Topical Day on “Numerical Simulation of Localised Corrosion,” SCK•CEN, Belgium, October 16, 2002
Experimental and Computational
Studies of IG-SCC of AA7050-T6
K. R. Cooper, R. G. Kelly
Center for Electrochemical Science & Engineering
Dept. of Materials Science and Engineering
University of Virginia
Charlottesville, VA 22904-4745
USA
Acknowledgements: Alcoa Foundation, J. Moran, E. Colvin, J. Staley
R. Gangloff, J. Scully, L.Young, G. Young
Al-Zn-Mg-(Cu) Alloys Exhibit E-Dependent IG EAC
• Strong Potential
Dependence
– Limited at high
potentials by free
surface pitting
• Mechanisms
– HE and AD
• Crack environment
unknown
– Potential (E)
– X+/– pH
Speidel (1975)
EAC Mechanisms & the Crack Environment
• Anodic Dissolution – “Echem knife”
da
M
=
⋅ iTip
dt nFρ
3e3e-
Al
H
HHH
H
• Hydrogen
Embrittlement
da
= f (θ , C H , DEff )
dt
Al → Al3+ + 3eAl3+ + H2O → AlOH2+ + H+
H+ + e- → H
H2O + e- → H + OH-
Al3+
H+
H
Al3+
A-
H2
H2O
e-
Potential
[X+/-]
Distance from Crack Tip
Objectives
• Characterize crack environment as f (da/dt)
– Local electrode potential (ECrack)
– Chemistry → pH, [X+/-]
• Modeling
–
–
–
–
Crack tip opening displacement (CTOD)
Conductivity (κ)
Tip corrosion height
iTip → dissolution-based crack advance
• Mechanistic interpretation of EAC process
– Relative contribution of AD and HE
Material & Properties
Alloy Zn
Mg
Cu
Si
Fe
Cr
Ti
Zr
Al
7050 6.09 2.14 2.19 0.05 0.09 < 0.01 0.02 0.11 bal.
AA 7050-T6
YS *
MPa
TS *
MPa
K Ic**
MPa √ m
530
596
21.5
*
S-orientation
** S-L
S
T
200 µm
L (RD)
Experimental Approach
• Crack growth rate measured using fracture mechanics
• Bulk Environment:
0.5 M CrO42- + 0.05 M Cl-, pH 9.2
Reservoir
Reference
electrode
• Crack Environment
– In situ – mini-RE, pH, Cl– Crack Chemistry → post test
extraction and analysis of
solution
Pump
Micro
electrodes
Take Away Points
•
Crack tip chemistry controls IG crack growth
kinetics (da/dt) in AA7050-T6
–
–
•
Acidity is main driver
Crack tip potential can decouple from Eapp
Major Influences on E(x) and [Al3+](x):
–
–
–
Crack tip opening
Presence of a resistive crack tip film
Minor:
•
•
crack wall current
EPFM does not predict crack tip for IGSCC
–
Crack tips are much tighter
Phenomenology
• At EAPP = -0.445 VSCE, slow da/dt (5.9x10-7 mm/s)
• ↓ -0.545 V SCE, high
da/dt (2.3x10-5
mm/s) maintained
7050-T651 @ 121 oC (#50-108)
0.5 M Na2CrO 4 + 0.05 M NaCl, pH 9.2
30
Crack Length, mm
• Spontaneous
transition to fast
da/dt (1.5x10-4
mm/s) after
incubation
32
da/dt in mm/s
ETrans ∼ -0.5 VSCE
28
2.3x10-5
-0.545
26
1.5x10-4
24
-0.445 VSCE
22
5.9x10
-7
L. Young ‘99
20
0.0
0.5
1.0
1.5
Time, days
2.0
2.5
Phenomenology
1 mm / 5 min
7050-T6
10
-3
-3
• Hysteresis in Rate
0.5 M Na2 CrO4 + 0.05 M NaCl
K = 10-15 MPa√m
1 mm / hour
