Galvanic Corrosion Induced Fatigue Crack Initiation and Propagation Behavior in AA 7050-T7451
Noelle Easter Co, James T. Burns
Center for Electrochemical Science and Engineering
Department of Materials Science and Engineering, University of Virginia
Motivation
Experimental Approach
Aluminum alloys are used for aerospace applications due to their high strength-to-weight
ratio. Assembly of complex structures requires the use of high strength stainless steel
fasteners.
A
galvanic
couple
between
Features that Initiate Cracking
Galvanic Couple
aluminum
and
stainless
steel
is
1
from USAF Teardown
Critical Corrosion Sites
created when the surface coatings
are breached enabling ingress of
Mechanical
damage
trapped electrolyte. Such couples
lead
to
corrosion
damage.
The
20%
structural integrity of the aluminum
components
are
affected
by
this
80%
corrosion damage due to its
Corrosion
propensity to initiate fatigue
2-4
damage
cracks.
The electrochemical and mechanical interactions of these galvanic couples are poorly
understood. As such, a collaborative effort aims to quantify local galvanic environments,
determine the corrosion morphology associated with such environments, and how such
morphologies influence the fatigue behavior of AA 7050-T7451.
Step 1:
Geometry dependent modeling to
determine the the chemistry, pH and potential
distribution for a AA 7050-CRES304 galvanic
couple (C Liu/RG Kelly)
Step 2: Study the microstructure interactions and
establish the corrosion morphology associated
with these conditions. (V Rafla/JR Scully)
AA7050-T7541
Pure Al 99.99%
-phase Al7Cu2Fe
0.0
SS
-0.2
1M NaCl pH:7 (not adjusted)
-0.2
1M NaCl pH:8
-0.3
S-phase Al2CuMg
1M NaCl pH:9
-0.4
1M NaCl pH:10
Mg2Si
-0.5
-0.6
Pure Cu 99.99%
SHT 7050 480C 4hrs
-0.6
E vs SCE (V)
E vs. SCE (V)
-0.4
-0.8
-1.0
AA7050
-1.2
-0.7
-0.8
T
Sample Preparation
AA7050-T7451 fatigue
specimens polished to 600
grit
Corrosion Generation
2
mm
4
area in the reduced
gage section (LS surface)
exposed to electrochemical
conditions
3D profile and top view of
generated pits obtained
using interferometer and
optical microscope
-1.0
-1.1
-1.6
-1.2
-1.3
-1.8
S
Discrete Pits:
1.5-hour or 5-hour potential
hold at -700 mV with 0.5 M
NaCl + 8x10-5 M NaAlO2 (pH 8)
Surface Recession:
72-hour potential hold at -700
mV with 0.5 M NaCl + 8x10-5
M NaAlO2 (pH 8)
Inter Granular
Corrosion (IGC):
168-hour hold inside the RH
chamber at 96% RH and 30oC
with droplet of 1 M NaCl +
0.022 M AlCl3 + 0.05 M
K2S2O8 on top of the exposed
area
Image Analysis
-0.9
-1.4
L
3D profile obtained using
white light interferometer
Top view obtained using
optical microscope
-1.4
-12
10
-11
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
-9
10
-8
10
-7
-6
10
-5
10
10
-4
10
-3
10
2
i (A/cm )
2
i (A/cm )
Step 3: Determine the influence of varying morphology on the fatigue behavior
and structural integrity of AA 7050-T7451
Knowledge Gaps
1. How do corrosion morphologies typical of galvanic couples influence overall fatigue life
behavior in AA 7050-T7451?
2. What features of the corrosion morphology influence the fatigue crack formation and
small crack growth behavior of AA 7050-T7451?
Objectives
1. Develop a corrosion protocol that will produce damage replicating corrosion
morphologies in aluminum galvanically coupled with stainless steel
2. Quantify fatigue behavior (total fatigue life, initiation life) associated with the corrosion
morphology
3. Determine the effects of corrosion morphology on small crack kinetics
4. Correlate crack formation with features of the corrosion morphology
Fatigue Test
Specimens with pits in the
reduced gage section
subjected to fatigue test with
a pre-determined loading
protocol at 90% RH
Fractography
Fracture surfaces
investigated using the
scanning electron
microscope
Fatigue specimen loaded in hydraulic frame with
flexi-glass chamber to control humidity; loading
direction is along L; Maximum load = 200 Mpa.
AA 7050-T7451
3D microstructure cube: 2” plate (S/8)
T
S
L
Grain width:
L: 22-1230 μm
S: 12-112 μm
T: 14-264 μm
Composition:
Element
Wt %
Al
Balance
Zn
6.1
Cu
2.2
Mechanical Properties:
Ultimate Tensile Strength: 530 MPa
Tensile Yield Strength: 470 MPa
Modulus of Elasticity: 71.7 GPa
Mg Zr
2.2 0.11
Fe
0.08
Si
0.04
Ti
0.02
KIC (S-L): 28 MPa√m
KIC (T-L): 31 MPa√m
KIC (L-T): 35 MPa√m
Data Analysis
da/dN vs crack length (a)
determined using the marker
band spacing
Fracture surface containing marker bands
Results
Corrosion morphologies effect on fatigue:
Macro-scale features for different corrosion morphologies:
0
0
0
*Dots on the graphs represent crack formation location
20
40
50
50
80
100
100
100
0.5 mm
µm
µm
0
µm
0
10
10
20
30
20
20
100
40
150
40
30
50
60
200
50
80
250
60
40
50
µm
Discrete Pit, 1.5 H
0.1 mm
Discrete Pit, 5 H
350
80
0.1 mm
0.1 mm
µm
µm
Surface Recession, 72 H
Different corrosion morphologies are
obtained by exposing the surface to
different electrochemical conditions.
