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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: uros.zupanc@i-var.si (U. Zupanc), janez.grum@fs.uni-lj.si (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
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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) 11971202

Contents lists available at ScienceDirect

Journal of Materials Processing Technologyjournal homepage: www.elsevier.com/locate/jmatprotec

Effect of pitting corrosion on fatigue performance of shot-peened aluminium alloy 7075-T651U. Zupanc a , J. Grum b,a b

Welding Institute, Ptujska 19, SI-1000 Ljubljana, Slovenia University of Ljubljana, Faculty of Mechanical Engineering, Aker eva 6, SI-1000 Ljubljana, Slovenia s c

a r t i c l e

i n f o

a b s t r a c tPitting corrosion has a major inuence 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 inuence of shot-peening treatment on corrosion fatigue properties. Induced compressive residual stresses in the surface layer retard the initiation 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.

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

1. Introduction Surface corrosion has a major inuence on aging of structural elements made of high-strength aluminium alloys. Aluminium alloy 7075 in alloy system AlZnMgCu contains a high number of intermetallic particles, i.e. constituent particles, where heterogeneity of a microstructure has an essential inuence on corrosion properties. Birbilis et al. (2006) electrochemically analysed isolated constituent particles of 7075 aluminium alloy. Large constituent particles Al7 Cu2 Fe, Al23 CuFe4 , and Al2 CuMg had different electrochemical 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 107 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: uros.zupanc@i-var.si (U. Zupanc), janez.grum@fs.uni-lj.si (J. Grum). 0924-0136/$ see front matter 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2010.03.004

of pitting in high-cycle fatigue testing upto 108 cycles, where a decrease of the threshold stress intensity by about 20% was observed. DuQuesnay et al. (2003) stated that the depth of corrosion 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 critical 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 tensile 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. Peening improved fatigue life by a factor of 1.26 depending on choosing

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the surface to pitting corrosion, the specimens were rotated every 24 h during the test.

2.4. Surface integrity characterizationFig. 1. Fatigue specimen details and dimensions (in mm).

different rework specications. Rodopoulos et al. (2004) indicated improved fatigue life of peened aluminium 2024-T351 by about 90% in axially fatigue testing. Benedetti et al. (2009) improved fatigue strength on aluminium 7075-T651 in reverse bending mode at 5 million cycles upto 46% by SP treatment. In the present research series of tests were performed to analyze the effects of SP treatment on surface corrosion properties. The objective of the study was to investigate the effect of SP treatment on electrochemical stability and corrosion fatigue properties of high-strength aluminium alloy 7075-T651 in chloride environments. Fatigue behaviour of prior-corroded as-machined and SP-treated specimens was evaluated to quantify the fatigue life changes. Characterization of the surface corrosion degradation and evaluation of pit-to-crack transition on fatigued specimens were made. Additionally, electrochemical tests in a chloride solution were conducted to investigate the effect of SP on surface corrosion properties. 2. Experimental details 2.1. Material A wrought plate of high-strength aluminium alloy 7075-T651 of 20 mm in thickness was delivered with the chemical composition (in wt.%): Al5.78Zn2.56Mg1.62Cu 0.21Cr0.05Mn0.04Ti0.09Si0.18Fe. Static mechanical properties of the tested material were: Rm = 585 MPa, Rp02 = 532 MPa and A50 = 12%. Specimens for fatigue testing were prepared in a long traverse (LT) direction (Fig. 1). As-machined specimens were ultrasonically cleaned in ethanol. In research the prepared specimens 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 & Corr). 2.2. Shot peening The specimens were SP-treated from all sides at the Metal Improvement Company in Germany using an air-blast machine. Cast steel-shot MI-170H with hardness of 55 HRC and a nominal diameter of 0.40 mm was chosen. In order to avoid medium collision, the angle of nozzle inclination was shifted by 5 with regard to the vertical axis. A constant specimen distance from the nozzle of around 120 mm was maintained. Surface coverage was set to 150%. Comparative Almen intensity value of 12A was achieved. 2.3. Salt spray tests The specimens for fatigue testing were placed in a salt spray chamber for surface corrosion tests in accordance with ASTM B117 for 168 h (7 days). Before the corrosion process, the SP-treated specimens were cleaned with concentrated HNO3 acid to avoid surface contamination by possible steel-shot residues. The prepared solution had a pH value of 6.5 and the inner chamber temperature was set to 35 C. Specimens were placed individually, parallel to each other, on plastic supports at an angle of indentation of 20 with regard to the vertical axis. In order to provide uniform exposure of

