibitor amount

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Corrosion behavior ofnoble coatings

Nickel

Corrosion

Fe

Corrosion

Fe

Zinc

Metallic coating: Sacrificial coatings

Corrosion behavior ofactive coating

- Nickel coatings have to be extremely protective (withoutdefects) to act as corrosion protection for iron

- Zinc coating will just corrode instead of Iron16


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Metallic coating: use and problems- Due to the galvanic coupling occurring between the coating and

the underlaying substrate, use of metallic coating is limited tospecific field of application.

- Noble coatings like Ni only if the coating quality can be very goodor in absence of chloride.

- Noble coating containing Au, Ag, Pt, if the esthetic aspect of the

surface is the dominant factors (jewelry, watches).

A very important aspect related to electrochemical cathodic metaldeposition is that it is very often accompanied by cathodic hydrogenreduction. Substrate susceptible to hydrogen induced stress

corrosion cracking are not indicated for these processes.

- Sacrificial Zinc anodic coating is the most important application ofmetallic coatings !

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Pretreatment Degreasing, etching (possibly sandblasting) clean, roughening, better adhesion

Possible coating: Zinc, Aluminum, Tin, lead

Etching through molten metallic salt

Wetting, roughening, even better adhesion

Dipping process Base metal and molten coating material reacttogether Formation of an alloyed intermediate layerCoating thickness depends on immersion time,bath temperature and composition, dimensionand base materials composition

Post-treatment Remove excess coatingProtect with chromate or phosphate conversioncoating

Metallic coating: surface processing

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1 Mio t Zinc to protect 10000 t SteelSignificance

Coating structure

Influence of theSi content in the

Steel

Main danger:Liquid metalEmbrittlement

Metallic coating: Hot (dip) galvanized steel

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From the surface In a cross section

Pore

Inorganic coating: anodic oxide growth on Al alloys

The principal types of galvanostatic(applied current density) anodizingprocesses:

1) chromic acid

2) sulfuric acid

Produces a thick porous film (forpractical use, between 5 -20 m,

hardcore process 50 m )

The surface need to be sealed (close thepores) to eliminate the path between the

underlying metal and the environment.Usually slight acidified hot water is used

Such coating are serviceable in a pHrange between 4 to 8.5. 20


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Al 5XXX serie alloys : rolled sheets, thickness: 1.2mm

Alloy Full name Si* Fe* Cu* Mn* Mg* Zn* Fe:Si

2-B57s Alloy 2 (commercial) 0.11 0.24 0.020 0.021 0.85 0.004 2.1

1-Base Alloy 1 (Base) 0.26 0.13 0.001 0.001 0.50 0.005 0.5

3-Hi Cu Alloy 3 (Hi Cu) 0.26 0.11 0.170 0.002 0.49 0.005 0.4

5-Hi Fe Alloy 5 (Hi Fe) 0.30 0.63 0.001 0.005 0.54 0.005 2.1

6-Lo Si Alloy 6 (Lo Si) 0.03 0.12 0.001 0.001 0.46 0.005 3.9

Raffinal Aluminium 0.0017 0.0004 0.0012 < 0.0001 0.0002 < 0.0005

Alkaline Etching Solutions: comparable to commercial etchant

Amount Molarity Ingredient Major impurities

100 g/l 2.5 M NaOH Fe: < 5 ppmCu: < 0.2 ppm

10 g/l 0.05 M Sodium gluconate Fe: < 0.001%

Pb: < 0.002%

20 g/l 0.77 M Alloy 2 (commercial)

Anodizing: the pretreatment question

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SEM and AFM images of polished and etched surfaces.

Alloy 2(commercial)

Alloy 3(Hi Cu)

Alloy 5(Hi Fe)

polished etched 10 s @ 60C etched 60 s @ 60C

5 m 5 m

etched 60 s @ 60C

20 m

Etching is very sensitive to small alloy composition changes

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FIB: Ion-beam image

100

80

60

40

20

0

Potential[V]

6005004003002001000

Time [s]

80

60

40

20

0

10-3C

urrent[A]

Potential

Current

Sample 2

200

150

100

50

0

Potential(V)

5004003002001000

Time (s)

16

14

12

10

8

6

4

2

Currentdensity(mA/cm

2)

pH13, 0.05M Na3PO4

3M NaOH / 0.6M KF /1.1M NaAlO2 / 0.21M Na3PO4

Thin, dense

Thickness:1.2-1.8 nm/V

Thick, porous

Platinum layer for FIB sectioning

Mg hydroxide / Phosphate

Mg alloy

Surface protection: thin dense vs. thick porous on Mg alloys

i= 15mA/cm2

i= 80 mA/cm2

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Corrosion resistant surface

1) Additional layer: porous anodizedoxides serves as bases for goodadhesion of polymer top coats

