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Journal of Materials Processing Technology 210 (2010) 1197–1202 Contents lists available at ScienceDirect Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec Effect of pitting corrosion on fatigue performance of shot-peened aluminium alloy 7075-T651 U. Zupanc a , J. Grum b,a Welding Institute, Ptujska 19, SI-1000 Ljubljana, Slovenia b University of Ljubljana, Faculty of Mechanical Engineering, Aˇ skerˇ ceva 6, SI-1000 Ljubljana, Slovenia article info Article history: Received 20 October 2009 Received in revised form 5 February 2010 Accepted 4 March 2010 PACS: 46.50 62.20.Me 81.40.Np 81.40.Ef Keywords: Aluminium alloys Pitting corrosion Fatigue Crack initiation abstract Pitting corrosion has a major influence on aging of structural elements made of high-strength aluminium alloys as corrosion pits lead to earlier fatigue crack initiation under tensile dynamic loading. A cause of fatigue crack initiation in a corrosive medium is a stress concentration at a corroded area. In order to improve material resistance to corrosion fatigue it is necessary to reduce pit-tip stresses. To eliminate or reduce pit stresses, cold surface hardening by shot peening was proposed. The objective of the present study was to investigate the effect of surface hardening by shot peening on electrochemical stability and corrosion fatigue properties of high-strength aluminium alloy 7075-T651 in the corrosive environment of a chloride solution. The results obtained show a favourable influence of shot-peening treatment on corrosion fatigue properties. Induced compressive residual stresses in the surface layer retard the initia- tion of fatigue cracks, and so the fatigue life improvement of structural elements made of high-strength aluminium alloys was observed. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Surface corrosion has a major influence on aging of structural elements made of high-strength aluminium alloys. Aluminium alloy 7075 in alloy system Al–Zn–Mg–Cu contains a high number of intermetallic particles, i.e. constituent particles, where hetero- geneity of a microstructure has an essential influence on corrosion properties. Birbilis et al. (2006) electrochemically analysed isolated constituent particles of 7075 aluminium alloy. Large constituent particles Al 7 Cu 2 Fe, Al 23 CuFe 4 , and Al 2 CuMg had different electro- chemical potential compared to the aluminium matrix. Due to electrochemical reactions in a corrosive environment, dissolved corroded areas lead to earlier fatigue crack initiation. Numerous studies relate corrosion surface damage to reduced material strength under dynamic loading. Pao et al. (2000) reported on the formation of fatigue cracks at corrosion pits on aluminium 7075-T7351. A decrease in the fatigue crack initiation threshold by 50% in axial fatigue testing was observed. Genel (2007) stated a degradation of the fatigue strength limit at 10 7 cycles by 60% of aluminium 7075-T6. Wang et al. (2003) investigated the effect Corresponding author. Tel.: +386 1 2809 442; fax: +386 1 2809 422. E-mail addresses: [email protected] (U. Zupanc), [email protected] (J. Grum). of pitting in high-cycle fatigue testing upto 10 8 cycles, where a decrease of the threshold stress intensity by about 20% was observed. DuQuesnay et al. (2003) stated that the depth of cor- rosion pits is the most important factor affecting a fatigue-exposed material. With longer exposure to a corrosive environment the pit depth increases. Jones and Hoeppner (2005) experimented a crit- ical pit depth for fatigue crack initiation measuring between 40 and 60 m. Furthermore, subsurface pit growth or tunnelling was reported. A cause of fatigue crack initiation in a corrosive medium is a stress concentration at pit locations. Sankaran et al. (2001) related a pit depth and crack growth rate under dynamic loading to a stress intensity factor K at aluminium 7075-T6 using AFGROW software. Pidaparti and Patel (2008), using FEM, made an analysis of local ten- sile stress environment around pits at aluminium 2024-T3. Stress levels at a pit area are in the magnitude of material plane strain fracture toughness, from which it is possible to estimate fatigue crack initiation. In order to improve material resistance to corrosion fatigue it is necessary to reduce pit-tip stresses in the corroded areas. Shot peening (SP) as intense plastic deformation in a thin surface layer affects fatigue properties by inducing favourable compressive residual stresses. Sharp and Clark (2001) showed that SP-treated specimens increased material resistance to crack initiation. Peen- ing improved fatigue life by a factor of 1.2–6 depending on choosing 0924-0136/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2010.03.004
6