After the exposure of the specimens in the salt spray chamber, the specimen surface was covered with white corrosion products consisting mostly of Al(OH)3 . For the analysis of surface roughness and assessment of the pit size at the surface, the corrosion products were cleaned with a hard polymer wire brush in a water-diluted nitric acid (10% HNO3 ) at room temperature. The evaluation of the tested specimens comprised surface properties and residual-stress measurement in the thin hardened layer. Measurement of highresolution surface roughness was made with a Taylor Hubson Form Talysurf Series 2 device. The residual-stress measurements were made with a semi destructive hole-drilling method in accordance with ASTM 837.

2.5. High-cycle fatigue testing Fatigue testing was carried out without preliminary removal of the corrosion products from the surface. Bending fatigue testing of the specimens was carried out with a Rumul Cracktronic device at room temperature. A constant amplitude bending stress was applied in the range of the maximum applied stresses, i.e. those ranging between 15% and 65% of delivered-material tensile strength Rm. The testing resonant stress frequency was 107 Hz using a sinusoidal waveform at a stress ratio R of 0.05. A criterion of specimen failure was a drop of inherent oscillation by more than 3%, where fatigue cracks occurred in a depth of upto 4 mm. In the present study a run-out criterion as a limit of fatigue strength was set at 10 million cycles. Fractured surfaces of all fatigued specimen were further evaluated using a scanning electron microscope (SEM).

2.6. Electrochemical testing Electrochemical potentiodynamic testing was conducted in a 0.1 M NaCl solution, made from analytical grade chemical and distilled water. The pH value was 6.5. For electrochemical testing additional as-machined and SP-treated specimens were sectioned in the form of discs of 15 mm in diameter in the longitudinal (L) direction. A Gamry Potentiostat/Galvanostat PC3 with a three-electrode corrosion cell was used, with the working electrode embedded in a Teon holder. The exposed area measured 0.785 cm2 . A saturated calomel electrode (SCE) served as a reference 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: linear polarisation, 10 mV vs. OCP, using a scan rate of 0.1 mV/s and potentiodynamic curves, starting from 250 mV vs. OCP upto 1 V using a scan rate of 1 mV/s. All potentials are reported with respect to SCE scale. Prior to potentiodynamic measurements electrochemical impedance spectroscopy (EIS) measurements were done with a perturbation signal of 10 mV (10 points per decade) in the frequency range from 65 kHz to 5 mHz.

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

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Fig. 2. SEM images of corroded as-machined (left) and SP-treated material (right). The pits analysed are marked with an arrow.