L. Young, ‘99
10
-4
– Incubation
• ∆t from 4 - 50 h
– Transition to HighRate
– Lowering of E does
not return rate to
incubation levels
LogCrack(da/dt),
mm/s
Growth Rate (mm/sec)
-4
1 mm / day
10
-5
-5
1 mm / week
-6
10-6
1 mm / month
(#1)
(#2)
(#3)
(#4)
(#5)
(#6)
10
-7
-7
10-8
-0.9
-0.9
-0.8
-0.7
-0.7
-0.6
-0.5
-0.5 -0.4
Potential (V vs. SCE)
Potential, VSCE
-0.3
-0.3-0.2
1 mm / year
• Coincident with da/dt
increase:
– E vs. t
28
2x10-5
Change EAPP
26
24
Probe Hole
22 6x10 -7 mm/s
20
Applied and Crack Potential vs. Time
-0.3
-0.4
Applied E
-0.5
-0.6
Crack E
-0.7
-0.8
• Large IR drop to tip
pH and [Cl -] vs. Time
10
Bulk pH
8
pH
– pH, [Cl-] vs. t
– Drop in pH, rise in [Cl-]
7050 #50-103
o
12 hour at 163 C
0.5 M Na2CrO4 + 0.05 M NaCl, pH 9.2
30
1
Cl -
6
Bulk Cl-
4
0.1
[Cl-], M
– ca. 40 h incubation
Crack Length, mm
• a vs. t
Crack Length vs. Time
32
Potential, V vs. SCE
In-Situ Crack
Measurements
pH
2
0.01
0
2
4
6
8
10
12
Time, days
14
16
18
Control of Cracking Rate by
Crack Tip Chemistry
Crack Length, mm
32
7050-T6 @ 121o C (50-110)
0.5 M Na2CrO4 + 0.05 M NaCl, pH 9.2
-0.545 V SCE
30
28
Start
injection
Stop
• 65x increase in da/dt
26
24
-6
2.3x10 mm/s
• EAPP < ETrans (-0.5
VSCE)
• Effect is rapid (∼10
min)
1.5x10-4
Hole
22
20
0.6
0.8
1.0
1.2
Time, days
1.4
Injection Solution:
0.2 M AlCl3 + 1 M Cr6+ as
CrO3 + Na2CrO4, pH 3.1
Injecting Aggressive Solution at T6 Crack Tip
Initiates High-Rate da/dt
• EAPP < ETrans (-0.5 VSCE)
10-3
Crack Growth Rate (mm/sec)
T6 (L. Young '99)
10-4
10-5
• Competitive processes:
10-6
10-7
10-8
-0.7
• Incubation is related to
development of critical crack
tip environment, not time to
get to grain boundary or
critical EAPP
ETrans
-0.6
-0.5
-0.4
-0.3
Applied Potential (V vs. SCE)
– Al3+ Production (corr.) +
Hydrolysis + Migration (Cl-)
– Diffusion
Injection Solution:
0.2 M AlCl3 + 1 M Cr 6+ as
CrO3 + Na2CrO4 , pH 3.1
Aggressive Tip Chemistry Required
for High-Rate da/dt
Injecting corrosion inhibiting solution decreases da/dt
Crack Length, mm
23.4
o
7050-T6 @ 121 C (50-111)
0.5 M Na2CrO4 + 0.05 M NaCl, pH 9.2
23.2
#2
Inj. #1
23.0
22.8
-0.445 V SCE
1.9x10-4
1.5x10-5
2x10-5
22.6
1x10-6
22.4
22
24
ETrans ~ -0.5 VSCE
26
Time, hours
28
30
Injection Solution:
0.5 M Na2CrO4, pH 9.2
Inj #1 = 390 µL
Inj #2 = 180 µL
Aggressive Tip Chemistry Required
for High-Rate da/dt
10-3
Crack Growth Rate (mm/sec)
ETrans
10-4
• Inj #1 - Transition
out of incubation
Inj #2
10-5
– Can fully suppress
transition
Inj #1
10-6
10-7
10-8
-0.7
LY T6 S-free
LY T6 S-rich
T6 @ 121 o C, S-rich (50-109)
T6 @ 121 o C, S-rich (50-110)
T6 @ 121 o C, S-rich (50-111)
-0.6
-0.5
Injection Solution:
0.5 M Na2CrO4
pH 9.2
-0.4
Applied Potential (V vs. SCE)
-0.3
• Inj #2 - Fully
established, highrate da/dt