Effect of Pitting Corrosion on Fatigue Performance of Shot-peened Aluminium Alloy 7075-T651

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Page 1: Effect of Pitting Corrosion on Fatigue Performance of Shot-peened Aluminium Alloy 7075-T651

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Journal of Materials Processing Technology 210 (2010) 1197–1202

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

journa l homepage: www.e lsev ier .com/ locate / jmatprotec

ffect of pitting corrosion on fatigue performance of shot-peened aluminiumlloy 7075-T651

. Zupanca, J. Grumb,∗

Welding Institute, Ptujska 19, SI-1000 Ljubljana, SloveniaUniversity of Ljubljana, Faculty of Mechanical Engineering, Askerceva 6, SI-1000 Ljubljana, Slovenia

r t i c l e i n f o

rticle history:eceived 20 October 2009eceived in revised form 5 February 2010ccepted 4 March 2010

ACS:6.502.20.Me1.40.Np1.40.Ef

a b s t r a c t

Pitting corrosion has a major influence on aging of structural elements made of high-strength aluminiumalloys as corrosion pits lead to earlier fatigue crack initiation under tensile dynamic loading. A cause offatigue crack initiation in a corrosive medium is a stress concentration at a corroded area. In order toimprove material resistance to corrosion fatigue it is necessary to reduce pit-tip stresses. To eliminate orreduce pit stresses, cold surface hardening by shot peening was proposed. The objective of the presentstudy was to investigate the effect of surface hardening by shot peening on electrochemical stability andcorrosion fatigue properties of high-strength aluminium alloy 7075-T651 in the corrosive environmentof a chloride solution. The results obtained show a favourable influence of shot-peening treatment oncorrosion fatigue properties. Induced compressive residual stresses in the surface layer retard the initia-tion of fatigue cracks, and so the fatigue life improvement of structural elements made of high-strengthaluminium alloys was observed.

eywords:luminium alloys

© 2010 Elsevier B.V. All rights reserved.

itting corrosionatiguerack initiation

. Introduction

Surface corrosion has a major influence on aging of structurallements made of high-strength aluminium alloys. Aluminiumlloy 7075 in alloy system Al–Zn–Mg–Cu contains a high numberf intermetallic particles, i.e. constituent particles, where hetero-eneity of a microstructure has an essential influence on corrosionroperties. Birbilis et al. (2006) electrochemically analysed isolatedonstituent particles of 7075 aluminium alloy. Large constituentarticles Al7Cu2Fe, Al23CuFe4, and Al2CuMg had different electro-hemical potential compared to the aluminium matrix. Due tolectrochemical reactions in a corrosive environment, dissolvedorroded areas lead to earlier fatigue crack initiation.

Numerous studies relate corrosion surface damage to reducedaterial strength under dynamic loading. Pao et al. (2000) reported

n the formation of fatigue cracks at corrosion pits on aluminium

075-T7351. A decrease in the fatigue crack initiation thresholdy 50% in axial fatigue testing was observed. Genel (2007) stateddegradation of the fatigue strength limit at 107 cycles by 60%

f aluminium 7075-T6. Wang et al. (2003) investigated the effect

∗ Corresponding author. Tel.: +386 1 2809 442; fax: +386 1 2809 422.E-mail addresses: [email protected] (U. Zupanc), [email protected]

J. Grum).

924-0136/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2010.03.004

of pitting in high-cycle fatigue testing upto 108 cycles, wherea decrease of the threshold stress intensity by about 20% wasobserved. DuQuesnay et al. (2003) stated that the depth of cor-rosion pits is the most important factor affecting a fatigue-exposedmaterial. With longer exposure to a corrosive environment the pitdepth increases. Jones and Hoeppner (2005) experimented a crit-ical pit depth for fatigue crack initiation measuring between 40and 60 �m. Furthermore, subsurface pit growth or tunnelling wasreported.

A cause of fatigue crack initiation in a corrosive medium is astress concentration at pit locations. Sankaran et al. (2001) relateda pit depth and crack growth rate under dynamic loading to a stressintensity factor �K at aluminium 7075-T6 using AFGROW software.Pidaparti and Patel (2008), using FEM, made an analysis of local ten-sile stress environment around pits at aluminium 2024-T3. Stresslevels at a pit area are in the magnitude of material plane strainfracture toughness, from which it is possible to estimate fatiguecrack initiation.

