Investigation on the corrosion and hydrogen evolution for AZ91D magnesium alloy in single and anion-containing oxalate solutions

Corrosion characterization of AZ91D alloy was studied in aqueous sodium oxalate solutions with various concentrations using different electrochemical techniques (open circuit potential, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV)). The corrosion rate and consequently the rate and extent of hydrogen evolution were found to increase significantly with increasing oxalate anion concentration and temperature or with decreasing the pH of solution. Increasing additions of various anions over the lower concentration range (0.001–1.0 mM) in the blank oxalate solution increases to a varying extent the corrosion rate of the alloy and hence increases the hydrogen evolution rate and decreases surface film stability in the following order Cl− > SO42− > F−. On the other hand, addition of phosphate anion exhibits a reverse trend, where the active corrosion rate decreases with increasing PO43− anion concentration, implying that this anion acts as a passivator for AZ91D alloy. The obtained electrochemical results are further confirmed by scanning electron microscopy (SEM) analysis.