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 poten tial (E-pit), repassivation potential (E-rep), and pit nucleation transient s. E-pit in 0.5 M sodium chloride (NaCl) decreased with increased Zn conten t, 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 off,,, with Zn content. Anal ysis of passive current noise at the pA level in buffered solutions, with o r without chloride (Cl-), showed noise caused by Cl- started at roughly the lowest potential where pitting occurred and had a similar potential depend ence, irrespective of Zn content. Therefore, it was considered that pit nuc leation 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 enhanc e dissolution kinetics in the local environment, facilitating the transitio n to stable pitting. A previously proposed model for the atomistics of acti vation was detailed.