In order to improve material resistance to corrosion fatigueit is necessary to reduce pit-tip stresses in the corroded areas.

Shot peening (SP) as intense plastic deformation in a thin surfacelayer affects fatigue properties by inducing favourable compressiveresidual stresses. Sharp and Clark (2001) showed that SP-treatedspecimens increased material resistance to crack initiation. Peen-ing improved fatigue life by a factor of 1.2–6 depending on choosing
Page 2: Effect of Pitting Corrosion on Fatigue Performance of Shot-peened Aluminium Alloy 7075-T651

1198 U. Zupanc, J. Grum / Journal of Materials Proce

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Fig. 1. Fatigue specimen details and dimensions (in mm).

ifferent rework specifications. Rodopoulos et al. (2004) indicatedmproved fatigue life of peened aluminium 2024-T351 by about 90%n axially fatigue testing. Benedetti et al. (2009) improved fatiguetrength on aluminium 7075-T651 in reverse bending mode at 5illion cycles upto 46% by SP treatment.In the present research series of tests were performed to ana-

yze the effects of SP treatment on surface corrosion properties.he objective of the study was to investigate the effect of SP treat-ent on electrochemical stability and corrosion fatigue properties

f high-strength aluminium alloy 7075-T651 in chloride envi-onments. Fatigue behaviour of prior-corroded as-machined andP-treated specimens was evaluated to quantify the fatigue lifehanges. Characterization of the surface corrosion degradation andvaluation of pit-to-crack transition on fatigued specimens wereade. Additionally, electrochemical tests in a chloride solutionere conducted to investigate the effect of SP on surface corrosionroperties.

. Experimental details

.1. Material

A wrought plate of high-strength aluminium alloy075-T651 of 20 mm in thickness was delivered with thehemical composition (in wt.%): Al–5.78Zn–2.56Mg–1.62Cu–.21Cr–0.05Mn–0.04Ti–0.09Si–0.18Fe. Static mechanical proper-ies of the tested material were: Rm = 585 MPa, Rp02 = 532 MPand A50 = 12%. Specimens for fatigue testing were prepared inlong traverse (LT) direction (Fig. 1). As-machined specimensere ultrasonically cleaned in ethanol. In research the prepared

pecimens were evaluated in four different research combinations:a) as-machined (AM); (b) as-machined and corroded (AM & Corr);c) shot-peened (SP); and (d) shot-peened and corroded (SP &orr).

.2. Shot peening

The specimens were SP-treated from all sides at the Metalmprovement Company in Germany using an air-blast machine.ast steel-shot MI-170H with hardness of 55 HRC and a nominaliameter of 0.40 mm was chosen. In order to avoid medium colli-ion, the angle of nozzle inclination was shifted by 5◦ with regard tohe vertical axis. A constant specimen distance from the nozzle ofround 120 mm was maintained. Surface coverage was set to 150%.omparative Almen intensity value of 12A was achieved.

.3. Salt spray tests

The specimens for fatigue testing were placed in a salt sprayhamber for surface corrosion tests in accordance with ASTM B117or 168 h (7 days). Before the corrosion process, the SP-treated spec-mens were cleaned with concentrated HNO3 acid to avoid surface

ontamination by possible steel-shot residues. The prepared solu-ion had a pH value of 6.5 and the inner chamber temperature waset to 35 ◦C. Specimens were placed individually, parallel to eachther, on plastic supports at an angle of indentation of 20◦ withegard to the vertical axis. In order to provide uniform exposure of

ssing Technology 210 (2010) 1197–1202

the surface to pitting corrosion, the specimens were rotated every24 h during the test.

2.4. Surface integrity characterization

After the exposure of the specimens in the salt spray chamber,the specimen surface was covered with white corrosion productsconsisting mostly of Al(OH)3. For the analysis of surface roughnessand assessment of the pit size at the surface, the corrosion productswere cleaned with a hard polymer wire brush in a water-dilutednitric acid (∼10% HNO3) at room temperature. The evaluation of thetested specimens comprised surface properties and residual-stressmeasurement in the thin hardened layer. Measurement of high-resolution surface roughness was made with a Taylor Hubson FormTalysurf Series 2 device. The residual-stress measurements weremade with a semi destructive hole-drilling method in accordancewith ASTM 837.