3.1. Surface integrity analysis SP treatment and pitting corrosion of tested specimens essentially changed the surface roughness properties. Results of the measurement of arithmetic average (Ra) and root mean square (Rq) surface roughness after SP treatment and the exposure in the salt chamber are given in Table 1. Reference surface roughness of the asmachined specimens is a result of nal grinding with emery paper of granulation of upto 1000. The as-machined specimens exposed to corrosion in the salt chamber essentially increased roughness by four times due to the surface corrosion damage (see Table 1). The size and shape of the corrosion pits formed at the asmachined and SP-treated specimens was evaluated. Surface SEM images are shown in Fig. 2. The pits were lined up in the rolling direction in agreement with local dissolution zones at constituent particles. The dimension along the rolling direction was designated as the pit length and perpendicular to rolling direction designated as the pit width. The pit lengths at the as-machined and corroded material measured in length upto 500 m and in width to 1040 m (Fig. 2a). Separate local pits combined in elongated corroded lines at the area of local dissolution. Furthermore, the corrosion pits grew not only in the rolling direction but did also coalescent with the pits in the direction perpendicular to rolling. Approximately 15 pits/mm2 at the corroded surfaces with a depth greater than 25 m were found. Maximal individual pit depths amounted to upto 45 m. Surface roughness changes at the SP-treated specimens were affected by the absorbed medium kinetic energy. The typical diameter of the dimples ranged between 150 and 200 m. Subsequent salt spray exposure of the peened specimens increased the surface roughness by 10%. The pits at the SP-treated specimens were not oriented with regard to the rolling direction as observed at the ground as-machined specimens (Fig. 2b). This can be contributed to local plastic deformation of the constituent particles in the peened surface layer. After SP treatment the number and size of pits at the surface decreased perceptibly. The SP-treated specimens exposed to the corrosive environment contained fewer surface pits. An average corrosion pit length at the SP-treated specimens measured upto 100 m, and their width upto 3050 m. The pit density

at the surface was essentially smaller and amounted to around 57 pits/1 mm2 . Due to surface roughness measuring machine limitations and the fact that pits can be much larger beneath the surface as observed on the surface, the true pit dimensions were evaluated further in the post fatigue fractographic analysis. However, some individual pit depths of the SP-treated material measured upto 60 m. The experimental results of surface pit properties are comparable to those found in other authors studies on heat-treated high-strength aluminium alloys. Curtis et al. (2003) observed less but deeper pits on shot-penned aluminium 2024. On the contrary, Prevey and Cammett (2004) reported shallower pits by one-third on low plasticity burnished aluminium 7075 compared to as-machined material. Trdan et al. (2009) concluded lower pit density on laser shock-processed 6xxx series aluminium alloys. All heat-treated aluminium alloys had induced compressive residual stresses by using different surface treatment techniques. 3.2. Residual stresses Residual stresses are one of key inuences on material fatigue resistance, also in a corrosive environment. Fig. 3 shows the measured residual stresses as a function of depth. Prior to SP treatment the as-machined specimens showed residual stresses in the thin surface layer amounting to around 50 MPa, induced most probably due to the specimen preparation. Relatively small-magnitude measured stress of the as-machined specimens was neglected in further evaluation. The residual compressive stresses after SP treatment amounted to around 320 MPa, i.e., to nearly 55% of the ultimate tensile strength of the material delivered. Depths of the

Table 1 Surface roughness measurement results. Treatment Ra ( m) Rq ( m) AM 0.32 0.47 AM & Corr. 1.24 2.02 SP 5.81 7.09 SP and Corr. 6.27 7.81

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

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

induced residual stresses after SP treatment of upto 500 m are greater than the depth of typical corrosion pits; therefore, residual stresses should inuence local stress concentrations at the pit area in fatigue testing. Due to the surface corrosion pits at the SP-treated specimens a relief of residual stresses was observed. Relaxation of the residual stresses at the surface amounted to around 23% of the residual stresses after SP treatment. No signicant changes in residual-stress measurement of the as-machined and corroded specimens were noticed. 3.3. Bending fatigue testing The semi-logarithmic SN curves generated for the fatigued specimens in different research combinations are shown in Fig. 4. Fatigue results for the as-machined specimens presented a baseline for further comparison of fatigue properties. As the tests were limited in the number of specimens and the applied fatigue load

