SIandAII Intergranular Corrosion Lecture

8/22/2019 SIandAII Intergranular Corrosion Lecture

http://slidepdf.com/reader/full/siandaii-intergranular-corrosion-lecture 1/43

Intergranular corrosion:

Specific material microstructure

+

- pitting

- galvanic- crevice

1



Some facts about intergranular corrosion

Observation: For a given materials, grain boundaries orareas near grain boundaries are less noble / less stable(different composition)

Result: The corrosive attack is localized at these “less noble”areas

Damage: Attack not only dissolves grain boundary areas butalso result in severe falling out of entire grains

Example for steel:

Chrome carbideformation at the grainboundary and

subsequent formationof chrome depletedzones

2



Intergranular corrosion: observed damages

°Locally increased corrosion at grain boundaries

°“Electrochemical” material removal evolves at relatively smallrate

°Removal of undermined grains is the most dangerous aspectregarding fast damages

a) Schematic view b) metallographic section

3



SensitizationFor steel:

°Intergranular corrosion is the result of sensitization of thematerial due to “inadequate” heat treatment

°During heat treatment: chromium reacts with carbon to producesmall carbides:

23 Cr + 6C Cr23C6

Diffusion is faster at grain boundary, so they arepreferentially formed in these areas

°A certain temperature domain is especially dangerous:

450°to 850°C4



Stainless steel: carbide formation

a) Chromium is diffusing from the inside of the grain to formcarbidesb) Depleted zones are formed along the grain boundaries

Chromium depleted zoneChromium carbide

Higher Cr-content

Cr- contentbefore heattreatment

Local Crcontent after

sensitization5



Influence of chrome depletion

°Lower chromium content of the alloy result in increase of thecritical current density for passivation

°Loss of passivation in the grain boundary areas

Typical potentiodynamic polarization curves for Cr-Ni steel

a) the grain is passivein a given electrolyte

Ecorr

icorr

icrit

E

6



°The grain boundary domain has a higher critical current

density as a result of lower chromium content

b) The grain boundarystays active

unfavorable area ratio (galvanic coupling)

very fast in depth propagation of the attack

Ecorr

icorr

icrit

E

7



Important parameter

Heat treatment temperature and time

The critical combination of temperature and heat treatmenttime is a well know relation.

Right estimation of the intergranular corrosionsusceptibility

T r e

a t m e n t t e m

p e r a t u r e ( ° C

)

Heat treatment time (hours)

Grain falling out

8



Influence of applied potential°Grain boundaries and matrix have different electrochemical

behavior°Active-active, active-passive and passive-passive element are

possible for intergranular corrosion

°Type of attackdepends of the

potential onthe surface

Active/active active/passive passive/passive type

Large crevice small crevice (grain falling out) attack

heavy weak grain attack

C u r r e n t d e n s i t y

Potential

Grain

Grainboundary

9



a) Active – active intergranular corrosion

°Broad crevices with pitting type attack in the grains

°Penetration depth: middle up the strong attack

10



b) Active – passive intergranular corrosion

°Very narrow crevices and grains falling out (classicalintergranular attack)

°Penetration depth: very deep

11



°Very narrow crevice

°Penetration depth: small (surface effects)

Metallographiccross sectionof attackedsurface

c) Passive – passive intergranular corrosion

12



To avoid intergranular corrosion of steel

1) Materials

°Heat treatment at higher temperature (1050-1100°C)followed by quenching

°Decrease of the carbon content of steel°Addition of Titanium, Niobium, Tantalum (higher affinity forcarbide formation than Chromium)

2) Construction

°Control the temperature flowduring welding

Heat affected zones are oftensusceptible to intergranularcorrosion

13



Precipitates in 2024T3

Example of Aluminum alloys

a) b)

TEM image of 2024-T3 microstructure showing:a) plate like S phase (Al2CuMg) precipitates,

b) rod-like precipitate at the grain boundary and matrix

The structure (shown in b) at the submicrometer range is

controlling the intergranular corrosion susceptibility of Al alloys14

Resin

Al- Alloy



°Example of alloy 2XXX: Copper depletion at grain boundariesand formation of small high potential intermetallic phases

-0.69 VCuAl2 : -0.640 V

Low Cu content : -0.750 V

Grain boundary structure for Al alloys

15



Al alloys: plane identification

16

Sections:

- Longitudinal (L)- Long transverse (LT)- Short transverse (ST)



Al alloy grain boundaries: influence of heat treatment on thecorrosion susceptibility

Subsequent homogenization heat treatment:

16 hours at 170 °C 20 hours at 190°C

Insufficient sufficient

Grain boundary attack

Influence of heat treatment

Pits

17

Pi i d i l i



Pitting and intergranular corrosion

°Potentiodynamic polarization curves of different section of amaterials

°For Al alloys, there is a competition between pitting and

intergranular corrosion, pitting starts first, but not always atthe grain boundary.

