The Effect of Galvanically Induced Corrosion Damage on
the Fatigue Crack Formation Behavior of AA 7050-T7451
Noelle Easter Co and Prof. James Burns
Center for Electrochemical Science and Engineering
Motivation
Results
The use of stainless steel fasteners in aircraft with aluminum substructure
creates a galvanic couple when exposed to atmospheric conditions, T
leading to the formation of galvanic corrosion damage.
Collaborative research program is in place to quantify, understand and
model this behavior.
S
Step 2: Study the microstructure interactions
Step 1: Geometry dependent modeling to
determine the the chemistry, pH and
potential distribution for a AA 7050CRES304 galvanic couple (C Liu/RG Kelly)
and establish the corrosion morphology
associated with these conditions. (V Rafla/JR
Scully)
µm
0
L
100
CORROSION DAMAGE CHARACTERIZATION
S
T
µm
0
0
50
100
150
1 mm
Discrete
Pits
1 mm
200
µm
200
50
300
100
400
150
500
200
600
250
700
300
800
350
1 mm
900
Surface
Recession
NM
400
NM
IGC
1000
L
Discrete Pits
Water layer
IGC
T
Stainless steel
fastener
Primer/Top Coating
Surface Recession
Top view images of the corroded LS surfaces using the white light interferometer (top) and optical
microscope (bottom)
The white light interferometer is able to capture the 3D features as well
as the true depths of discrete pits and surface recession. However it is L
not capable of determining the true depth of the IGC fissures.
AA 7050 - T7451
substructure
S
Corroded LS surfaces(top) and their corresponding fracture surfaces (bottom)
QUANTIFICATION OF CRACK FORMATION AND SMALL CRACK
GROWTH BEHAVIOR
Step 3: Determine the influence of varying morphology on the
fatigue behavior and structural integrity of AA 7050-T7451
IDENTIFICATION OF CORROSION DAMAGE FEATURE
*52 μm
*Average pit depth where primary
crack initiates
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?
*165 μm
*633 μm
Objectives
Histogram of pit depths and crack initiation sites for discrete pits (left) and for IGC (right)
Taking all the pit depth measurements, cracks do not initiate at the
deepest pit for both discrete pits and IGC. However, among all
initiation sites (primary and secondary), the initiation site of the
primary crack is the deepest. Most secondary cracks initiated at
shallow discrete pits (lower tail end of the histogram).
1. Characterize the corrosion damage induced under electrochemical
Plot of total fatigue life and initiation life to create a 10 um
Pit depth does not dictate the location of the initiation site,
condition representative of a galvanic crevice in atmospheric
crack size (right)
conditions
this
points
toward
a
strong
effect
of
micro-geometry
or
Fracture surface with marker bands is used to quantify the
2. Quantify the crack formation and small crack growth behavior for
microstructure.
microstructurally small scale fatigue crack growth (left)
Initiation site for secondary crack
different galvanically induced corrosion morphologies
Samples with discrete pits have the highest total fatigue life, whereas
3. Identify the salient features of the corrosion damage that govern the samples with surface recession have the shortest total fatigue life.
crack formation behavior for each morphology
Samples with longer initiation life have higher total fatigue life.
(1)
(2)
0
50
100
150
200
250
300
Experimental Approach
AA7050-T7451 fatigue
specimens polished to 600 grit
Wt %
Balance
S
6.1
Mg
Zr
Fe
2.2 2.2 0.11 0.08
L
Fatigue specimen
Si
0.04
Ti
0.02
1.00E-03
1.00E-04
A1
Discrete Pits
A2
A3
Surface Recession
1.00E-05
B2
1 mm
1 mm
B3
(5)
IGC
C1
C2
C3
500
1000
1500
(1) SEM image of corroded surface (2) optical
image of corroded surface (3) SEM image of the
fracture surface (4) white light interferometer
image of the corroded surface (5) 3D image of
the corrosion damage
2000
Plot of crack growth rate (da/dN) versus crack length (a) for all fatigue samples with crack lengths
7-day hold inside the obtained from marker band spacing
RH chamber at 96%
RH and 30oC with Once crack extends 50-100 μm beyond the corrosion damage, the
droplet of 1 M NaCl + growth rates merge and are consistent with each condition, thus
0.022 M AlCl3 + 0.05
M K2S2O8 on top of supporting the conclusion that crack formation life and corrosion
the exposed area feature depth dominate any secondary effect of crack propagation
generates IGC.
behavior.
