Electrochemical behavior of AZ91D magnesium alloy in phosphate medium: Part II. Induced passivation

Abstract

Induced passivation of AZ91D magnesium alloy in phosphate solution was carried out both chemically, using various inorganic oxidants, namely, molybdate, vanadate and iodate, as well as electrochemically by anodizing the alloy under various controlled overpotentials within the range 0.1–3.4 V. In acidic phosphate (pH 4.5), molybdate and vanadate anions exhibit similar behavior, as they show a dissolution effect at lower concentrations and passivation at higher concentrations. On the other hand, iodate anions shows critical behavior with a passivation effect up to 0.1 mM and depassivation for higher concentrations. Generally, over the concentration domain (0.01–1.0 mM) the results reveal small inhibitive effects with maximum values of 19.7% for ${\text{IO}}_{3}{}^{ - }$ and 24–25% for ${\text{MoO}}_{4}{}^{2 - }$ and ${\text{VO}}_{3}{}^{ - }$ manifesting weak propensities for these inorganic species to enhance the corrosion resistance of AZ91D alloy in acidic phosphate medium. The effect of anodic potential on the characteristics of surface films formed on the alloy in alkaline phosphate solution (pH 11.9) indicates that higher forming overpotential induces better passivation due to the formation of rather thicker and more resistive anodic films. The stability of the films is greater in alkaline as compared to acidic phosphate solutions.