The effect of surface contamination on corrosion performance of ultrasonic shot peened 7150 Al alloy

Abstract

Surface contamination layer mainly containing Fe, Ti and O with a thickness of ~20 μm was induced on AA 7150 surface during ultrasonic shot peening (USSP) treatment. The contaminated layer and its exfoliation evolution in solution containing HCl was investigated using immersion test, electrochemical impedance spectra (EIS), scanning kelvin probe force microscopy (SKPFM), SEM with energy-dispersive X-ray spectroscopy, XRD and TEM. USSPed AA 7150 with contamination layer showed ~3 times higher corrosion rate than that of controlled sample. However, after the exfoliation of the contamination layer, the corrosion rate of USSPed alloy actually is lower than that of its untreated counterpart. Additionally, as revealed by SKPFM, surface contamination layer is cathodic relative to the substrate, indicating that surface contamination layer is detrimental for the substrate alloy when galvanic corrosion occurs. Nonetheless, it should be noted that even in the presence of contamination layer, the localized corrosion resistance of peened alloy was greatly improved, which might be due to surface nanocrystallization, disappearance of rolled 〈220〉 texture and residual compressive stress effects caused by USSP.