Formation of localized corrosion fissure in a thick plate of 7475 aluminum alloy

Constant immersion corrosion test and electrochemical measurement in NaCl solution were carried out to clarify localized corroison behavior in a thick plate of 7475 alloy comparing with 7075 alloy. The effect of aging treatments on the formation of localized corrosion fissure was investigated metallurgically and electrochemically. The T6 tempered alloy showed a high susceptibility to localized corrosion, and the extension law in the direction of fissure depth was estimated as a function of immersion time. Two pitting potentials were found out on the anodic polarization curve of 7475-T6, and the less noble potential was corresponding to the onset of localized corrosion. The preferential dissolution path was shown to agree with the region involving both the grain boundary and outside area of cored structure resulted from microsegregation of Cr or Ti. The presence of Fe-bearing second phase particles played a part to facilitate the formation of localized corrosion fissure.