Microstructurally small fatigue crack growth behavior
becomes independent of the micro-feature when the crack
extends away from the initiation point.
3D profile obtained using white
light interferometer
Conclusions
Top view obtained using
optical microscope
LOADING PROTOCOL:
Constant maximum stress: 200 MPa
Baseline cycle: R=0.5, f=20 Hz
Marker cycle: R=0.1, f=10 Hz
Fatigue specimen loaded in
hydraulic frame with flexi-glass
chamber to control humidity;
loading direction is along L.
FRACTOGRAPHY
Fracture surfaces investigated
using the scanning electron
microscope
References
Load induced fracture marks (marker bands) are produced on
the fracture surface.
The micro-feature of the crack initiation site for discrete pits and
surface recession corrosion damage can be characterized by the
combination of 2D and 3D imaging techniques. XCT will be used for
IGC. Even for surface recession, cracks do not initiate at the deepest
portion of the damage pit.
Combination of 2D and 3D imaging techniques is necessary
to identify the micro-features where crack initiates.
Future Work
x-ray
computed
1. Successfully developed and characterized corrosion damage typical 1. Use
tomography (XCT) to locate
of the galvanic couples
secondary
cracks
and
2. Crack formation life and feature size dominate the total fatigue life;
constituent particles with
morphology influence on small scale crack propagation diminishes
respect to the corrosion
after 50-100 μm beyond corrosion damage.
damage (particularly for IGC)
3. Macro-scale corrosion features do not fully capture the crack
2. Use EBSD to
formation behavior; 2D-3D techniques have been successfully
determine the
utilized to characterize micro-geometry features of surface
influence
of
recession/pits.
crystallographic
4. XCT and EBSD techniques have been initially employed to
orientation on
characterize IGC morphology and to identify microstructure
the crack growth
features pertinent to crack formation.
behavior
DATA ANALYSIS
da/dN vs crack length (a) plot
determined using marker band
spacing
600
Initiation site for primary crack
B1
3D profile and top view of
generated pits obtained using
interferometer and optical
microscope
Specimens with pits in the
reduced-gage section subjected
to fatigue test with a predetermined loading protocol at
90% relative humidity
550
Crack length a (μm)
72-hour potential
hold at -700 mV
with 0.5 M NaCl +
8x10-4 M NaAlO2
(pH 8) generates
surface recession.
450
µm
0
1.5-hour potential
hold at -700 mV
with 0.5 M NaCl +
8x10-4 M NaAlO2
(pH 8) generates
discrete pits.
400
(3)
1.00E-06
IMAGE ANALYSIS
FATIGUE TEST
1 mm
500
CORROSION GENERATION
2mm x 2mm area in the
reduced-gage section
(LS surface) of the fatigue
specimen exposed to different
electrochemical conditions
1 mm
(4)
Grain width:
L: 22-1230 μm
S: 12-112 μm
T: 14-264 μm
S
L
350
1.00E-02
T
T
1.00E-01
da/dN, (um/cycle)
SAMPLE PREPARATION
AA 7050- T7451 Composition:
Element
Al
Zn Cu
Crack growth rate vs crack length
1.
2.
Burns, J.T., J.M. Larsen, and R.P. Gangloff, Effect of initiation feature on microstructure-scale fatigue crack propagation in Al–Zn–Mg–Cu. International Journal of
Fatigue, 2012. 42(0): p. 104-121.
Spear, A.D., Li, Shiu Fai, Lind, J.F., Suter, R.M. and 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 Materialia Inc.
1mm
XCT
image
(top), fracture
surface
(bottom left),
EBSD image
(bottom right)
Acknowledgement
This work is funded by the US Office of Naval Research (B. Nickerson).