Mechanism of Activation of Aluminum by Low-Melting Point Elements: Part 2 — Effect of Zinc on Activation of Aluminum in Pitting Corrosion

Abstract To clarify the mechanism of pitting corrosion of Al, the effect of a large amount of Zn (up to 2%) on Al pitting was studied in terms of pitting potential (Epit), repassivation potential (Erep), and pit nucleation transients. Epit in 0.5 M sodium chloride (NaCl) decreased with increased Zn content, and Erep was 50 mV to 60 mV below Epit, independent of Zn content. The shift of the anodic polarization curve in 6 M hydrochloric acid ([HCl] pit-like solution) was consistent with the change of Epit with Zn content. Analysis of passive current noise at the pA level in buffered solutions, with or without chloride (Cl−), showed noise caused by Cl− started at roughly the lowest potential where pitting occurred and had a similar potential dependence, irrespective of Zn content. Therefore, it was considered that pit nucleation had the same potential dependence, and possibly the same frequency, in Al-Zn alloys and pure Al and that the effect of Zn simply was to enhance dissolution kinetics in the local envi...