An electrochemical impedance study of bare and anodized AZ31 Mg alloy in dilute Harrison solution

Anodic oxides were grown on AZ31 Mg alloy by micro-arc anodic oxidation in phosphate-based alkaline solutions with and without silicate and aluminate ions. The anodic oxides were characterized by SEM and XRD. Impedance spectra were recorded before and after single cycle anodic polarization in dilute Harrison solution (DHS), which is constituted by a mixture of ammonium sulfate and sodium chloride. Addition of silicate and aluminate oxyanions to the phosphate-based anodizing solution provided anodic oxides with improved morphology and thickness. Nevertheless, the protection performance in DHS is relatively poor due to the high reactivity of Mg in the presence of NH4+. The dissociation of this species into NH3 and H+ as local pH increases promotes further metal corrosion and hinders precipitation of Mg(OH)2. The likely better protection indicated only silicate-rich oxide results from favored film compaction and blocking effect of Mg(OH)2 due to interactions between the oxide bulk phase and NH4+.