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
120
1 mm
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
0.1 mm
50
0.1 mm
µm
Discrete Pit, 1.5 H
Discrete Pit, 5 H
100
300
70
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
µm
Total cycles to failure
Initiation life to 10 m
1600000
1400000
*Average depth where
primary crack initiates
1200000
Number of cycles, N
120
1000000
53 m
800000
600000
165 m
400000
216 m
200000
633 m
0
50
100
150
200
0
250
Pit depths are obtained using the white light interferometer. Crack does not initiate
at the deepest pit (or largest area or largest volume). Crack does not initiate at the
location of the densest area.
500
250
R1
R1 Initiation
R2
R2 Initiation
R3
R3 Initiation
200
150
Maximum Area Valley Depth
0
0
Peak Density
0.75 mm
Surface Recession
150
Discrete Pits
60
100
50
400
300
200
R1
R1 Initiation
R2
R2 Initiation
R3
R3 Initiation
100
0
0
0
50
100
150
200
Root Mean Square
250
0
50
100
150
200
250
Root Mean Square
300
P1 P2 P3
350
400
450
A1 A2 A3
B1 B2 B3
C1 C2 C3
D1 D2 D3
The broadly corroded surface are divided into smaller areas. Individual root mean
Initiation life is strongly reduced to squares of the divisions are obtained to represent a measure of surface roughness.
near constant values after little Neither the root mean square, nor peak density, maximum valley depth, nor a
corrosion damage but there is still a combination of these metrics control the location of the crack initiation.
small decrease in total life for the
surface recession.
The influence of damage size plateaus
after a sharp initial decrease.
Pristine
500
550
600
µm
Discrete Pit, 5 H Surface Recession, 72 H
IGC
Combination of 2D and 3D imaging
techniques allows the determination of
the exact crack initiation location.
Discrete Pit, 1.5 H IGC, 168 H
1e-1
da/dN (um/cycles)
1e-2
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
Average Depth:
633 μm
1e-5
216 μm
165 μm
53 μm
1e-6
0
500
1000
1500
2000
2500
Crack length, a (um)
Fissure depths were obtained using x-ray computed tomography. Fissure depths,
total fissure length per plane, and the number of fissures per plane, do not dictate
the location of the crack initiation. Combination of total fissure depth and number
of fissures per plane tend to correlate with crack initiation.
Macro-scale features represented by the metrics above do not control the
location of crack initiation except for the combination of total fissure depth and
number of fissures per plane for IGC.
On-going
The starting size of the crack is much larger for
corrosion damage with greater depths leading to 1. Determination of constituent particle
the slightly lower lives for surface recession
location and distribution with respect
Crack growth converge to comparable values away from the corrosion damage.
to crack initiation point using XCT and
SEM
back
scatter
imaging
Conclusions
2.
Characterization
of
the
local
geometry
1. Corrosion morphology characterization and combinations of unique imaging
of
the
corrosion
surface
using
XCT
and
techniques for the determination of crack initiation location are presented.
white
light
interferometer
2. The total fatigue life is highly influenced by the initiation life.
3.
Identification
of
grain
orientation
3. Microstructurally small fatigue crack growth behavior becomes independent of
effect
on
crack
growth
by
overlaying
the corrosion feature when the crack extends away from the initiation point.
marker
bands
on
the
EBSD
images
of
4. Severe “macro-scale metrics” do not correlate with the observed crack
the
fracture
surface
formation location except for the combination of total fissure depth and number
4.
Identification
of
local
plasticity
effects
of fissures per plane for IGC.
on
the
location
of
fatigue
crack
5. These efforts motivate the investigation of the microstructure and micro-scale
initiation
(Collaboration with M. Sangid, Purdue)
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).
work
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.