SIandAII Localized Corrosion Lecture

8/22/2019 SIandAII Localized Corrosion Lecture

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Aspects of localized corrosion:

- pitting- galvanic

- crevice

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Stainless Steel: conducting passive film

Anodic reaction: in the pit

Cathodic reaction: on the passive film + in the pit

Cathodic area is much larger than anodic area(cathodic reaction in the pit can be neglected)

Galvanic couplinginducesfast localized attack

Combination: Pitting Galvanic corrosion

Active Pits

IntactsemiconductingPassive layer

Stainless steel

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Aluminum

Aluminum alloys: insulating passive film

Anodic reaction: in the pit

Cathodic reaction: in the pit (or on intermetallic phases)

No reactions on passive film

External galvaniccoupling withintermetallics

can inducefast localizedattack

Active Pits

Intact insulatingpassive layer

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Are all the passive oxide conducting ?

Photoelectrochemistry allows to answer this question

- Monochromatic light of different energies (wavelength) is shinedon the material surface under electrochemical control

- The additional current as function of the light energy is recorded

Experimental setup

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+-

Charge injection at semiconducting surface

At an oxide-solution interface, an equilibrium betweenthe redox potential of the reduction reaction and theFermi level of the oxide is established

When the surface is

sufficiently polarized orlight is shining on thesurface, electrons canbe promoted in the

conduction band

W: space charge domain

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Emitted Photocurrent from a passive film

- The fundamental model concerning the description of photoelectrochemicaleffect in a bulk semiconductor has been derived by W. Grtner.

with : Incoming light intensity

: Absorption coefficientL : Diffusion length of the charge carriersW: Space charge domain width

- Obviously not all the incoming photons can successfully be converted inelectron-hole pairs, the Quantum efficiency h of a process can be formulated:

Nel = Number of generated electronsNphot = Number of generated electrons

Iph = q 1-e-W

1+ L

=Nel

Nphot=

Iph h

q


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7

Emitted Photocurrent from a passive film II

- The wavelength dependence of the photocurrent and the band gap Eg determination

with h = hc/

- For very thin semiconducting film the exponential term can be developed in a Taylor-Series

- For a constant incoming light wavelength, the Grtner theory provides a direct relationbetween the electrochemically applied potential and the photocurrent (Ufb: flat bandpotential)

W = (U-Ufb)1/2

= Ah - Eg

n

h

Iph resp. h

Iph = q W


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1.4x10-4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Photostrom[

A]

450400350300250

Wellenlnge [nm]

Insulating Ni oxideTitanium oxide

1.4x10-4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Photostrom[

A]

500450400350300250

Wellenlnge [nm]

- Thin Ti passive oxides are semiconducting (band gap: 3eV )- Thin passive Ni oxides are isolating

Some example of passive surfaces

Band gap

energy

Maximalphotocurrent

Photocurrent(A

)

Wavelength (nm)

Ph

otocurrent(A)

Wavelength (nm)8


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Substrate

Metal - Matrixwith TiO2-Nanoparticles

TiO2

TiO2TiO2 TiO2

TiO2 TiO2

TiO2

TiO2

TiO2

TiO2

Environmental interactions:

cleaning and bactericideeffects

Surface functionalizing with TiO2 nanoparticles

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0.0

0.1

0.2

0.3

0.4

0.5

250 300 350 400 450

Wellenlnge des Lichts [nm]

Photos

trom-Intens

itt[mA]

abgeschiedene Nickelschicht

Abscheidung mit Titandioxid

verbesserter Abscheidungsprozess

0.40 mA

0.15 mA

0.01 mA

- Pure Ni (with surface oxide) does not show photoelectrochemical effects

- Very important photoelectrochemical effect is obtained upon integration of TiO2nanoparticles (intensity smaller for agglomerated particles)

Photoc

urrentIntensity[mA]

Light wavelength [nm]

Ni electro-deposition

Ni+ TiO2 (agglomerated)

Ni + TiO2 (dispersed)

Band gapenergy

0.1 M Na2SO4 solution

Photo-activity of TiO2 in metallic matrix

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Semiconducting behavior of passive Steel

All the stainless steels have semiconducting surface oxides

The properties are

changing with Moaddition

Solution:

1M Na2SO4300 mV SCE 11


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CrNisteels

I X5CrNi18-10X4CrNi18-12

X6CrNiTi18-10X2CrNi19-10

1.43011.4303

1.45411.4306

1818

1819

humid areas

CrNiMosteels

II X5CrNiMo17-12-2X6CrNiMoTi17-12-2

X2CrNiMo17-12-2X3CrNiMo17-13-3X2CrNiMo18-14-3X2CrNiMoN17-11-2X2CrNiMoN17-13-3

1.44011.4571

1.44041.44361.44351.44061.4429

2424

2426273032

mild outdoor climate,weathered

III X2NiCrMoCu25-20-5X2CrNiMoN17-13-5

X2CrNiMoN22-5-3

1.45391.4439

1.4462***)

3537

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outdoor climate, unweatheredindustrial atmosphere,weathered

IV X1NiCrMoCuN25-20-7X1CrNiMoCuN20-18-7X2CrNiMnMoNbN25-18-5-4

1.45291.45471.4565

474850

aggressive mediaindoor swimming pools,tunnels, sewage treatmentplants

Specialmaterials

NiCr21Mo14WNiMo16Cr15WNiMo16Cr16Ti

2.46022.48192.4610

(66)(68)69

Combination of aggressivemediachemicals industr

Type ofsteel

Corrosionresistanceclass*)

Abbreviated name Materialno.

