Surface Layer Dissolution Kinetics of Aluminum Alloy 7075 in Various Tempers

The breakdown behavior of aluminum alloy (AA)7075 (UNS A97075) samples at various tempers was studied using an in-situ observation system. All under-aged and peak-aged samples showed surface layer dissolution and transient current peaks associated with a breakdown phenomenon at potentials lower than the potential required for sustained localized corrosion. However, the nature of the current peaks depended on the temper. The microstructure in the bulk and the surface layer was studied by focused ion beam sectioning and transmission electron microscopy (TEM). TEM analysis showed that the layer thickness decreased with the extent of aging. The kinetics of surface layer dissolution during potentiostatic measurements were determined using image analysis. The differences in the polarization curves for different tempers were explained by differences in hardness and, thus, surface layer thickness and surface layer dissolution rate. The surface layer dissolution kinetics were different in directions along and perpendicular to the final polishing direction. This observation was explained by anisotropy in the surface layer microstructure.