Mechanism of localized corrosion of 7075 alloy plate

Abstract

Investigation of localized corrosion in 7075 aluminum alloy plate shows that the mechanism of corrosion can be rationalized on the basis of potentiokinetic anodic polarization behavior. 7075 alloy in cold water quenched W temper has been shown to be susceptible to both pitting and intergranular corrosion. Aging to peak strength condition (T651 temper) increases the susceptibility to intergranular corrosion. The preferential anodic path is presumed to be solute enrichment (Zn and Mg) in the vicinity of grain boundaries. This is substantiated by the observation of two distinct pitting potentials in the anodic polarization curve for the T651 temper. Overaging to the T7351 temper decreases susceptibility, both by the reduction in the difference between the two pitting potentials and by the reduction in the amount of solute atoms segregated. Metallographic cross-sections of potentiostatically polarized specimens substantiate the proposed mechanism.