– Did not fully
suppress transition
Crack Growth Rate = f (Crack Tip [Al3+], pH)
• Critical chemistry
• Aggressiveness of
crack tip solution
– Dissolution /
repassivation
kinetics
– Hydrogen
production /
absorption
Crack Growth Rate, mm/s
– pH < 4
– Al3+ > 0.2 M
CGR vs. Crack Tip pH
10-3
0.5 M Na 2CrO4 + 0.05 M NaCl, pH 9.2
K = 10 to 15 MPa m 1/2, OCP = -950 to -850
Potentials in mV vs. SCE
10-4
Bulk
OCP
10-5
10-6
Incubation
10-7
AA7075-T6, 1 M NaCl (Nguyen '82)
AA7050-T651
AA7050 Near T651 (6hr @ 163oC)
AA7050 Slightly OA (12hr @ 163oC)
10-8
0
2
4
6
8 10
Crack Tip pH
12
14
Probing the Potential Dependence:
In Situ Crack Potential Measurements
• Net anodic current
mini-reference
probe hole
12.7 mm
Crack growth direction
Crack Potential, V vs. SCE
• Low TIP Potential
-0.4
-0.5
Applied Potential
-0.6
-0.7
-0.8
-0.9
0
2
4
6
8 10
Distance to Crack Tip, mm
In Situ Crack Potential Measurements
• Crack wake is fully polarized
• Local potential is complex f (EAPP and Distance
to tip)
Distance to crack tip = 10.5 mm
micro-reference
probe hole
12.7
mm
Potential, V vs. SCE
-0.3 6 hr @ 163 oC
-0.4
OCP
-0.5
-0.6
-0.7
Dist. to crack tip = 1.0 mm
-0.8
Crack growth
direction
0
60
120
Time, minutes
180
In-Situ Crack Potential Measurements
• Crack wake fully polarized
Crack Potential, mVSCE
– Define wake as > 3 mm from tip
-400 Distance from crack tip
at time of measurement
in mm
-500
4.90
-600
14.35
Crack Wake
-700
-800
4.78
No IR Drop
-900
-800
-700
-600
-500
-400
Applied Potential, mV SCE
Crack tip E independent or weakly
dependent on EAPP
If ETIP ≠ f (EAPP), as ↑ EAPP , ↑ ∇E è ↑ [X+/-] at tip
Crack Potential, mVSCE
-400
-500
Distance from crack tip 14.35
at time of measurement
in mm
4.90
-600
-700
3.36
4.77
No IR Drop
0.45
-800
0.31
-900
-800
0.80
0.81
0.34 0.48
0.26
-700
-600
-500
-400
Applied Potential, mVSCE
As crack
grows,
probe is
further in
wake
Modeling – Crack Potential
∆rj 1
Rj =
Aj κ j
∆V j = I j ⋅ R j
• Steady State
• Assumptions
j
I j = I Tip + 2 ∑ iWall , x ⋅ AWall , x
– Constant κ
– Equal D for all
species
x =0
I Tip
da
M
=
⋅
dt diss nFρ B ⋅ wTip
B
Tip corr.
height, wTip
iWall,j
CMOD
ITip
θ
L
rj
z
CTOD
x
y
How Wide are Crack Tips?
In Situ Bolt-loaded & Measured CSOD
• TG Fatigue → Blunt tip
• IG EAC → Sharp tip
K = 13 MPa√m
Fatigue
EAC
203 nm
2.8 µm
B. Gable
D. Hass
Crack Tip Shapes Modeled
Crack Opening Displacement, µm
5
Blunt
4
Dean and Hutchinson
3
2
250 nm
Sharp, δ Tip =
50 nm
Sharp, δ Tip =
1
0
0
20
40
60
Distance from crack tip, µm
80
100
Exptl. Air Fatigue δ
Exptl. Air Fatigure δ - Fit (r2 = 0.92)
Exptl. Corrosion Fatigue δ
Exptl. Corrosion Fatigue δ - Fit (r2 = 0.92)
Exptl. EAC δ
2
Exptl. EAC δ - Fit (r = 0.999)
Calc. δ for quais-static crack (Dean et. al.)