cycles of upto 107 cycles, a more accurate fatigue endurance limit at applied stresses in a range of 5 108 or even in a range of 109 cycles cannot be conrmed without additional testing. The presence of a corrosive chloride environment has a major inuence on fatigue properties of high-strength aluminium alloy 7075. Low material corrosion resistance signicantly decreased the fatigue life of the corroded specimens. The fatigue life at higher applied stresses decreased by a factor of about 10 compared to the baseline. The fatigue stress limit of the corroded specimens of 85 MPa at 107 cycles amounted to only 45% of the fatigue stress limit of the baseline at 189 MPa. Local stress concentrations at the degraded area resulted in much faster fatigue crack initiation. The fractured surfaces were examined to evaluate critical pit(s) for fatigue crack initiation. Two typical fractured surfaces of the corroded as-machined specimens are shown in Fig. 5. A fatigue crack mainly nucleated from an individual pit at higher applied stresses (Fig. 5a). The critical pit depth of the specimens fatigued at 143 MPa was around 110 m. At lower fatigue stresses two or more crack-nucleating pits were found. A minimal critical pit depth to initiate a fatigue crack at the as-machined polished specimen was around 70 m (Fig. 5b). A favourable inuence of SP treatment on material fatigue resistance was found. SP treatment nearly doubled the maximal cycles to failure at the higher applied stresses when compared to the untreated specimens. The fatigue limit of the SP-treated specimens increased to 218 MPa at 107 cycles. Furthermore, the SP-treated specimens outperformed the as-machined parent material when exposed to corrosive chloride environment by a factor of 2. The fatigue stress limit increased to 165 MPa. The experimental data assumed an increase of fatigue strength of the SP-treated material due to the compressive residual-stress ability to inuence crack nucleation. Strain hardening by shot peening retarded the crack propagation. Increased resistance to plastic deformation and the residual-stress prole so provided a corresponding fatigue crack

Fig. 5. Fractured surface of fatigued corroded as-machined specimen from individual pit (left) and multiple pits-to-crack nucleation (right). The critical pits are marked with a dotted line.

Fig. 6. Fractured surface of fatigued SP-treated specimens (left) and also in corrosive conditions (right). The crack initiation sites are marked with an arrow.

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Fig. 7. Corroded specimen cross-sections analysis of fatigue cracks formed from pits.

closure. The induced compressive residual stresses also retarded fatigue crack initiation in the pit area that resulted in better fatigue properties. Fatigue crack initiation was observed at much greater depths in the SP-treated specimens than in the as-machined specimens. The SEM images of fractured surfaces of the SP-treated specimens are shown in Fig. 6. Typical initiation depths were found upto 370 m below the surface, depending on maximal applied fatigue stresses, whereas with the as-machined specimens initiation was, at maximum, at 100 m beneath the surface. The compressive residual-stress layer pushed the crack region beneath the surface. Fatigue cracks of the corrosion-exposed SP-treated specimens showed the same behaviour. Cracks also initiated at sub-supercial sites. Even at a highly corroded area at the edge of the corroded SP-treated specimen a fatigue crack was observed at 246 m beneath the surface (Fig. 6b). In the surface integrity analysis and further in the fatigue performance evaluation, pitting corrosion was dened as a type of corrosion damage. But to characterize a true corrosion nature, corresponding cross-sectional microstructural images of fatigue specimens were studied. They are shown in Fig. 7. The crosssections were polished and etched using Kellers reagent. Multiple fatigue cracks initiated from a highly degraded surface of the asmachined specimen fatigued at 110 MPa (Fig. 7a). Individual surface layers of exposed microstructural grain bodies were entirely corroded due to an intergranular corrosion attack. The corrosive attack was oriented in agreement with local dissolution zones of the constituent particles lined in the LT direction. A fatigued SP-treated and corroded specimen is shown in Fig. 7b where crack initiated at the surface dimple. A residue of aluminium hydroxide Al(OH)3