2.5. High-cycle fatigue testing

Fatigue testing was carried out without preliminary removalof the corrosion products from the surface. Bending fatigue test-ing of the specimens was carried out with a Rumul Cracktronicdevice at room temperature. A constant amplitude bending stresswas applied in the range of the maximum applied stresses, i.e.those ranging between 15% and 65% of delivered-material ten-sile strength Rm. The testing resonant stress frequency was 107 Hzusing a sinusoidal waveform at a stress ratio R of 0.05. A criterionof specimen failure was a drop of inherent oscillation by more than3%, where fatigue cracks occurred in a depth of upto 4 mm. In thepresent study a run-out criterion as a limit of fatigue strength wasset at 10 million cycles. Fractured surfaces of all fatigued speci-men were further evaluated using a scanning electron microscope(SEM).

2.6. Electrochemical testing

Electrochemical potentiodynamic testing was conducted in a0.1 M NaCl solution, made from analytical grade chemical and dis-tilled water. The pH value was 6.5. For electrochemical testingadditional as-machined and SP-treated specimens were sectionedin the form of discs of 15 mm in diameter in the longitudi-nal (L) direction. A Gamry Potentiostat/Galvanostat PC3 with athree-electrode corrosion cell was used, with the working elec-trode embedded in a Teflon holder. The exposed area measured0.785 cm2. A saturated calomel electrode (SCE) served as a refer-ence electrode and two stainless-steel rods as counter electrodes.Following a 1-h stabilization at open circuit potential (OCP),measurements were performed in the following order: linearpolarisation, ±10 mV vs. OCP, using a scan rate of 0.1 mV/s andpotentiodynamic curves, starting from −250 mV vs. OCP upto 1 Vusing a scan rate of 1 mV/s. All potentials are reported with respectto SCE scale. Prior to potentiodynamic measurements electrochem-ical impedance spectroscopy (EIS) measurements were done witha perturbation signal of 10 mV (10 points per decade) in the fre-quency range from 65 kHz to 5 mHz.

3. Results and discussion

The results of the surface integrity analysis, residual-stress mea-surement, fatigue testing with post fatigue fractographic analysis,and electrochemical testing are presented in following sections.

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U. Zupanc, J. Grum / Journal of Materials Processing Technology 210 (2010) 1197–1202 1199

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ably due to the specimen preparation. Relatively small-magnitudemeasured stress of the as-machined specimens was neglected infurther evaluation. The residual compressive stresses after SP treat-ment amounted to around −320 MPa, i.e., to nearly 55% of theultimate tensile strength of the material delivered. Depths of the

Fig. 2. SEM images of corroded as-machined (left) and SP-tre

.1. Surface integrity analysis

SP treatment and pitting corrosion of tested specimens essen-ially changed the surface roughness properties. Results of the

easurement of arithmetic average (Ra) and root mean square (Rq)urface roughness after SP treatment and the exposure in the salthamber are given in Table 1. Reference surface roughness of the as-achined specimens is a result of final grinding with emery paper

f granulation of upto 1000. The as-machined specimens exposedo corrosion in the salt chamber essentially increased roughness byour times due to the surface corrosion damage (see Table 1).

The size and shape of the corrosion pits formed at the as-achined and SP-treated specimens was evaluated. Surface SEM

mages are shown in Fig. 2. The pits were lined up in the rollingirection in agreement with local dissolution zones at constituentarticles. The dimension along the rolling direction was desig-ated as the pit length and perpendicular to rolling directionesignated as the pit width. The pit lengths at the as-machinednd corroded material measured in length upto 500 �m and inidth to 10–40 �m (Fig. 2a). Separate local pits combined in elon-

ated corroded lines at the area of local dissolution. Furthermore,he corrosion pits grew not only in the rolling direction but didlso coalescent with the pits in the direction perpendicular toolling. Approximately 15 pits/mm2 at the corroded surfaces withdepth greater than 25 �m were found. Maximal individual pit

epths amounted to upto 45 �m. Surface roughness changes athe SP-treated specimens were affected by the absorbed mediuminetic energy. The typical diameter of the dimples ranged between50 and 200 �m. Subsequent salt spray exposure of the peenedpecimens increased the surface roughness by 10%. The pits athe SP-treated specimens were not oriented with regard to theolling direction as observed at the ground as-machined specimensFig. 2b). This can be contributed to local plastic deformation of theonstituent particles in the peened surface layer.

