Electrochemical behaviour of the Mg alloy AZ91D in borate solutions

Corrosion and passivation behaviour of Mg-based alloy AZ91D was investigated in aqueous sodium borate solutions (pH 9.2) in relation to some test parameters, using electrochemical techniques. Increasing borate concentration (0.01–0.10 M) or temperature up to 298 K leads to increase the corrosion rate of the alloy. However, at temperatures higher than 298 K borate anions have stronger propensity to passivate the alloy, thereby decreases its corrosion rate. For a fixed borate concentration increasing Cl⁻ addition is correlated with a more negative corrosion potential and a higher corrosion rate, as well as increase the vulnerability of the anodic passive film for breakdown. The influence of oxidizing potentials over the range −1.5 V to 2.75 V (SCE) on the performance of the alloy in the most aggressive borate solution (0.10 M) reveals that higher potentials, induces better passivation due to formation of a rather thick and more protective n-type semiconducting film. A modified Randles circuit including Warburg impedance to account for the diffusion of reactants or products through the surface film was adopted to analyse the EIS data, that gave impedance parameters in good agreement with the results of open circuit potential and dc polarization measurements.