Electrochemical behavior of the active surface layer on rolled aluminum alloy sheet

The highly deformed, micrograined layer on the outermost surface of a rolled Al-Fe-Si-Mn model alloy was electrochemically characterized. The thickness of this deformed surface layer in a 1.0 mm thick sheet was approximately 1 mm. Polarization curves in 5% NaCl solution at pH 3.0 and 11.5 were obtained at different depths from the surface using controlled sputtering in a glow discharge optical emission spectrometer for sample preparation. Both the anodic and the cathodic reactivity of the deformed surface layer were significantly higher than that of the bulk. Consistent with this, image analysis of scanning electron microscopy backscattered images revealed an increased number of fine intermetallic particles in the surface layer as compared with the bulk of the material. The corrosion morphology of the outermost surface was characterized by a high density of fine pits, while fewer and larger pits were observed in the bulk. The results highlight the importance of heavily deformed surface layers in controlling corrosion behavior of rolled aluminum products.