After SP treatment the number and size of pits at the sur-

ace decreased perceptibly. The SP-treated specimens exposed tohe corrosive environment contained fewer surface pits. An aver-ge corrosion pit length at the SP-treated specimens measuredpto 100 �m, and their width upto 30–50 �m. The pit density

able 1urface roughness measurement results.

Treatment AM AM & Corr. SP SP and Corr.

Ra (�m) 0.32 1.24 5.81 6.27Rq (�m) 0.47 2.02 7.09 7.81

aterial (right). The pits analysed are marked with an arrow.

at the surface was essentially smaller and amounted to around5–7 pits/1 mm2. Due to surface roughness measuring machine limi-tations and the fact that pits can be much larger beneath the surfaceas observed on the surface, the true pit dimensions were evalu-ated further in the post fatigue fractographic analysis. However,some individual pit depths of the SP-treated material measuredupto 60 �m. The experimental results of surface pit properties arecomparable to those found in other author’s studies on heat-treatedhigh-strength aluminium alloys. Curtis et al. (2003) observed lessbut deeper pits on shot-penned aluminium 2024. On the con-trary, Prevey and Cammett (2004) reported shallower pits byone-third on low plasticity burnished aluminium 7075 comparedto as-machined material. Trdan et al. (2009) concluded lower pitdensity on laser shock-processed 6xxx series aluminium alloys. Allheat-treated aluminium alloys had induced compressive residualstresses by using different surface treatment techniques.

3.2. Residual stresses

Residual stresses are one of key influences on material fatigueresistance, also in a corrosive environment. Fig. 3 shows the mea-sured residual stresses as a function of depth. Prior to SP treatmentthe as-machined specimens showed residual stresses in the thinsurface layer amounting to around −50 MPa, induced most prob-

Fig. 3. Distribution of residual stresses vs. depth for 7075-T651 aluminium.

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1200 U. Zupanc, J. Grum / Journal of Materials Proce

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Fa

Fig. 4. Corrosion fatigue life for 7075-T651 aluminium.

nduced residual stresses after SP treatment of upto 500 �m arereater than the depth of typical corrosion pits; therefore, residualtresses should influence local stress concentrations at the pit arean fatigue testing. Due to the surface corrosion pits at the SP-treatedpecimens a relief of residual stresses was observed. Relaxationf the residual stresses at the surface amounted to around 23%f the residual stresses after SP treatment. No significant changesn residual-stress measurement of the as-machined and corrodedpecimens were noticed.

.3. Bending fatigue testing

The semi-logarithmic S–N curves generated for the fatigued

pecimens in different research combinations are shown in Fig. 4.atigue results for the as-machined specimens presented a base-ine for further comparison of fatigue properties. As the tests wereimited in the number of specimens and the applied fatigue load

ig. 5. Fractured surface of fatigued corroded as-machined specimen from individual pit (dotted line.

Fig. 6. Fractured surface of fatigued SP-treated specimens (left) and also in corro

ssing Technology 210 (2010) 1197–1202

cycles of upto 107 cycles, a more accurate fatigue endurance limit atapplied stresses in a range of 5 × 108 or even in a range of 109 cyclescannot be confirmed without additional testing. The presence of acorrosive chloride environment has a major influence on fatigueproperties of high-strength aluminium alloy 7075. Low materialcorrosion resistance significantly decreased the fatigue life of thecorroded specimens.

The fatigue life at higher applied stresses decreased by a factorof about 10 compared to the baseline. The fatigue stress limit ofthe corroded specimens of 85 MPa at 107 cycles amounted to only45% of the fatigue stress limit of the baseline at 189 MPa. Local stressconcentrations at the degraded area resulted in much faster fatiguecrack initiation. The fractured surfaces were examined to evaluatecritical pit(s) for fatigue crack initiation. Two typical fractured sur-faces of the corroded as-machined specimens are shown in Fig. 5.A fatigue crack mainly nucleated from an individual pit at higherapplied stresses (Fig. 5a). The critical pit depth of the specimensfatigued at 143 MPa was around 110 �m. At lower fatigue stressestwo or more crack-nucleating pits were found. A minimal criticalpit depth to initiate a fatigue crack at the as-machined polishedspecimen was around 70 �m (Fig. 5b).