18

E id f I t l i



Evidence of Intergranular corrosion

°Attack at different applied potential domains:- At low potential pitting- At higher potential: intergranular corrosion

19

X R h t i t h



20

2mm

X- Ray synchrotron microtomography

Spectroscopy is used to determine the

characteristics of chemical bonding andelectron motion

Scattering is commonly used to determine thestructures of crystals and large molecules such

as proteins

Imaging is used in diverse research areassuch as cell biology, lithography, infraredmicroscopy, radiology and x-ray tomography

C t t i t h



21

I = I 0 ∗ exp − µ ( x, y, z)dxdydz path

∫

µ = linear attenuation coefficient=f(ρ, E, Z)

E = Photon energyZ = atomic numberρ = density

Visible: Atomic number contrast alwaysmixed with density difference B. L. Henke, E. M. Gullikson, and J. C. Davis, “X-Ray

Interactions:Photoabsorption, Scattering, Transmission, and Reflectionat E = 50–30,000 eV, Z = 1–92,” At. Data Nucl. Data Tables 54,

181 (1993)

x

I0 I

µ(x,y1)y

y=y1

Contrast in tomography

X R i t h



720 images for reconstructionMax rotation: 180°X-ray energy: 17kV

Min. acquisition time: ~3s per frame

Resolution: ~1µm3

/voxel

SLS, Tomcat, 0.7 x 0.7 mm FOV, 2048x2048 CCD,20xZeiss lens, 350 nm/pixel

2mm

QuickTime™ andaTIFF (Uncompressed) decompressor

are neededto see this picture.

X-Ray microtomography

500 µm

scr een

X-ray source

X-Raysource

22

Intergranular corrosion (IGC) of Aluminum



In depth corrosion propagationcan be followed online in a “non-destructive” mode

beamcamera

6016 / IGC in 7h 2.5MHCl

Intergranular corrosion (IGC) of Aluminum

SLS X04SA MaterialsScience Beamline atthe Paul ScherrerInstitute (PSI), Villigen

23

Microtomography and hidden corrosion !



24

6111, 0.7 M HCl, 7h exposed

- IGC (Intergranular Corrosion)

- Channeling “Exfoliation like attack” ELA

- Surface deformed layer undamaged

=> IGC and channeling interactions

Microtomography and hidden corrosion !

Surface deformed layer for Aluminum



25

Characteristics of deformed surface layers: Nano-crystalline (grains < 50 nm) Second-phase inclusions (oxides,

lubricant) Intermetallic particle distribution

different from bulk.

Deformed surface layers are genericfeatures of all rolled aluminium sheet products.

In principle more susceptible to corrosion

Initiation but can show other advantages !100 nm

TEM micrograph of transverse section (AA5754 H18)

4 5 0 n m

TEM of ultramicrotomed cross sections

Surface deformed layer for Aluminum

To avoid intergranular corrosion in aluminum



To avoid intergranular corrosion in aluminum

1) Materials

°Choose the appropriate heat treatment to avoid

formation of cathodic intermetallic particles and purealuminum areas along the grain boundaries

°This process is unfortunately less controllable (v erymuch dependent on treatment temperature) then forsteel because of the complex

2) Construction

°Control the temperature flow during welding

26

Mathematical modeling of localized corrosion



• Modelling of anodic dissolution of pure aluminium

• Mass transport equations

• Modelling of anodic dissolution of the S phase (AlCuMg)of Al 2024 alloy

The work was supported by the EU Commission 6thFramework Program Project “SICOM” (SimulationBased Corrosion Management)