Table 2 Corrosion current densities. Treatment jcorr (A/cm )2

As-machined 2.1 106

Shot-peened 5.2 106

Ratio 1:2.5

at the surface and secondary fatigue crack propagation were also visible. 3.4. Electrochemical testing Electrochemical potentiodynamic testing was conducted to investigate effects of SP treatment on the material electrochemical characteristics. Different electrochemical properties were evaluated: corrosion potential, corrosion current density, and corrosion rate. Potentiodynamic polarisation curves and electrochemical impedance properties are shown in Fig. 8. Results show nearly the same free corrosion potential (Ecorr ) of the as-machined and the SP-treated specimens with values near 700 mV (Fig. 8a). A comparison of the corrosion current densities (Icorr ) was performed with a Tafel analysis. The average values of ve individual tests for corrosion current densities are given in Table 2. Higher corrosion current density (Icorr ) on the SP-treated specimens by a factor of 2.5 with reference to the as-machined specimens was observed. The increase of corrosion current density indicated a higher pit growth rate. The increased surface roughness of the SP-treated specimens and possible remains of the steel medium at the aluminium surface could have inuenced the electrochemical properties although the treated surface was chemically

Fig. 8. Polarisation curves of as-machined and SP-treated specimens in 0.1 M NaCl (a). Electrochemical impedance spectroscopy results shown as Bode plots (b).

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cleaned. Electrochemical impedance data show similar corrosion susceptibility. The polarisation resistance (Rp ) is estimated from the impedance response in the Bode plot as a total impedance value at the peak of the curve (Fig. 8b). The SP-treated material had a lower Rp . The evaluated Rp was 4.3 k cm2 for the asmachined material, while the value for the SP-treated specimens was 3.4 k cm2 . The decrease of impedance response at lower frequencies refers to a dissolution process of the tested aluminium surfaces in the chloride solution. Overall, both surfaces are corrosion susceptible, but the SP-treated specimens showed slightly lower polarisation resistance. Curtis et al. (2003) observed similar detrimental higher corrosion current density of the SP-treated aluminium 2024. An increase in corrosion current density by a factor of 5 was reported. The specimens were electrochemically tested in the same microstructural L direction as in present research. Liu and Frankel, 2006 showed the same effect of increasing corrosion current density in low plasticity burnished aluminium 7075, experimented in an isolated longitudinal (L) direction. But the results obtained in the short traverse (ST) directions were in a contrast where the corrosion current density decreased by a factor of about 3. To estimate the real fatigue life improvement of the peened material it is important to evaluate the benecial compressive residual-stress eld compared to critical pit depths on the one hand and the possible detrimental electrochemical changes on the other hand. Future work will combine effects of crack-nucleating pits, induced compressive residual-stress proles, and electrochemical potentiodynamic properties of SP-treated material in all three material microstructural planes. Complex interactions between residual-stress distribution, microstructure orientation, and grain sizes in different microstructural directions are expected. Development of such a probabilistic model is under investigation to understand these phenomena and predict structural integrity of elements made of high-strength aluminium alloys in a corrosive environment. 4. Conclusions To determine the effects of pitting corrosion on the fatigue properties of the SP-treated aluminium alloy 7075-T651, a series of tests were performed. The research results demonstrate positive effect of the SP treatment of structural elements exposed to the corrosive chloride environment. Based on the study of the inuence of the SP treatment on corrosion fatigue resistance of aluminium alloy 7075-T651 in the 5% NaCl solution fog chamber and in 0.1 M NaCl solution, following conclusions can be drawn: Fatigue resistance of the corroded specimens drastically decreased in comparison with the parent material due to material pitting corrosion. A decrease of fatigue life by a factor of 10 was observed with individual fatigue stresses. The fatigue stress limit of the as-machined and corroded specimens of 85 MPa amounted

