siandaii intergranular corrosion lecture
TRANSCRIPT
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Intergranular corrosion:
Specific material microstructure
+
- pitting
- galvanic- crevice
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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
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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
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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
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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
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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
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°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
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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
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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
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a) Active – active intergranular corrosion
°Broad crevices with pitting type attack in the grains
°Penetration depth: middle up the strong attack
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b) Active – passive intergranular corrosion
°Very narrow crevices and grains falling out (classicalintergranular attack)
°Penetration depth: very deep
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°Very narrow crevice
°Penetration depth: small (surface effects)
Metallographiccross sectionof attackedsurface
c) Passive – passive intergranular corrosion
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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
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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
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°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
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Al alloys: plane identification
16
Sections:
- Longitudinal (L)- Long transverse (LT)- Short transverse (ST)
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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
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Pi i d i l i
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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.
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E id f I t l i
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Evidence of Intergranular corrosion
°Attack at different applied potential domains:- At low potential pitting- At higher potential: intergranular corrosion
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X R h t i t h
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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
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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
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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
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Intergranular corrosion (IGC) of Aluminum
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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
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Microtomography and hidden corrosion !
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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
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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
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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
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Mathematical modeling of localized corrosion
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• 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
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Generalized mass transport equations
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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
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Homogeneous reaction and Al solution chemistry
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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
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Hydrolysis reactions
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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
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Complexation reactions
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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
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Geometry and boundary conditions
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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
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Model for Al with most relevant species
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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
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Localized attack of 2024 alloy: experimental input
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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
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• 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
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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
p o t e n t i a l [ V ]
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
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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
p o t e n t i a l [ V ]
pH, pit 1pH, pit 2
potential, pit 1
potential, pit 2
37
p p
Modeling of intergranular corrosion (IGC)
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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
8/22/2019 SIandAII Intergranular Corrosion Lecture
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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
8/22/2019 SIandAII Intergranular Corrosion Lecture
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• 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
8/22/2019 SIandAII Intergranular Corrosion Lecture
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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"
cathode "in" and "out"
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"
cathode "in" and "out"
bulk: pH = 7, [Cl-] = 0.5 M
41
pH evolution as function of the cathode location
8/22/2019 SIandAII Intergranular Corrosion Lecture
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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
8/22/2019 SIandAII Intergranular Corrosion Lecture
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• 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