A favourable influence of SP treatment on material fatigue resis-tance was found. SP treatment nearly doubled the maximal cyclesto failure at the higher applied stresses when compared to theuntreated specimens. The fatigue limit of the SP-treated specimensincreased to 218 MPa at 107 cycles. Furthermore, the SP-treatedspecimens outperformed the as-machined parent material whenexposed to corrosive chloride environment by a factor of 2. Thefatigue stress limit increased to 165 MPa. The experimental data

due to the compressive residual-stress ability to influence cracknucleation. Strain hardening by shot peening retarded the crackpropagation. Increased resistance to plastic deformation and theresidual-stress profile so provided a corresponding fatigue crack

left) and multiple pits-to-crack nucleation (right). The critical pits are marked with

sive conditions (right). The crack initiation sites are marked with an arrow.

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U. Zupanc, J. Grum / Journal of Materials Processing Technology 210 (2010) 1197–1202 1201

s analysis of fatigue cracks formed from pits.

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Table 2Corrosion current densities.

Fig. 7. Corroded specimen cross-section

losure. The induced compressive residual stresses also retardedatigue crack initiation in the pit area that resulted in better fatigueroperties. Fatigue crack initiation was observed at much greaterepths in the SP-treated specimens than in the as-machined spec-

mens. The SEM images of fractured surfaces of the SP-treatedpecimens are shown in Fig. 6. Typical initiation depths were foundpto 370 �m below the surface, depending on maximal appliedatigue stresses, whereas with the as-machined specimens ini-iation was, at maximum, at 100 �m beneath the surface. Theompressive residual-stress layer pushed the crack region beneathhe surface. Fatigue cracks of the corrosion-exposed SP-treatedpecimens showed the same behaviour. Cracks also initiated atub-superficial sites. Even at a highly corroded area at the edgef the corroded SP-treated specimen a fatigue crack was observedt 246 �m beneath the surface (Fig. 6b).

In the surface integrity analysis and further in the fatigue per-ormance evaluation, pitting corrosion was defined as a type oforrosion damage. But to characterize a true corrosion nature,orresponding cross-sectional microstructural images of fatiguepecimens were studied. They are shown in Fig. 7. The cross-ections were polished and etched using Keller’s reagent. Multipleatigue cracks initiated from a highly degraded surface of the as-

achined specimen fatigued at 110 MPa (Fig. 7a). Individual surfaceayers of exposed microstructural grain bodies were entirely cor-

oded due to an intergranular corrosion attack. The corrosive attackas oriented in agreement with local dissolution zones of the con-

tituent particles lined in the LT direction. A fatigued SP-treatednd corroded specimen is shown in Fig. 7b where crack initiatedt the surface dimple. A residue of aluminium hydroxide Al(OH)3

Fig. 8. Polarisation curves of as-machined and SP-treated specimens in 0.1 M NaCl (

Treatment As-machined Shot-peened Ratio

jcorr (A/cm2) 2.1 × 10−6 5.2 × 10−6 1:2.5

at the surface and secondary fatigue crack propagation were alsovisible.

3.4. Electrochemical testing

Electrochemical potentiodynamic testing was conducted toinvestigate effects of SP treatment on the material electrochemicalcharacteristics. Different electrochemical properties were evalu-ated: corrosion potential, corrosion current density, and corrosionrate. Potentiodynamic polarisation curves and electrochemicalimpedance properties are shown in Fig. 8. Results show nearly thesame free corrosion potential (Ecorr) of the as-machined and theSP-treated specimens with values near −700 mV (Fig. 8a). A com-parison of the corrosion current densities (Icorr) was performedwith a Tafel analysis. The average values of five individual testsfor corrosion current densities are given in Table 2.

Higher corrosion current density (Icorr) on the SP-treated spec-imens by a factor of 2.5 with reference to the as-machined

specimens was observed. The increase of corrosion current densityindicated a higher pit growth rate. The increased surface rough-ness of the SP-treated specimens and possible remains of the steelmedium at the aluminium surface could have influenced the elec-trochemical properties although the treated surface was chemically

a). Electrochemical impedance spectroscopy results shown as Bode plots (b).