PREn**) Applications


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Oxides and pitting susceptibilityWhen the potential of the sample surface is modified due to a

polarization, the charge in the space charge layer can also beswitched

a) Adsorption of Cl

-impossible

b) Adsorption of Cl-

favorable

Amount of metastable pits isinfluenced by this parameter 13


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Mg AZ alloys and microstructure obtained

100m

200m

AZ71

as cast

T4420C/30h

Nominal composition:

Mg: balance Al: 7 % (6.91% from ICP-AES) Zn: 1 % (0.84%)

Mn: 0.11 % Ni: 0.001 % Fe.


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- Large photocurrent intensities can beobtained for pure Mg and semi-conductingproperties of the hydroxide formed can be

investigated

- For the alloy, the presence of Aluminum inthe surface hydroxide suppresses thephotoelectrochemical current.

- Al hydroxide is insulating

200x10 -6

150

100

50

0

Ph

otocurrentnorm

alized(A)

500400300200

Wavelength (nm)

AZ 71 NRC T4

Directly after Im.+ 1 hour+ 3 hours+ 3 days

200x10-6

150

100

50

0

Photocurren

tnormalized(A)

500400300200

Wavelength (nm)

Mg 99.95% pure

Directly after im.

+ 1 hour+ 3 hours

+ 3 days

Mg oxides and influence of Al in 0.01 M NaOH

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Summary

Most of the passive surface layers (hydroxide oroxides) are semiconductors. This is the case for Tioxide (very good catalyst), Fe, Cr, Mg (when it isstable)

They all have a small band gap energy (2-3 eV) andgalvanic coupling of the intact passive layer withactive pits is an important factor

There are some examples of insulating hydroxides:Al , Ni

These passive layers are not having a galvaniccoupling influence

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A locally increase of corrosion attack in an existinggeometrical feature (recess, hole, scratch, crack, joiningarea, welding area)

The whole crevice surface is attacked but quite

often the bottom region are more damaged

Due to the corrosive attack, the crevice is widenedpossibly until breaking of the part (often solutionleakage)

Crevice corrosion: observation

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E l d i i


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Examples: macro and micro-crevice

Initial state

SSa

Mn

S

Passivefilm

Dissolution of MnS

SSb

MnS

S speziesMn2+

Formation of Passivefilmon top of MnS

SSc

MnS

Precipitation ofMn Oxide(s) and

pushout of part of MnS

Mn2+

MnS

S spezies

SSd

Macroscopic crevice corrosion

are often at the joining of tubes orweld

Microscopic crevice are the result

of dissolution of defects like theMnS example

Crevice corrosion susceptibility of corrosion resistant passive materials isalways higher than pitting corrosion because the unfavorable geometry isalready present

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M h i ( l ith itti )


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1) Crevice corrosion is the result of an active-passiveelement

- The crevice (geometrical recess) is active

- The material surface is passive

a) Stagnant solution or dirt particles adsorbing Cl- anions dorexample are increasing locally the Cl- concentration

b) High chloride contents are reached inducing pitting

c) Hydrolysis of the corrosion products induces pH drop

d) Autocatalytic mechanism

Mechanisms (see analogy with pitting)

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A l t f i i i t d t t d( l )


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Example: Airplane structure

Different areas at the bottom ofthe planes are at location where

condensation takes place andsubjected to crevice corrosion

White areas: mostly dryRed areas: often humid

A lot of crevices in riveted structured(macroscale)

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Comple cre ice corrosion problems


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Corrosion found at fastenersholes and joints

Here, not only crevice are

present, but screws (steel)and structures (Al alloy) aremade of dissimilar materials !

