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

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 (E_pit), repassivation potential (E_rep), and pit nucleation transients. E_pit in 0.5 M sodium chloride (NaCl) decreased with increased Zn content, and E_rep was 50 mV to 60 mV below E_pit, 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 E_pit 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 environment, facilitating the transition to stable pitting. A previously proposed model for the atomistics of activation was detailed.