On the first breakdown in AA7075-T6

The first breakdown in AA7075-T6 was studied by an in situ observation system in which a combination of a magnified image of the surface and the instantaneous polarization curve allowed determination of the corrosion process as a function of potential. As-polished (to 1 mu m) AA7075-T6 clearly exhibited dissolution of a thin surface layer corresponding to a sharp increase of current just above the first breakdown potential. No surface layer dissolution was observed for samples that were either ion milled or chemically etched to remove the effects of polishing. This susceptible surface layer was apparently the result of the mechanical polishing process. The surface microstructure of an as-polished sample was analyzed by TEM and several distinct features were found: (1) a unique thin layer with thickness of 100 nm on average; (2) many fewer fine hardening particles in the thin surface layer than in the bulk matrix, which suggests that the fine particles were destroyed and eliminated by polishing; (3) high aspect ratio nano-grains elongated along the final polishing direction; (4) a high concentration of Zn and Mg at the nano-grain boundaries. The attack of the surface layer might initiate at the active nano-grain boundary followed by nano-grain dissolution. (C) 2007 Elsevier Ltd. All rights reserved.