to only 45% of the fatigue stress limit of the parent material at 189 MPa. Local stress concentrations at the degraded surface pits area resulted in much faster fatigue crack initiation. After the SP treatment the number of surface pits was considerably reduced. The SP-treated specimens outperformed the parent material when exposed to the corrosive chloride environment by a factor of 2. The fatigue stress limit of the corroded SP-treated specimens increased to 165 MPa and thus approached 87% of the baseline result. The experimental data assumed an increase of fatigue strength of the peened material due to the residual-stress ability to retard crack propagation. In electrochemical potentiodynamic testing, corrosion current density higher by a factor of 2.5, with reference to the as-machined material, was observed with the SP-treated specimens. The increase of corrosion current density indicated a higher pit growth rate. ReferencesASTM B117-07a Standard Practice for Operating Salt Spray (Fog) Apparatus. ASTM E837-08 Standard Test Method for Determining Residual Stresses by the HoleDrilling Strain-Gage Method. Birbilis, N., Cavanaugh, M.K., Buchheit, R.G., 2006. Electrochemical behavior and localized corrosion associated with Al7 Cu2 Fe particles in aluminum alloy 7075T651. Corros. Sci. 48, 42024215. Benedetti, M., Fontanari, V., Scardi, P., Ricardo, C.L.A., Bandini, M., 2009. Reverse bending fatigue of shot peened 7075-T651 aluminium alloy. Int. J. Fatigue 31, 12251236. Curtis, S.A., Rios, E.R., Rodopoulos, C.A., Romero, J.S., Levers, A., 2003. Investigating the benets of controlled shot peening on corrosion fatigue of aluminium alloy 2024 T351. In: Proceedings of 8th International Conference on Shot Peening (ICSP-8), Germany, pp. 1620. DuQuesnay, D.L., Underhill, P.R., Britt, H.J., 2003. Fatigue crack growth from corrosion damage in 7075-T6511 aluminum alloy under aircraft loading. Int. J. Fatigue 25, 371377. Genel, K., 2007. The effect of pitting on the bending fatigue performance of highstrength aluminum alloy. Scripta Mater. 57, 297300. Jones, K., Hoeppner, D.W., 2005. Pit-to-crack transition in pre-corroded 7075-T6 aluminum alloy under cyclic loading. Corros. Sci. 47, 21852198. Liu, X., Frankel, G.S., 2006. Effects of compressive stress on localized corrosion in AA2024-T3. Corros. Sci. 48, 33093329, doi:10.1016/j.corsci.2005.12.003. Pao, P.S., Gill, S.J., Feng, C.R., 2000. On fatigue crack initiation from corrosion pits in 7075-T7351 aluminum alloy. Scripta Mater. 43, 391396. Pidaparti, R.M., Patel, R.R., 2008. Correlation between corrosion pits and stresses in Al alloys. Mater. Lett. 62, 44974499. Prevey, P.S., Cammett, J.T., 2004. The inuence of surface enhancement by low plasticity burnishing on the corrosion fatigue performance of AA7075-T6. Int. J. Fatigue 26, 975982. Rodopoulos, C.A., Curtis, S.A., Rios, E.R., Solis Romero, J., 2004. Optimization of the fatigue resistance of 2024-T351 aluminium alloys by controlled shot peeningmethodology, results and analysis. Int. J. Fatigue 26, 849856. Sankaran, K.K., Perez, R., Jata, K.V., 2001. Effect of pitting corrosion on the fatigue behavior of aluminum alloy 7075-T6: modelling and experimental studies. Mater. Sci. Eng. A297, 223229. Sharp, P.K., Clark, G., 2001. The effect of peening on the fatigue life of 7050 aluminium alloy. Research Report DSTO-RR-0208, accessed at: http://hdl.handle.net/1947/3292. Trdan, U., Grum, J., Porro, J.A., Ocana, J.L., 2009. Analysis of residual stress and corrosion resistance of laser shock-processed 6012 and 6082 aluminium alloys. In: 17th International Symposium on Gas Flow and Chemical Lasers, SPIE, vol. 7131. Wang, Q.Y., Kawagoishi, N., Chen, Q., 2003. Effect of pitting corrosion on very high cycle fatigue behavior. Scripta Mater. 49, 711716.