Corrosion Performance of Anodized AZ91D Magnesium Alloy: Effect of the Anodizing Potential on the Film Structure and Corrosion Behavior

Die-cast AZ91D magnesium alloy samples have been submitted for anodizing at different potentials. Anodizing was conducted in an environmentally friendly solution which comprised 3 M KOH + 1 M Na₂SiO₃ at room temperature. The surface treatment was performed electrolytically at four different potentials: 3, 5, 8, and 10 V. The corrosion resistance was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization curves obtained after 7 days of immersion in a 3.5 wt.% NaCl solution at room temperature. The porosity of the anodic films was estimated by means of the linear polarization method. SEM images revealed that the surface oxide was thicker for the anodic films obtained at 3 and 5 V. The films obtained at these potentials were more porous than those formed at 8 and 10 V. The results showed that the thickness of the anodic film had a significant effect on the corrosion behavior of the AZ91D, whereas the influence of the initial porosity was not significant.