Corrosion inhibition of AZ31 Mg alloy by aqueous selenite (SeO₃ ²⁻)

An investigation was undertaken to study aqueous selenite as a possible corrosion inhibitor for magnesium alloys. Exposure experiments, anodic and cathodic polarization studies, electrochemical impedance spectroscopy (EIS) studies, morphological analysis, X-ray photoelectron spectroscopy (XPS) and Raman surface analysis were performed on alloy AZ31 during and after exposure in different concentrations of sodium selenite (0.5 mM to 50.0 mM) dissolved in 0.1 M NaCl. Formation of a Se⁰ film consisting of both amorphous and crystalline Se⁰ was observed after exposure to solutions independent of the selenite concentration. In the case of the highest concentration, precipitation of MgSeO3 was observed on the surface of the Mg alloy samples. Both anodic and cathodic inhibition was observed across the range of concentrations examined. Cathodic inhibition was observed independent of selenite concentration and appeared to arise from the presence of a surface film formed on both the matrix and on Al-Mn particles that inhibited water reduction. Anodic inhibition in electrochemical measurements was evident at lower selenite concentrations. Characterization of the film morphology suggested that film structure was detrimentally affected as concentration of the inhibitor increased.