Complex crevice corrosion problems

Crevice + galvaniccorrosion is possible

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Other e ample of cre ice corrosion (microscale)


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Other example of crevice corrosion (microscale)

Poor quality ofsoldering orwelding processes

Crevice corrosion Pitting

0.1M NaCl solution

Electrochemicalcell with O-rings

For Stainless Steel

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Crevice corrosion and coatings DLC


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Properties of DLC

1500-3500 HV

temperature stability < 350 C

very low friction and wear

stable against acids and alkaline mediabiocompatible, haemocompatible

Deposition by PA-CVDfrom C

2H

2gas

Commercial applications:

DLC coated hard disk, diesel compression cam plate and injector

DLC coated screw compressor, wrist pin and gear

Crevice corrosion and coatings DLC

Diamond-like Carbon (DLC) coatings are ideal for applications where low friction isaimed at

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Failure of coated medical implants


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BUECHEL-PAPPAS HIP REPLACEMENT SYSTEM

Its not the coating which is bad, itsthe interface that needs to beanalyzed

Adhesion/delaminationin corrosive media ?

cracks

DLC

CoCrMoor

TiAlV

interlayer

Failure of coated medical implants

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DLC coating local failure in vivo (in patients)


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101 patients DLC/Polyethylene(PE)

101 patients Al2O

3/PE

8.5 year fallow-up50% of DLC/PE failed

Retrieved DLC-head: Numerous pits revealing the metallicsubstrate, severe PE wear

2 m DLC

2 m DLC-Sigradient

ca. 60 nm Si

Ti-Al-V

Multilayered structuredetermined by XPS depthprofiling

G. Taeger et al., Mat.-wiss. u. Werkstofftech. 34 (2003) 1094

DLC coating local failure in-vivo (in patients)

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Focused Ion Beam section on defect


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Overview:

Left picture: A crack at the right end of the delamination has been followedRight picture: Enlargement, several cuts needed to find the end of the

delamination crack.

During cutting a part of DLC delaminated (right picture).

Focused Ion Beam section on defect

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Detail of the crevice corrosion attack (nanoscale)


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- The first 50 nm above the TiAlV substrate are more or less totally corroded away(the smooth round shape of the corroding tip indicate that it is not a crack growth)

- Silicon is not stable in vivo due to crevice corrosion and the related local aggressive

media that can be established. It is important to note that this very well knownphenomenon is not predictable by simple thermodynamic consideration !

TiAlV

DLC-Si

Si

Detail of the crevice corrosion attack (nanoscale)

Silicon Pourbaix Diagram

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Artificial crevice setup


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In order to investigate the corrosion mechanisms in pitsand crevices, it is possible to use a model experimentalsetup called artificial crevice.

It is a plate (foil)

of the material of interest(thickness: t, width: w)pressed between twotransparent plastic sheets

This crevice (depth ) isconnected to anelectrochemical system

(working electrode)

Artificial crevice setup

Front View Side View

Al alloysheet

Plastic

sheet

t

w

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Potential evolution in crevice


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The total potential drop between the reference electrode and

the bottom of the crevice can be formulated by

Potential evolution in crevice

iRsrtot EEEE ++=

Plastic Plastic

Metal

Plastic PlasticMetal

depth

width

tot: total potential drop

r: potential difference betweenreference electrode and noncorroding surface

s: surface overpotential

iR: ohmic potential drop

Ref.ele

ctrode

r

Ref.ele

ctrode

i

R

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The different potentials in detail


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The different potentials in detail

)/log(oasiiE =

The surface overpotential is simply given by the Butler Volmer relation

The iR drop contribution is given by the ohms law

The measurable potential difference can be then

expressed in terms of all the parameters

RiE netiR =

RiiAE netnettot ++= )85.0/log(

)log( or iEA = 30

Experimental investigation: example of Aluminum


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Experimental investigation: example of Aluminum

It is not possible to measure all the components of thepotential individually

But if the net current can be determined, then the d ifferent

components can be discussed

The following procedureis used.

Apply a certain potential.In this case, it was startedat 0V SCE and the potentialwas gradually decreased

The dissolution rate canbe measured optically through

the transparent walls

10-6

10-5

10-4

10-3

10-2

10-1

100

101

-1 -0.8 -0.6 -0.4 -0.2 0 0.2

Curre

ntDensity(A

/cm2)

Potential V (SCE)

99.99 Al

Al - 1.9 Cu

Al - 3.9 Cu

0.2 Cu

0.022 Cu

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The potential evolution: part II


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The potential evolution: part II

The dissolution current is related to the dissolve d depththrough the total charge Qano:

o is the dissolved depth for a given condition

The measured charge

is different

Because of the cathodicReaction occuring in thecrevice

MnFwtQano /0=

cathanonetQQQ =

anonet QQ = 85.00

0.2

0.4

0.6

0.8

1

1.2

-2 -1.5 -1 -0.5 0 0.5 1 1.5

Curren

tDensity(A/cm

2)

Potential V (SCE)

Data

iR

s

Al

0.02 Cu

r

0.2 Cu

1.9 Cu

3.9 Cu

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Avoid macroscopic crevice corrosion


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Avoid macroscopic crevice corrosion

The same precautions as for pitting corrosion areapplicable

Avoid the presence of deposits, dirt, lime

Conception mistakesavoid deep and narrowrecesses as well areas with stagnant solution

To avoid

solution

Crevice in welds

Optimized process !

Welded from both sides

solution

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SIandAII Localized Corrosion Lecture

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