2) Color pigments or inhibitors can beintegrated in the pores

3) Disadvantage of thick porous oxidesis that they are brittle and crack veryeasily. Corrosion protection is locallysuppressed

FIB sectionActiveAgents

releasedfrom the

pores

Anodizing: additional information's

Anodizing is possible on any metals and alloys that can produce aninsulating oxide: Al, Ti, Mg, Ta, Zr (not exhaustive list)

For semiconducting oxides (example: steel), the applied anodic current isused to dissociate water ! No anodizing possible

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Conversion coating: principle

The result is a rapid and technicallysimple process

+ Other advantages compared to otherprocesses

Is it a perfect corrosion protectionmethod ?

no, chromate is carcinogenic andneeds to be replaced sooner or later

The term conversion coating comes from the fact that film growth is the resultof reduction of an high valence ion present in solution

Typical : Chromate, Phosphate, Titanate, Zirconate

At the same time, metallic atoms from the surface are oxidized to form a mixed

oxide. For example Al-Cr mixed oxide for Al alloys.

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Reactions taking place during a conversion coating process

Al0

Al+3

slow

Cr2O7-2

Cr(III)

Fe(CN)6-4

Fe(CN)6-3

Al+3

Al0

fast fast

Cr2O7-2

Cr(III)

L. Xia, R. L. McCreery,,J. Electrochem Soc

. 1999,146

, 3696-3701

AA 2024-T3

Cr2O72-

Cr2O7-2, HCrO4

-

Cr (III) hydroxide

Cr(III)/Cr(VI) mixed oxide

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Reduction of Cr(VI) and role of intermediate species

AA2024-T3 very slowly

reduces Cr2O72-AA2024-T3 rapidly

reduces Fe(CN)63-

Fe(CN)64- rapidly

reduces Cr2O72-

IrCl62- can substitute

as mediator

immersion time (seconds)

0 50 100 150 200 250 300 350

0

200

400

600

800

1000

1200

1400

Rama

n860cm-1peakhe

ight(AU)

Fe(CN)63-

IrCl62-

V3+

Fe3+

No mediator

AlCr(III)

Cr2O72- Fe(CN)6

4-

Fe(CN)63-

Al3+AA2024-T3:

L. Xia, R. L. McCreery,,J. Electrochem Soc

. 1999,146

, 3696-3701

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The coating (or primer) provides a stablereservoir of Cr6+

Cr6+ remains stable (for some significant

time)

Cr6+ is leached by contact with solution

Cr6+ hydrolyzes to an anionic speciesthat migrates to defects

At the defect, chromate is reduced andprecipitated as a mixed Cr(III)/Cr(VI)oxide where it inhibits cathodic rxns, or

At the defect chromate absorbs intoaluminum hydroxide gel stabilizing it andpreventing further corrosion.

HCrO4-/CrO42-/Cr2O72-solution

Al alloy substrate

labile Cr6+

conversioncoating

mixed Cr(III)/Cr(VI)oxide deposit

chromatefrom primer coat

Zhao, JES, 145, 2258 (1998).

Self healing properties of chromate conversion coating

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originally bare alloy surface defect

originally coated alloy surface inhibitor reservoir

incipient protective layer due to action of inhibitor

10 to 5000 m0.5 to 0.05M NaCl

The cell permits examination for:

Release of active agent into solution(UV absorption or ICP-OES)

Deposition of the active agent on the defect(Auger microscopy or Electron Microprobe)

Corrosion resistance of the simulated defect Corrosion resistance of the original coating.

(Electrochemical Impedance spectroscopy EIS)

Zhao, McCreery, Frankel, JES, 145, 2258 (1998).

Reduction of Cr(VI) and role of intermediate species

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Pourbaix-Diagram of Ce[M. Pourbaix, Atlas of Electrochemical Equilibria Aqueous

Solutions (Houston, Tx: NACE, 1974), p.183]

Solubility-Diagram of Ce[Buchheit RG, M.S., Schmutz P, Guan H, Active corrosionprotection in Ce-modified hydrotalcite conversion coatings.

CORROSION, 2002. 58(1): p. 3-14.]

