Effect of Microstructure on the Localized Corrosion and Stress Corrosion Behaviours of Plasma-Electrolytic-Oxidation-Treated AA7075 Aluminum Alloy Forging in 3.5？wt.？% NaCl Solution
The influence of metallurgical heterogeneities such as coring and intermetallic phases on the corrosion and stress corrosion cracking behaviours of AA7075 aluminium alloy forging was examined in 3.5？wt. % NaCl solution with and without plasma electrolytic oxidation coating. Electrochemical test results demonstrated significant improvement in the corrosion resistance of the alloy after PEO coating. Stress corrosion results show that the metallurgical heterogeneities resulted in a loss in elongation of the uncoated sample in NaCl (11.5%) when compared to the one tested in air (12.9%). The loss in elongation of the uncoated sample was shown to be due to localized corrosion-assisted mechanical cracking rather than true stress corrosion based on preexposure tensile tests followed by posttest metallographic observation of the stress corrosion tested samples. This was further confirmed by the fractographic examination of the failed samples, which exhibited a typical ductile cracking morphology for all the coated and uncoated specimens. 1. Introduction High-strength, precipitation hardened aluminium alloys such as AA7075 are widely used in aerospace industries in view of the good stress corrosion cracking (SCC) resistance in T7 temper conditions [1–5]. Although this treatment offered good improvement in the SCC resistance, localized corrosion such as pitting remains a problem in commercial aluminum alloys containing intermetallic phases such as Al7Cu2Fe, (Al,Cu)6 (Fe,Cu), and Mg2Si. Hence several studies were directed towards understanding the influence of these phases on the galvanic corrosion of aluminum. Among these phases, Al7Cu2Fe was reported to be the most cathodic phase, which results in the development of pitting at the particle-matrix interface [6]. These pits were the initiation sites for fatigue crack resulting in premature failure of AA7075-T7351 alloy [7]. On this aspect, surface modification techniques such as laser surface melting (LSM) and plasma electrolytic oxidation (PEO) are widely employed for improving the corrosion resistance of light alloys such as aluminum and magnesium. Yue et al. [8] obtained significant improvement in the SCC resistance for 7075 aluminum alloy using excimer laser treatment. With respect to PEO, although this treatment was well proven towards enhancing the corrosion and wear resistance of aluminum alloys, the influence of such coating on the SCC resistance was not studied in detail. However, available results on the magnesium alloys indicated good improvement in the SCC resistance of AM50 magnesium alloy [9]. The
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