Calc. δ for static crack (Anderson)
50
• Air Fatigue
reasonable match to
blunt crack per
EPFM
• Corrosion Fatigue
matches trapezoid
with 2 µm tip
• IGSCC crack is
very, very tight
40
35
30
25
20
15
10
5
0
0
500
1000
1500
Distance from Crack Tip, µm
2000
20
30
40
50
60
70
Distance from Crack Tip, µm
80
2500
8
Crack Opening Displacement (δ), µm
Fit of Crack
Shapes to Models
Crack Opening Displacement (δ), µm
45
7
6
5
4
3
2
1
0
0
10
90
100
Modeling Crack E - CTOD and iwall
Model CTOD
50 nm
250 nm
E-P Analysis (~2 µm)
• CTOD contols ECrack
-0.9
0
-0.8
Iwall = 0
-0.8
-0.6
Iwall = 5 µ A/cm2
CTOD = 250 nm
-0.7
ETip, VSCE
• iwall = 5
increases IR-drop ~
10%
-0.6
i Wall = 0
µA/cm2
-0.5
Crack Potential, V vs. SCE
– “Blunt” CTOD does
not give match to
E(x)
– Sharp (50 - 250 nm)
CTOD produces
steep ∇E
-1.0
1
2 3 4 5 6 7
8
Distance from Crack Tip, mm
9
2-D Simulation of Near Tip
For CTOD of interest, centerline position can be approximated by 1-D
1.0 -0.70
-0.70
1.0 -0.70
Potential, V vs. SCE
CTOD = 50 nm
0.9
0.9
-0.70
CTOD = 5 µm
Potential, V vs. SCE
-0.72
-0.72
0.8
0.7
-0.72
0.6
0.5
0.4
-0.74
-0.74
0.3 -0.72
0.2-0.70
0.1
0.0
-6
-0.72
-0.76
-0.76
-0.78
-0.80
-0.82
-0.84
-0.86
-0.78
-0.80
-0.82
-0.84
-0.86
-4
-0.88
-2
0
2
Distance from centerline, mm
4
-0.70
Distance from crack tip, mm
Distance from crack tip, mm
0.8
-0.74
0.7
-0.76
0.6
-0.78
0.5
-0.80
-0.74
-0.74
0.4
-0.82
-0.82
-0.76
-0.84
-0.72
0.2 -0.78
-0.84
-0.72
-0.78
-0.86
-0.86
0.3
0.1
-0.76
-0.80
-0.80
-0.88
0.0
6
-6
-4
-2
0
2
Distance from centerline, mm
4
6
Validation of 1-D Model for
Centerline
-0.65
-0.70
Potential, V vs. SCE
CTOD (δ
δ) = 50 nm
200 nm
-0.75
1 µm
5 µm
-0.80
-0.85
1-D Model
2-D Model at Centerline
-0.90
-0.95
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Distance from crack tip, mm
0.9
1.0
1.1
Modeling Crack Potential –
Film Effects
-0.4
– Must be thin (10’s nm)
– Salt film (AlCl3-like)
• Moderate κ ∼ 10-2
– 100’s µm
– Al-Cl-OH
– H2 bubbles (Pickering
‘84)
-0.5
Crack Potential, V vs. SCE
• Low κ < 10-4 1/Ω-cm)
-0.6
-0.7
Constant κ = 0.0614 1/Ω-cm
20 nm film κ = 0.0001 1/Ω-cm
1 µm film κ = 0.01 1/Ω-cm
1 µm film κ = 0.0001 1/Ω-cm
10 µm film κ = 0.01 1/Ω-cm
10 µm film κ = 0.0001 1/Ω-cm
100 µm film κ = 0.01 1/Ω-cm
100 µm film κ = 0.0001 1/Ω-cm
-0.8
-0.9
-1.0
-1
0
1
2
3
4
5
6
7
8
Distance from Crack Tip, mm
9
10
Crack Potential at 1 mm from Tip, V vs. SCE
-0.50
-0.55
-0.60
-0.70
-0.75
100 µm film κ = 0.01 1/Ω-cm
10 µm film κ = 0.0001 1/Ω-cm
10 µm film κ = 0.01 1/Ω-cm
1 µm film κ = 0.0001 1/Ω-cm
1 µm film κ = 0.01 1/Ω-cm
20 nm film κ = 0.0001 1/Ω-cm
100 µm film κ = 0.0001 1/Ω-cm
-0.65
Constant κ = 0.0614 1/Ω-cm
