Passivation and potential fluctuation of Mg alloy AZ31B in alkaline environments

The passivation and corrosion behaviors of Mg alloy AZ31B were investigated in chloride-free NaOH solutions with pH range from 10 to 14. The results indicate that various phases behave differently at different pH levels. Al-Mn intermetallics serve as strong cathode at pH ≤ 11, but dissolve at pH ≥ 13. Ambient oxygen is responsible for the potential rise at pH ≥ 13 and subsequent significant potential oscillation (>1 V) is attributed to the passive film breakdown at the intergranular boundaries in the oxide film. Optimal passivation is achieved at pH 12 with thin uniform passive film formed across the surface.