Galvanic Corrosion Induced Fatigue Crack Initiation and Propagation Behavior in AA 7050-T7451

Noelle Easter Co, James T. BurnsCenter for Electrochemical Science and Engineering

Department of Materials Science and Engineering, University of Virginia

MotivationAluminum alloys are used for aerospace applications due to their high strength-to-weightratio. Assembly of complex structures requires the use of high strength stainless steelfasteners.

Knowledge Gaps1. 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?

Step 1: Geometry dependent modeling todetermine the the chemistry, pH and potentialdistribution for a AA 7050-CRES304 galvaniccouple (C Liu/RG Kelly)

Step 2: Study the microstructure interactions andestablish the corrosion morphology associatedwith these conditions. (V Rafla/JR Scully)

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

80%

20%

Features that Initiate Cracking from USAF Teardown1

Mechanical damage

Corrosion damage

Objectives

The electrochemical and mechanical interactions of these galvanic couples are poorlyunderstood. As such, a collaborative effort aims to quantify local galvanic environments,determine the corrosion morphology associated with such environments, and how suchmorphologies influence the fatigue behavior of AA 7050-T7451.

10-9

10-8

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-1.4

-1.3

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-1.0

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-0.2

E v

s S

CE

(V

)

i (A/cm2)

AA7050-T7541

Pure Al 99.99%

-phase Al7Cu

2Fe

S-phase Al2CuMg

Mg2Si

Pure Cu 99.99%

SHT 7050 480C 4hrs

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1M NaCl pH:7 (not adjusted)

1M NaCl pH:8

1M NaCl pH:9

1M NaCl pH:10

E v

s. S

CE

(V

)

i (A/cm2)

SS

AA7050

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 kinetics4. Correlate crack formation with features of the corrosion morphology

AA 7050-T7451

Element Al Zn Cu Mg Zr Fe Si Ti

Wt % Balance 6.1 2.2 2.2 0.11 0.08 0.04 0.02Grain width:L: 22-1230 μmS: 12-112 μmT: 14-264 μm

Experimental Approach

Sample Preparation

AA7050-T7451 fatigue specimens polished to 600

grit

Corrosion Generation

4 mm2 area in the reduced gage section (LS surface)

exposed to electrochemical conditions

Image Analysis

3D profile and top view of generated pits obtained using interferometer and

optical microscope

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

Data Analysis

da/dN vs crack length (a) determined using the marker

band spacing

1.5-hour or 5-hour potentialhold at -700 mV with 0.5 MNaCl + 8x10-5 M NaAlO2 (pH 8)

72-hour potential hold at -700mV with 0.5 M NaCl + 8x10-5

M NaAlO2 (pH 8)

168-hour hold inside the RHchamber at 96% RH and 30oCwith droplet of 1 M NaCl +0.022 M AlCl3 + 0.05 MK2S2O8 on top of the exposedarea

Discrete Pits:

Surface Recession:

Inter Granular Corrosion (IGC):

Fatigue specimen loaded in hydraulic frame withflexi-glass chamber to control humidity; loadingdirection is along L; Maximum load = 200 Mpa.

3D profile obtained usingwhite light interferometer

Top view obtained usingoptical microscope

Fracture surface containing marker bands

A galvanic couple betweenaluminum and stainless steel iscreated when the surface coatingsare breached enabling ingress oftrapped electrolyte. Such coupleslead to corrosion damage. Thestructural integrity of the aluminumcomponents are affected by thiscorrosion damage due to itspropensity to initiate fatiguecracks.2-4

T

S

L

3D microstructure cube: 2” plate (S/8) Composition:

Mechanical Properties:

T

S

L

Ultimate Tensile Strength: 530 MPaTensile Yield Strength: 470 MPaModulus of Elasticity: 71.7 GPa

KIC (S-L): 28 MPa√m

KIC (T-L): 31 MPa√m

KIC (L-T): 35 MPa√m

Galvanic Couple Critical Corrosion Sites

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Root Mean Square

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R1 Initiation

R2

R2 Initiation

R3

R3 Initiation

Results

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A1 A2 A3 B1 B2 B3 C1 C2 C3 D1 D2 D3

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Discrete Pit, 5 H Surface Recession, 72 HIGC, 168 H

Nu

mb

er

of C

ycle

s, N

Discete Pit, 1.5 H

Total cycles to failure

Initiation life to 10 um

Crack length, a (um)

0 500 1000 1500 2000 2500

da

/dN

(u

m/c

ycle

s)

1e-6

1e-5

1e-4

1e-3

1e-2

1e-1

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

References Acknowledgement

Conclusions Future Work

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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 location5. These efforts motivate the investigation of the microstructure and micro-scale

features.

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.

The broadly corroded surface are divided intosmaller areas. Individual root mean squares ofthe divisions are obtained to represent ameasure of surface roughness. Neither theroot mean square, nor peak density, maximumvalley depth, nor a combination of thesemetrics control the location of the crackinitiation.

Crack growth converge to comparable values away from the corrosion damage.

Combination of 2D and 3D imaging techniques allows the determination of

the exact crack initiation location.

Fissure depths were obtained using x-ray computed tomography. Fissure depths,total fissure length per plane, and the number of fissures per plane, as well as thecombination of their interaction do not dictate the location of the crack initiation.

Initiation life is strongly reduced tonear constant values after littlecorrosion damage but there is still asmall decrease in total life for thesurface recession.

Surface Recession

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.

Macro-scale features for different corrosion morphologies:Corrosion morphologies effect on fatigue:

1. 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. 187-207.2. 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).3. 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).4. 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).5. 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).6. 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).

The influence of damage size plateaus after a sharp initial decrease.

The starting size of the crack is much larger for corrosion damage with greater depths leading to the slightly lower lives for surface recession

Different corrosion morphologies areobtained by exposing the surface todifferent electrochemical conditions.

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Discrete Pit, 1.5 H Discrete Pit, 5 H Surface Recession, 72 H IGC, 168 H

Macro-scale features represented by the metrics above do not control the location of crack initiation.

*Dots on the graphs represent crack formation location

Root Mean Square

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Peak D

ensity

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R1

R1 Initiation

R2

R2 Initiation

R3

R3 Initiation

Dis

cret

e P

its

Surf

ace

Rec

essi

on

IGC

*53 μm

*165 μm*216 μm

*633 μm

*Average depth where primary crack initiates

Pit depths are obtained using the white light interferometer. Crack does not initiateat the deepest pit (or largest area or largest volume). Pit densities represent themeasure of probability of pit interaction. Crack does not initiate at the location ofthe densest area.