Page 6: Effect of Pitting Corrosion on Fatigue Performance of Shot-peened Aluminium Alloy 7075-T651

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leaned. Electrochemical impedance data show similar corrosionusceptibility. The polarisation resistance (Rp) is estimated fromhe impedance response in the Bode plot as a total impedancealue at the peak of the curve (Fig. 8b). The SP-treated materialad a lower Rp. The evaluated Rp was 4.3 k� cm2 for the as-achined material, while the value for the SP-treated specimensas 3.4 k� cm2. The decrease of impedance response at lower fre-

uencies refers to a dissolution process of the tested aluminiumurfaces in the chloride solution. Overall, both surfaces are cor-osion susceptible, but the SP-treated specimens showed slightlyower polarisation resistance. Curtis et al. (2003) observed similaretrimental higher corrosion current density of the SP-treated alu-inium 2024. An increase in corrosion current density by a factor

f 5 was reported. The specimens were electrochemically testedn the same microstructural L direction as in present research. Liund Frankel, 2006 showed the same effect of increasing corrosionurrent density in low plasticity burnished aluminium 7075, exper-mented in an isolated longitudinal (L) direction. But the resultsbtained in the short traverse (ST) directions were in a contrasthere the corrosion current density decreased by a factor of about

.To estimate the real fatigue life improvement of the peened

aterial it is important to evaluate the beneficial compressiveesidual-stress field compared to critical pit depths on the one handnd the possible detrimental electrochemical changes on the otherand. Future work will combine effects of crack-nucleating pits,

nduced compressive residual-stress profiles, and electrochemi-al potentiodynamic properties of SP-treated material in all threeaterial microstructural planes. Complex interactions between

esidual-stress distribution, microstructure orientation, and grainizes in different microstructural directions are expected. Devel-pment of such a probabilistic model is under investigation tonderstand these phenomena and predict structural integrity oflements made of high-strength aluminium alloys in a corrosivenvironment.

. Conclusions

To determine the effects of pitting corrosion on the fatigue prop-rties of the SP-treated aluminium alloy 7075-T651, a series of testsere performed. The research results demonstrate positive effect

f the SP treatment of structural elements exposed to the corro-ive chloride environment. Based on the study of the influence ofhe SP treatment on corrosion fatigue resistance of aluminium alloy075-T651 in the 5% NaCl solution fog chamber and in 0.1 M NaClolution, following conclusions can be drawn:

Fatigue resistance of the corroded specimens drasticallyecreased in comparison with the parent material due to mate-ial pitting corrosion. A decrease of fatigue life by a factor of 10 wasbserved with individual fatigue stresses. The fatigue stress limitf the as-machined and corroded specimens of 85 MPa amounted

ssing Technology 210 (2010) 1197–1202

to only 45% of the fatigue stress limit of the parent material at189 MPa. Local stress concentrations at the degraded surface pitsarea resulted in much faster fatigue crack initiation.

After the SP treatment the number of surface pits was consider-ably reduced. The SP-treated specimens outperformed the parentmaterial when exposed to the corrosive chloride environment bya factor of 2. The fatigue stress limit of the corroded SP-treatedspecimens increased to 165 MPa and thus approached 87% of thebaseline result. The experimental data assumed an increase offatigue strength of the peened material due to the residual-stressability to retard crack propagation.

In electrochemical potentiodynamic testing, corrosion currentdensity higher by a factor of 2.5, with reference to the as-machinedmaterial, was observed with the SP-treated specimens. The increaseof corrosion current density indicated a higher pit growth rate.

References

ASTM B117-07a Standard Practice for Operating Salt Spray (Fog) Apparatus.ASTM E837-08 Standard Test Method for Determining Residual Stresses by the Hole-

Drilling Strain-Gage Method.Birbilis, N., Cavanaugh, M.K., Buchheit, R.G., 2006. Electrochemical behavior and

localized corrosion associated with Al7Cu2Fe particles in aluminum alloy 7075-T651. Corros. Sci. 48, 4202–4215.

Benedetti, M., Fontanari, V., Scardi, P., Ricardo, C.L.A., Bandini, M., 2009. Reversebending fatigue of shot peened 7075-T651 aluminium alloy. Int. J. Fatigue 31,1225–1236.

Curtis, S.A., Rios, E.R., Rodopoulos, C.A., Romero, J.S., Levers, A., 2003. Investigatingthe benefits of controlled shot peening on corrosion fatigue of aluminium alloy2024 T351. In: Proceedings of 8th International Conference on Shot Peening(ICSP-8), Germany, pp. 16–20.

DuQuesnay, D.L., Underhill, P.R., Britt, H.J., 2003. Fatigue crack growth from corrosiondamage in 7075-T6511 aluminum alloy under aircraft loading. Int. J. Fatigue 25,371–377.

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