Influence of Electrolyte Composition on the Behavior of Magnesium at High Anodic Potentials

Abstract

Potentiodynamic curves were measured in electrolytes of 0.2Mol/L NaOH with the addition of compounds used in conventional and in recently developed anodizing baths. The curves showed as a characteristic behavior passivity breakdown at about +3V, high current densities of magnesium dissolution, followed by secondary passivation and breakdown of the anodic film at high potentials. The compounds were characterized according to their influence on the potential and the critical current density for secondary passivation, the passive current density and the breakdown potential of the anodic film. Compared with 0.2Mol/L NaOH, the addition of fluoride, phosphate, stannate and ethanol lowered the critical current density for secondary passivation. Al(OH) 3 and silicate showed no effect on the critical current density. No secondary passivation occurred in the presence of permanganate, while in the presence of silicate no sharp active-passive transition was found.