Al matrix

IMP

Cathodic reductionO2 + 2H2O + 4e

- 4OH-Anodic oxidationAl Al3+ + 3 e-

Al oxide

Cerium as inhibitor for possible chromate replacement

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not immersed to CeCl3 12min immersed to10mM CeCl3 + 1M NaCl

6016 Alloy, 350C, 15h, fresh polished

30m

Electron microprobe (EPMA) mapping of a surface immersedin 1M NaCl + 10mM CeCl3 for 12 min

Cerium blocks the local cathodic activity

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Electrochemical Impedance spectroscopy

Amplitude

E

I

Simple model-for electrochemical Interface

Rs

Rp

C

Rs : solution resistanceRp : polarization resistanceof the surface

C: capacitance of the surface

Ohms law gives a simple relation

E = I * R

When AC signal are involved the relation is

Eac = Iac * Z Z: impedance

- Apply voltage perturbation (10 mV)

- Measure the current response in functionof frequency

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103

104

105

106

0 2 4 6 8 10 12

Rc

(o

hmcm

2)

Time (hrs)

Bare 2024-T3 is exposed to Ce coatings and control samples in theartificial scratch cell for 48 h

Surfaces are removed and placed in contact with aerated 0.05 MNaCl

EIS spectra are collected vs. exposure time and Rp is computed

Ce-modified HT

Cerrate conversioncontrol

Rp

(ohm

*cm2)

EIS results on self-healing properties of a coating

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50 mm 1 m 10

CeO2MnO4-

VO3-

V10O286-

MoO42-

Fe(CN)64-/3-

NO3-

LiAl(OH)3+

ZnAl(OH)32+

Ce objectives: incorporate Ce oxyanion in the HT interlayer.

have HT release oxyanions upon contact withsolution.

exploit Ce redox chemistry to induceself-healing.

[Buchheit RG, M.S., Schmutz P, Guan H,

Active corrosion protection in Ce-modifiedhydrotalcite conversion coatings.CORROSION, 2002. 58(1): p. 3-14.]

Hydrotalcite structure can accommodate various species in their

structure

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AA 2024-T3

Amorphous, Field-Applied Metalic

Cladding

Composed of Al-TM-RE with a compositionthat retains amorphicity at slow cooling rates

Upon aging, amorphous structure transformsto nano-crystallite particulates within amorphousmatrix - amorphous character retained

Provides initial and most important line ofcorrosion protection

Also improves fatigue resistance

As-cast 270C

TEM BF micrographs of

Al90Co3Ce7 alloy

in the as-cast and heat

treated conditions.

Non-chromate Inhibitor Identification and Delivery-on-Demand

Solubility and signal for demand is controlled viamultiple mechanisms

(1) Incorporation into the galleries

of hydrotalcite crystals powders(2) Packaging of inhibitor solidsvia plasma polymerization

A- A-

(3) Containment within sol gelnano-capsules having poreswith controlled release gates.

Inh.

Smart combined coating to replace chromate

Polymer coating

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Filiform corrosion under organic coatings: optical observation

When an organic coating ismechanically damaged,undermining of the coating

occurs. Corrosion filamentspropagate at the interfacepolymer/ metal (or oxide)

FiliformCorrosionOn scratchedAl surface

Coated Steel

Coated Aluminum

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PolymerCoating

CorrosionProducts

Al-Alloy with

intermetallic

particles

active head

(AlCl3solution):

Al dissolution

back of head:

oxygen reduction(at the

intermetallicparticles ?)

Electrochemical Mechanism:Al is dissolved in the active head,

the anode. The electrons set freeby this reaction are used at thecathode, behind the active head, toreduce oxygen.

SEMof crosssection

Plane-viewphotograph

Corrodedarea

Filiform corrosion: damages

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Filiform corrosion mechanisms

Anode

(Acidic)

Membrane Cathode

(Alkaline)

Corrosion

products

Polymer

coating

Al alloy withintermetallic

particles

Filiform corrosion (FFC) is a special case of an aeration cell.

Requirement for initiation:- High relative humidity- Water and air can diffuse through the organic layer

- there must be a defect in a coating (scratch, pinhole)- Salt must be present in the defect

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Corrosion protection methods: summary

Metallic coatings provide good mechanical properties to the surface.

Disadvantage: conducting therefore risk of accelerating the substratecorrosion because of galvanic coupling (except sacrificial coatings)

Inorganic coating provide good corrosion resistance.

Disadvantage: anodized or thermal oxides are brittle and do not withstandapplied stress. Anodizing might also be an expensive process

Conversion coating: adherent surface layer of low solubility oxide,phosphate or chromate.

Disadvantage: the more efficient species are toxic

Organic coatings: Inert barrier layer, usually epoxy resins,

polyurethane top coat (essentially paint the surface)

Disadvantage: soft compared to the other type of coating. Can be easilydamaged and then corrosion can proceed

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siandaii corrosion protection lecture

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