The effect of temper on the first breakdown in AA7075

The first breakdown mechanism of AA7075 was studied by an in situ observation system in which a combination of a magnified image of the surface with the instantaneous polarization curve allowed determination of the corrosion process as a function of potential. In particular, the effect of alloy temper was examined. Underaged samples behaved in a similar fashion as the peak aged samples described previously; they exhibited surface layer dissolution associated with a transient current peak at lower potentials than required for initiation of stable localized corrosion and Zn rich bands were found in the near surface region. Surface layer dissolution and the transient current peak were not observed for samples in the solutionized + quenched condition because no comparable Zn enriched bands were present and the composition of the surface layer was not substantially different than the bulk matrix. Overaged samples also exhibited no layer attack and only one breakdown potential. Zn-rich bands were observed at the nano-grain boundaries for the overaged temper, but the localized attack propagated into the bulk matrix rather than laterally as layer attack.