• Modelling of pitting in matrix of Al 2024 alloy

Mathematical modeling of localized corrosion

• Modelling of intergranular corrosion of Al 2024 alloy

27

Generalized mass transport equations



zi – charge numberci - concentrationui - mobilityF – Faraday’s constant, 96487 C/equivΦ - electrostatic potential Di – diffusion coefficientv – bulk velocity R – gas constant, 8.31 J/molT – absolute temperature

ii

i Rt

c+⋅∇−=

∂

∂N

vN iiiiiii cc DFcu z +∇−Φ∇−=

iiRTu D =

Nernst-Einstein equation

Flux density of each dissolved species i

Material balance for each chemical component i

Assumption of electroneutrality of solution

∑ =i

iic z 0

flux density migration diffusion convection

accumulation net input production(in homogeneous reactions)

The model does not assume theequilibrium state in solution: allterms in homogeneous reactions, Ri, are treated explicitly using kineticconstants taken from the literature

Generalized mass-transport equations

28

Homogeneous reaction and Al solution chemistry



Hydrolysis of Al(III) Al3+ + H2O AlOH2+ + H+

H2O H+ + OH-

Species

Na+, Cl-, H+, OH-

Al3+, AlOH2+, Al(OH)2

+, Al2(OH)

2

4+

AlCl2+, Al(OH)Cl+, Al(OH)2Cl

3Al3+ + 4H2O Al3(OH)45+ + 4H+

Al3+ + 2H2O Al(OH)2+ + 2H+

Al3+ + 3H2O Al(OH)3(aq) + 3H+

Al3+ + 4H2O Al(OH)4- + 4H+

2Al3+ + 2H2O Al2(OH)24+ + 2H+

13Al3+ + 28H2O Al

13O

4(OH)

24

7+ + H+

Al3+ + Cl- AlCl2+

AlOH2+ + Cl- Al(OH)Cl+

AlCl2+ +2H2O Al(OH)2Cl + 2H+

Al(OH)Cl+ + H2O Al(OH)2Cl + H+

1E-13

1E-11

1E-09

1E-07

1E-05

0.001

1 2 3 4 5 6 7 8

pH

c o m c e n t r a t i o n [ M ]

Al

AlOH

Al(OH)2

Al(OH)3

Al(OH)4

Al2(OH)2

Al3(OH)4

Al13O4(OH)24

Chloride chemistry

Homogeneous reaction and Al solution chemistry

29

Hydrolysis reactions



Homogeneous reactions

Species

Na+, Cl-, H+, OH-, Al3+, AlOH2+, Al(OH)2+, Al2(OH)2

4+

k f k b Ref.

Al3+ + H2O AlOH2+ + H+ 1.09 · 105 s-1 4.4 · 109 M−1 s−1 1

AlOH2+ + H2O Al(OH)2+ + H+ 1.09 · 105 s-1 4.4 · 109 M−1 s−1 1

2Al3+ + 2H2O Al2(OH)24+ + 2H+ 10-2 M-1 s-1 108 M−2 s−1 1

H2O H+ + OH− 2.6 · 10−5 s−1 1.3 · 1011 M−1 s−1 2

1 L.P. Holmes, D.L. Cole, E.M. Eyring, J. Phys. Chem. 72 (1967), 301.2 M. Eigen, L. De Maeyer, Z. Elektrochemie 59 (1955), 986.

+++++ ⋅⋅−⋅== HAlOHbAlf HAlOH232 cck ck R R ++ −= 23 AlOHAl

R R

++ −=2

2Al(OH)AlOH

R R+++++ ⋅⋅−⋅==HAl(OH)bAlOHf AlOHAl(OH) 2

222

cck ck R R

2

H(OH)Alb

2

Alf H(OH)Al)()( 4

2234

22+++++ ⋅⋅−⋅== cck ck R R

++ −= 422

3(OH)AlAl

R R

−+−+ ⋅⋅−⋅==OHHbOHf OHH 2

cck ck R R M5.55OH 2=c

Hydrolysis reactions

30

Complexation reactions



Species

k f k b, s-1 Ref.