Modeling Crack Potential – Film Effects
Range of
Experimental
Observations
Modeling Crack Potential – Film Effects
-0.4
Crack Potential, V vs. SCE
-0.5
-0.6
-0.7
7050-T651, EApp = -0.495 V SCE
7050, 12 hr/163 C, -0.545
7050, 12 hr/163 C, -0.495
7050, 12 hr/163 C, -0.445
7050, 6 hr/163 C, -0.495
Constant κ = 0.0614 1/Ω-cm
First 2 mm κ = 0.10 1/Ω-cm
First 2 mm κ = 0.030 1/Ω-cm
-0.8
-0.9
-1.0
-1
0
1
2
3
4
5
6
7
Distance from Crack Tip, mm
8
9
10
-0.4
Crack Tip κ < Bulk κ
Potential, V vs . SCE
7050-T651, -0.495, 2x10-5
o
-7
7050, 12 hr/163 C, -0.545, 6x10
o
-7
7050, 12 hr/163 C, -0.495, 9x10
o
-6
7050, 12 hr/163 C, -0.445, 5x10
o
-5
7050, 6 hr/163 C, -0.445, 4x10
1-D Model, κ = 0.1 1/Ω-cm
1-D Model, κ = 0.01 1/Ω-cm
1-D Model, κ = 0.001 1/Ω-cm
-0.7
-0.8
-1.0
0
1
2
3
4
5
6
7
8
9
10
8
9
10
Distance from Crack Tip, mm
5.0
[Al3+] solubility in 1 M Cr(VI)
κ = 0.1 1/Ω-cm
κ = 0.01 1/Ω-cm
κ = 0.001 1/Ω-cm
4.5
4.0
3+
– Due to higher I
needed for ∆E
-0.6
-0.9
[Al V], vs.
M SCE
Potential,
• implies salt
film/H2 bubbles at
tip
• Higher κ leads to
ppt of AlCl3 for
over 1 cm from tip
-0.5
3.5
δTip = 200 nm
3.0
-7
iWall = 2x10 A/cm
ETip = -0.90 V SCE
2.5
2
2.0
1.5
1.0
0.5
0.0
0
1
2
3
4
5
6
7
Distance from Crack Tip, mm
So What
• Crack tip must be small in order to achieve
the sharp E-gradient near the tip
– Predicted CTOD ~ 250 nm,
– EPFM ~ 5 µm
– Measured CSOD ~ 200 nm
• The crack tip must be saturated over some
distance << 1 mm
– Not anhydrous AlCl3 – conductivity too low
– Saturated Al-Cl-OH-Cr(VI) = upper bound on κ
Implications - Crack Growth by AD
• Calculated da/dt due to tip dissolution
– Tip corrosion height < 100 nm (Newcomb et. al.,
2000)
– Effective crack κ - H2 (Pickering, 1984)
Fraction of observed da/dt due to tip dissolution
Effective
Crack
Conductivity,
1 /Ω -c m
0.1
0.01
0.001
5x10-5
Observed da/dt =
κBULK = 0.065 1/Ω-cm
Tip Corrosion Height,
nm
50
200
500
>> 10
5.6
0.4
> 10
1.4
0.1
> 5
0.6
0.04
mm/s
Model parameters: CTOD = 200
nm, CMOD = 110 µm, L = 17
mm, iwall = 1 µA/cm2, ∆E = 0.4 V
Summary of AA7050-T6 EAC Tip Conditions
• Crack tip likely has gelatinous Al-OH-Cl-Cr(VI) film
• Active crack tip potential is approximately -0.9 VSCE
– independent of the external potential
– majority of IR drop confined to the first 3 mm from the crack tip
• indicative of net anodic current in the near-tip region
– ∇E support [X+/-] at tip
• The CTOD for IG EAC is approximately 200 nm
– 10x smaller than for fatigue cracking
– Poorly characterized by EPFM analysis of a moving crack
• Quantifying dissolution at tip needs two key unknowns:
– effective crack conductivity
– tip corrosion height