IGC, 168 H
*53 μm
120
µm
Total cycles to failure
Initiation life to 10 um
800000
Number of Cycles, N
0.1 mm
100
300
70
*Average depth where
primary crack initiates
600000
400000
*165 μm
*216 μm
*633 μm
200000
0
50
100
150
200
250
0
300
350
400
A1 A2 A3
450
B1 B2 B3
C1 C2 C3
D1 D2 D3
500
550
Discete Pit, 1.5 H
600
IGC, 168 H
Discrete Pit, 5 H
Surface Recession, 72 H
µm
Initiation life is strongly reduced to
near constant values after little
corrosion damage but there is still a
small decrease in total life for the
surface recession.
The influence of damage size plateaus
after a sharp initial decrease.
Combination of 2D and 3D imaging
techniques allows the determination of
the exact crack initiation location.
1e-1
250
R1
R1 Initiation
R2
R2 Initiation
R3
R3 Initiation
200
150
100
50
0
0
50
100
150
200
250
Root Mean Square
500
The broadly corroded surface are divided into
smaller areas. Individual root mean squares of
the divisions are obtained to represent a
measure of surface roughness. Neither the
root mean square, nor peak density, maximum
valley depth, nor a combination of these
metrics control the location of the crack
initiation.
400
300
200
R1
R1 Initiation
R2
R2 Initiation
R3
R3 Initiation
100
0
0
50
100
150
200
250
Root Mean Square
1e-3
Discrete Pit, 1.5H R1
Discrete Pit, 1.5H R2
Discrete Pit, 1.5H R3
Discrete Pit, 5H R1
Discrete Pit, 5H R2
Discrete Pit, 5H R3
Surface Recession, R1
Surface Recession, R2
Surface Recession, R3
IGC, R1
IGC, R2
IGC, R3
1e-4
1e-5
IGC
da/dN (um/cycles)
1e-2
Pit depths are obtained using the white light interferometer. Crack does not initiate
at the deepest pit (or largest area or largest volume). Pit densities represent the
measure of probability of pit interaction. Crack does not initiate at the location of
the densest area.
Surface Recession
Peak Density
0
0
Maximum Area Valley Depth
0.75 mm
120
1 mm
Surface Recession
150
Discrete Pits
60
1e-6
0
500
1000
1500
2000
2500
Crack length, a (um)
Fissure depths were obtained using x-ray computed tomography. Fissure depths,
The starting size of the crack is much larger for
total fissure length per plane, and the number of fissures per plane, as well as the
corrosion damage with greater depths leading to combination of their interaction do not dictate the location of the crack initiation.
the slightly lower lives for surface recession
Macro-scale features represented by the metrics above do not control the
Crack growth converge to comparable values away from the corrosion damage.
location of crack initiation.
Conclusions
1. Corrosion morphology characterization and combinations of unique imaging
techniques for the determination of crack initiation location are presented.
2. The total fatigue life is highly influenced by the initiation life.
3. Microstructurally small fatigue crack growth behavior becomes independent of
the macro-feature when the crack extends away from the initiation point.
4. Severe “macro-scale metrics” do not correlate with the observed crack
formation location
5. These efforts motivate the investigation of the microstructure and micro-scale
features.
References
1.
2.
3.
4.
5.
6.
G.A. Shoales, S.A. Fawaz, M.R. Walters, in: M. Bos (Ed.) ICAF 2009 - Bridging the Gap Between Theory and Operational Practice, Springer, Rotterdam, The Netherlands, 2009, pp. 187-207.
Burns, J. T., Larsen, J. M. & Gangloff, R. P. Driving forces for localized corrosion-to-fatigue crack transition in Al-Zn-Mg-Cu. Fatigue Fract. Eng. Mater. Struct. 34, 745–773 (2011).
Sankaran, K. K., Perez, R. & Jata, K. V. Effects of pitting corrosion on the fatigue behavior of aluminum alloy 7075-T6: modeling and experimental studies. Mater. Sci. Eng. A 297, 223–229 (2001).
Gruenberg, K. M., Craig, B. a., Hillberry, B. M., Bucci, R. J. & Hinkle, a. J. Predicting fatigue life of pre-corroded 2024-T3 aluminum. Int. J. Fatigue 26, 629–640 (2004).
Burns, J. T., Larsen, J. M. & Gangloff, R. P. Effect of initiation feature on microstructure-scale fatigue crack propagation in Al-Zn-Mg-Cu. Int. J. Fatigue 42, 104–121 (2012).
Spear, A. D., Li, S. F., Lind, J. F., Suter, R. M. & Ingraffea, A. R. Three-dimensional characterization of microstructurally small fatigue-crack evolution using quantitative fractography combined with post-mortem X-ray
tomography and high-energy X-ray diffraction microscopy. Acta Mater. 76, 413–424 (2014).
Future Work
1. Determination of constituent particle location with
respect to crack initiation point using XCT and SEM
back scatter imaging
2. Characterization of the local geometry of the
corrosion surface using XCT and white light
interferometer
3. Identification of grain orientation effect on crack
growth by overlaying marker bands on the EBSD
images of the fracture surface.
Acknowledgement
This project is funded by the Office of Naval Research (grant number N00014-14-1-0012) with
Mr. William Nickerson as the project officer. Special thanks to my adviser, Dr. James T. Burns,
and to the Center for Electrochemical Science and Engineering in UVA.