Al3+ + Cl- AlCl2+ 226 M-1 s-1 75 - k f ([Al3+] –[AlOH2+] 1

AlOH2+ + Cl- Al(OH)Cl+ 1.9 · 104 M-1 s−1 5.7 · 103 – k f [AlOH2+] 1

AlCl2+ + 2H2O Al(OH)2Cl + 2H+ 4 · 10-6 s-1 2

Al(OH)Cl+ + H2O Al(OH)2Cl + H+ 4 · 10-6 s-1 2

Kinetic constants for homogeneous reactions

Geometry

hemispherical pit with a radius of 10 µm

capillary diameter 100 µm, capillary height 10 mm

1 R.T. Foley, T.H. Nguyen, J. Electrochem. Soc . 129 (1982), 464; 2 R.C. Turner, G.J. Ross, Can. J. Chem. 48 (1970), 723

+−++ ⋅−⋅⋅= 232 AlClbClAlf AlClck cck R

++ −== 2-3 AlClClAlR R R

++−++ ⋅−−⋅⋅=Al(OH)ClbAlClClAlOHf Al(OH)Cl

)( 22 ck ccck R++ −== 2-3

AlClClAlR R R

+⋅= 22 AlClf ClAl(OH) ck R ++ ⋅= 2

AlClf 21

H ck R

+⋅=Al(OH)Clf ClAl(OH) 2

ck RClAl(OH)H 2

R R =+

Na+, Cl-, H+, OH-, Al3+, AlOH2+, Al2(OH)24+, AlCl2+, Al(OH)Cl+, Al(OH)2Cl

Complexation reactions

31

Geometry and boundary conditions



Mechanism of Al dissolution

Al → Al3+ + 3 e-

Al bulk boundaryoxide film: insulation

Flux of species Al3+ at pit boundary

Boundary conditionsbulk concentrations for

all species 0, =Φ=

∞

iicc

F

j Al

3=⋅+ nN 3Al

0, =Φ= ∞

ii

cc

with j Al from 1 µA/cm2 to 5 A/cm2

Geometry

capillary radius 100 µm-10 mm, capillary height 10 mm, pit radius 10 µm

Capillary wall

• “no walls”: bulk concentrations for all species

• insulation

Condition for stable pit growth I pit /r pit > 10-2 A/cm

For a 10 µm-radius pit j Al > 1.6 A/cm2

Al3+

Al bulk

z

s y

m m e t r y a x i s

c

a p i l l a r y w a l l

Na+

Cl-

Al3+

Al bulk

z

Al3+

Al bulk

Al3+

Al bulk

z

s y

m m e t r y a x i s

c

a p i l l a r y w a l l

Na+

Cl-

Geometry and boundary conditions

32

Model for Al with most relevant species



Calculations with COMSOL

• adaptive meshes

• triangular Lagrange-quadratic elements

• active dissolution j Al = 4 A/cm2

(M. Verhoff, R. Alkire. J. Electrochem. Soc. 147

(2000), 1349)

Conclusions

• meshes with more than 5000

elements should be used

0.142

0.1425

0.143

0 5000 10000 15000 20000

number of elements

[ N a

+ ] , [ M ]

0.0499

0.05

0.0501

p o t e n t i a l [ V ]

[Na+]

potential

+

Boundary conditions: „no walls“

Species Na+, Cl-, H+, OH-, Al3+, AlOH2+,

Al2(OH)24+, AlCl2+, Al(OH)Cl+, Al(OH)2Cl

pH-value, chloride concentration and potentialat pit bottom

0

2

4

6

0.000001 0.0001 0.01 1

j Al [A/cm2]

p H o r [ C l - ] [ M ]

0

0.02

0.04

0.06

p o t e n

t i a l [ V ]

pH

Cl

potential

fitting pH

pH = -0.5351 log( j Al) + 2.8259

Bulk: cCl = 1 M; pH = 6

Model for Al with most relevant species

33

Localized attack of 2024 alloy: experimental input



Experimental input

F

jCu

2====⋅⋅⋅⋅++++ nN 2

Cu

F

j Mg

2====⋅⋅⋅⋅++++

nN2Mg

Mechanism of metal dissolution

F j Al

3nN 3

Al=⋅+Al → Al3+ + 3 e-

Cu→ Cu2+ + 2 e-

Mg → Mg2+ + 2 e-

20 randomly distributed pits with a radius of 0.2 µm each

3D Geometry

Capillary: radius 15 µm; height: 1mm

pit 1 pit 2

Alloy composition:Al: 95%, Cu: 3.5%, Mg: 1.5%

Approx. 20 pits with radii 0.1-0.3 µm

Max. measured current: 50 nA34

y p p

Localized corrosion of 2024 alloy



• Al-boundary: actively dissolving part: japassive Al-bulk surface: ja·10-6

z

s y m m e t r y

a x i s

c a p i l l a r y

w a l l

Al3+

Mg2+

Cu2+

Na+ Cl_

Al bulk

Al3+

z

s y m m e t r y

a x i s

c a p i l l a r y

w a l l

Al3+

Mg2+

Cu2+

Na+ Cl_

Al bulk

Al3+

s y m m e t r y

a x i s

c a p i l l a r y

w a l l

Al3+

Mg2+

Cu2+

Na+ Cl_

Al bulk

Al3+

Boundary conditions

• Capillary top: “no walls” (bulk concentrationsfor all species )

