The Initial Corrosion Behavior of AZ31B Magnesium Alloy in Chloride and Sulfate Solutions

Abstract

The initial corrosion of Mg alloys is important for the conversion coating treatment, by which the conversion coating is formed in a few minutes of immersion in the conversion solution. This study detailed the initial corrosion of AZ31B Mg alloy in chloride and sulfate solutions by acquiring the open circuit potential, optical image and hydrogen evolution, together with the post-immersion TEM and XPS characterization. Before the breakdown of the surface film, more hydrogen evolved in the SO4 2− solution than in Cl− solution, and the corrosion products formed in the former were thicker than in the latter. The initial corrosion of AZ31B was thus more severe in SO4 2− solution than in Cl− solution, which is different from the typical role of Cl− ions in metal corrosion. Local attack at nanometric scales was observed in Cl− solution, but not in SO4 2− solution. The presence of SO4 2− in Cl− solution mitigated local attack and the corrosion products were more uniform, due to the competitive adsorption effect of SO4 2−.