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Discrete Pits IGC

Corroded LS surfaces(top) and their corresponding fracture surfaces (bottom)

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The Effect of Galvanically Induced Corrosion Damage on the Fatigue Crack Formation Behavior of AA 7050-T7451

Noelle Easter Co and Prof. James BurnsCenter for Electrochemical Science and Engineering

Motivation

Knowledge Gaps

Objectives

The use of stainless steel fasteners in aircraft with aluminum substructurecreates a galvanic couple when exposed to atmospheric conditions,leading to the formation of galvanic corrosion damage.

Collaborative research program is in place to quantify, understand andmodel this behavior.

1. Characterize the corrosion damage induced under electrochemicalcondition representative of a galvanic crevice in atmosphericconditions

2. Quantify the crack formation and small crack growth behavior fordifferent galvanically induced corrosion morphologies

3. Identify the salient features of the corrosion damage that govern thecrack formation behavior for each morphology

1. How do corrosion morphologies typical of galvanic couples influenceoverall fatigue life behavior in AA 7050-T7451?

2. What features of the corrosion morphology influence the fatigue crackformation and small crack growth behavior of AA 7050-T7451?

Water layer

Stainless steelfastener

AA 7050 - T7451substructure

Primer/Top Coating

Pit depth does not dictate the location of the initiation site, this points toward a strong effect of micro-geometry or

microstructure.

Experimental Approach

Step 1: Geometry dependent modeling todetermine the the chemistry, pH andpotential distribution for a AA 7050-CRES304 galvanic couple (C Liu/RG Kelly)

Step 2: Study the microstructure interactionsand establish the corrosion morphologyassociated with these conditions. (V Rafla/JRScully) Surface Recession

SAMPLE PREPARATIONAA7050-T7451 fatigue

specimens polished to 600 grit

CORROSION GENERATION2mm x 2mm area in the

reduced-gage section (LS surface) of the fatigue

specimen exposed to different electrochemical conditions

IMAGE ANALYSIS3D profile and top view of

generated pits obtained using interferometer and optical

microscope

FATIGUE TESTSpecimens with pits in the

reduced-gage section subjected to fatigue test with a pre-

determined loading protocol at 90% relative humidity

FRACTOGRAPHYFracture surfaces investigated

using the scanning electron microscope

1.5-hour potentialhold at -700 mVwith 0.5 M NaCl +8x10-4 M NaAlO2

(pH 8) generatesdiscrete pits.

72-hour potentialhold at -700 mVwith 0.5 M NaCl +8x10-4 M NaAlO2

(pH 8) generatessurface recession.

7-day hold inside theRH chamber at 96%RH and 30oC withdroplet of 1 M NaCl +0.022 M AlCl3 + 0.05M K2S2O8 on top ofthe exposed areagenerates IGC.

Results

Element Al Zn Cu Mg Zr Fe Si Ti

Wt % Balance 6.1 2.2 2.2 0.11 0.08 0.04 0.02

AA 7050- T7451 Composition:

DATA ANALYSISda/dN vs crack length (a) plot

determined using marker band spacing

3D profile obtained using whitelight interferometer

Top view obtained usingoptical microscope

LOADING PROTOCOL:

Constant maximum stress: 200 MPaBaseline cycle: R=0.5, f=20 HzMarker cycle: R=0.1, f=10 Hz

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Load induced fracture marks (marker bands) are produced onthe fracture surface.

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Conclusions

References

Future Work1. Successfully developed and characterized corrosion damage typical

of the galvanic couples

2. Crack formation life and feature size dominate the total fatigue life;morphology influence on small scale crack propagation diminishesafter 50-100 μm beyond corrosion damage.

3. Macro-scale corrosion features do not fully capture the crackformation behavior; 2D-3D techniques have been successfullyutilized to characterize micro-geometry features of surfacerecession/pits.

4. XCT and EBSD techniques have been initially employed tocharacterize IGC morphology and to identify microstructurefeatures pertinent to crack formation.

Acknowledgement

1. Use x-ray computedtomography (XCT) to locatesecondary cracks andconstituent particles withrespect to the corrosiondamage (particularly for IGC)

Grain width:L: 22-1230 μmS: 12-112 μmT: 14-264 μm

Fatigue specimen loaded inhydraulic frame with flexi-glasschamber to control humidity;loading direction is along L.

Fatigue specimen

XCT image(top), fracturesurface(bottom left),EBSD image

(bottom right)

This work is funded by the US Office of Naval Research (B. Nickerson).1. 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. 104-121.2. 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.

Discrete Pits

Top view images of the corroded LS surfaces using the white light interferometer (top) and opticalmicroscope (bottom)

The white light interferometer is able to capture the 3D features as wellas the true depths of discrete pits and surface recession. However it isnot capable of determining the true depth of the IGC fissures.

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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 thedeepest pit for both discrete pits and IGC. However, among allinitiation sites (primary and secondary), the initiation site of theprimary crack is the deepest. Most secondary cracks initiated atshallow discrete pits (lower tail end of the histogram).

Step 3: Determine the influence of varying morphology on the fatigue behavior and structural integrity of AA 7050-T7451

Fracture surface with marker bands is used to quantify themicrostructurally small scale fatigue crack growth (left)

Plot of total fatigue life and initiation life to create a 10 umcrack size (right)

Samples with discrete pits have the highest total fatigue life, whereassamples with surface recession have the shortest total fatigue life.Samples with longer initiation life have higher total fatigue life.

Microstructurally small fatigue crack growth behavior becomes independent of the micro-feature when the crack

extends away from the initiation point.

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Crack length a (μm)

Crack growth rate vs crack length

A1 A2 A3

B1 B2 B3

C1 C2 C3

Surface Recession

Discrete Pits

IGC

The micro-feature of the crack initiation site for discrete pits andsurface recession corrosion damage can be characterized by thecombination of 2D and 3D imaging techniques. XCT will be used forIGC. Even for surface recession, cracks do not initiate at the deepestportion of the damage pit.

Combination of 2D and 3D imaging techniques is necessary to identify the micro-features where crack initiates.

Initiation site for primary crack

Initiation site for secondary crack

CORROSION DAMAGE CHARACTERIZATION

QUANTIFICATION OF CRACK FORMATION AND SMALL CRACK GROWTH BEHAVIOR

IDENTIFICATION OF CORROSION DAMAGE FEATURE

Plot of crack growth rate (da/dN) versus crack length (a) for all fatigue samples with crack lengthsobtained from marker band spacing

2. Use EBSD todetermine theinfluence ofcrystallographicorientation onthe crack growthbehavior

(1) SEM image of corroded surface (2) opticalimage of corroded surface (3) SEM image of thefracture surface (4) white light interferometerimage of the corroded surface (5) 3D image ofthe corrosion damage

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Once crack extends 50-100 μm beyond the corrosion damage, thegrowth rates merge and are consistent with each condition, thussupporting the conclusion that crack formation life and corrosionfeature depth dominate any secondary effect of crack propagationbehavior.

*Average pit depth where primarycrack initiates

*52 μm

*633 μm

*165 μm

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