• Capillary wall: “no walls”; insulating wall

0, =Φ= ∞ii cc F

jCu

2

====⋅⋅⋅⋅++++ nN 2Cu

F

j Mg

2====⋅⋅⋅⋅++++ nN 2Mg

Mechanism of metal dissolution

F

j Al

3====⋅⋅⋅⋅++++ nN 3Al

Al → Al3+ + 3 e-

Cu→ Cu2+ + 2 e-

Mg → Mg2+ + 2 e-

• AlMgCu-boundary: actively dissolving part:

• AlMgCu-boundary:passive inclusion surface:

active metal dissolution surface: magnesium part of the

inclusion dissolves first and fast ->

MgCuAl 9.0001.0099.0 j j j j ++=

4

MgCuAl

10)25.025.05.0( −⋅++= j j j j

( I = 10 nA gives approx. j = 0.037 A/cm2)

y

35

Results for attack at intermetallics: pH evolution



The pH, the chloride concentration and thepotential values at the groove bottom

Homogeneous reactionsSpecies

pH = 6, [Cl-] = 1 M

Na+, Cl-, H+, OH-, Cu2+, Mg2+, Al3+, AlOH2+,Al2(OH)2

4+, AlCl2+, Al(OH)Cl+, Al(OH)2Cl

Al3+ + H2O AlOH2+ + H+

2Al3+ + 2H2O Al2(OH)24+ + 2H+

H2O H+ + OH-

Al3+ + Cl- AlCl2+

AlOH2+ + Cl- Al(OH)Cl+

AlCl2+ +2H2O Al(OH)

2Cl + 2H+

Al(OH)Cl+ + H2O Al(OH)2Cl + H+

0

2

4

6

8

10

0.00001 0.001 0.1 10

j active [A/cm2]

p H o r [ C l - ] [ M ]

0

0.02

0.04

0.06


pH

Cl

potential

fitting pH

pH = -0.5885 log( j active) + 2.5497

pH profile for I = 10 nA ( j = 0.037 A/cm2)

Boundary condition: “no walls”

p

36

Experimental input: attack distribution and current



Centerline concentration and potential profilesupwards from the pit bottom calculated for two pitsassuming a dissolution current density of 0.2 A/cm2

Al3+ + H2O AlOH2+ + H+Homogeneous reaction

Species

pH = 6, [Cl-] = 1 M

Na+, Cl-, H+, Cu2+, Mg2+, Al3+, AlOH2+ Distribution of pH values overthe capillary bottom

pit 1 pit 2

Boundary condition: “no walls”

0.000001

0.00001

0.0001

0.001

0.01

0 1 2 3 4 5

z [µm]

s p e c i e s c o n c e n t r a t i o n [ M ]

Al, pit 1 AlOH, pit 1Cu, pit 1 Mg, pit 1

Al, pit 2 AlOH, pit 2

Cu, pit 2 Mg, pit 2

3.5

4

4.5

5

5.5

0 1 2 3 4 5

z [µm]

p H

0

0.00005

0.0001

0.00015

0.0002

0.00025

0.0003


pH, pit 1pH, pit 2

potential, pit 1

potential, pit 2

37

p p

Modeling of intergranular corrosion (IGC)



Sample cross section

Geometrical input

Electrochemical input: j ≈ 0.01 A/cm2

Al 2024

capillary end diam. = 40 µµµµm

0.001

0.01

0.1

1

10

100

1000

10000

100000

-700 -600 -500 -400 -300 -200 -100 0 100

E vs SCE (mV)

l o g [ a b s ( J ) ] ( J u n i t s µ µµ µ A / c m 2 )

Sample surfaceAnodic polarization curves for AA2024-T30.5M NaCl (pH 7)

g g ( )

38

IGC: Geometry and boundary conditions I



Capillary:

radius 25 µm, height 5 mm

Crevice:radius 0.05 µm, depth 1mmand 0.1 mm

Surrounding cathode “out”:at 10 µm from axis, width 0.1µm

Cylindrical cathode “in”:50 µm up from bottom, width2.5 µm

Anode: crevice bottom

anode

Al3+

O2

OH-

Na+ Cl-

H+ OH-

Al3+ AlOH2+ Al(OH)2+ Al2(OH)2

4+

AlCl2+ Al(OH)Cl+ Al(OH)2ClO2

cathode 1

O2

s y m m e t r y a x i s OH-

cathode 2

cathode “out“

cathode “in“1. cathode “out“ is active2. cathode “in“ is active3. cathodes “in“ and “out“

are active

grain boundary

39

IGC: Geometry and boundary conditions II



• Al-boundary: actively dissolving part: jAl

• passive Al-bulk surface: insulation

Boundary conditions

• Capillary top: “no walls”

bulk concentrations for all species0, =Φ= ∞

ii cc

Anode: metal dissolution

Al→ Al3+ + 3 e-

O2 + 2H2O + 4e- 4OH-

Cathode: oxygen reduction

F

j Al

3nN 3

Al=⋅+

F

j

4

2

2

O

O−=⋅nN

F

j2

-

O

OH =⋅nN

• Current at cathodic boundary:

)(21

2

cathodecathode

anode AlO

ss

s j j

+=

where sanode and scathode are anodic and cathodic areas

40

pH, oxygen and chloride evolution



Calculations for cases cathode “in” andcathodes “in” and “out” were not done for thehigher dissolution current densities because of

oxygen depletion in the crevice

2

4

6

8

10

1E-07 1E-06 0.00001 0.0001 0.001 0.01 0.1

jAl [A/cm2]

p H

cathode "out"

cathode "in"

cathode "in" and "out"

0

1

2

3

4

1E-07 0.000001 0.00001 0.0001 0.001 0.01 0.1

jAl [A/cm2]

c o n c e n t r a t i o n C

l - [ M ]

cathode "out"

cathode "in"


pH, concentrations of O2 and Cl- at crevice bottom as a function of the active dissolution

current density (with an active cathode shown)

0

0.00005

0.0001

0.00015

0.0002

0.00025

0.0003

1E-07 1E-06 0.00001 0.0001 0.001 0.01 0.1

jAl [A/cm2]

c o n c e n t r a t i o n O

2 [ M ]

cathode "out"

cathode "in"


bulk: pH = 7, [Cl-] = 0.5 M

41

pH evolution as function of the cathode location



Comparison of pH values at crevice bottom for models withdepths of 1 mm and 0.1 mm as a function of dissolution current densityfor different cases of active cathode

2

3

4

5

6

7

8

0.0000001 0.00001 0.001 0.1

jAl [A/cm2]

p H

0.1 mm

1 mm

7

7.5

8

8.5

9

9.5

10

0.0000001 0.000001 0.00001 0.0001

jAl [A/cm2]

p H

0.1 mm

1 mm

2

3

4

5

6

7

8

1E-07 0.000001 0.00001 0.0001 0.001

jAl [A/cm2]

p H

0.1 mm

1 mm

cathode “out“ cathode “in“ cathodes “in“ and “out“

42

Conclusions for the IGC model



• Mathematical model for simulating intergranular corrosionhas been developed. The model includes metallic ionicspecies resulting from electrochemical reactions at themetal-solution interface (heterogeneous reactions) and

reactions in solution (homogeneous reactions).

• Model includes one anodic site at crevice bottom and cathodic sites onsample surface (cathode “out”) and in crevice (cathode “in”).

It is shown that attack propagation requires electrical coupling between theanodic site and cathodic site on the sample surface (cathode “out”),

resulting in high dissolution rates. If only cathode “in” is active, the attackwill propagate with slower rates depending on crevice depth due to oxygendepletion in the crevice (ex. for 1-mm crevice 10-5 A/cm2 (0.11 mm/year)).

Active cathode “out”, or both, cathodes “in” and “out”, lead to acidic

conditions, and active cathode “in” to alkaline conditions in the crevice

anode

Al3+

O2

OH-

Na+ Cl-

H+ OH-

Al3+ AlOH2+ Al(OH)2+ Al2(OH)2

4+

AlCl2+ Al(OH)Cl+ Al(OH)2Cl

O2

cathode 1

O2

s y m m e t r y a x i s

OH-

cathode 2

cathode “out”

cathode“in”

anode

43

Documents

SIandAII Intergranular Corrosion Lecture