Effect of K₂ZrF₆ Concentration on the Two-Step PEO Coating Prepared on AZ91 Mg Alloy in Alkaline Silicate Solution

In this study, a two-step Plasma Electrolytic Oxidation (PEO) method with constant primary step parameters and varying K2ZrF6 concentration in the secondary electrolyte solution was carried out to obtain a hard and dense coating on AZ91D alloy. For lower concentrations of K2ZrF6, a mixture of granular and pancake structure with higher surface porosity was obtained. Increasing the concentration up to 6 g/L caused the granular structure to disappear and a significant decrease was observed in the surface porosity as well as inner layer damage. Due to the compact inner layer structure, significant improvement in the corrosion resistance and mechanical properties of the AZ91 was observed with K2ZrF6 concentration. Highest values of hardness and corrosion resistance, i.e., 1589.45 HV and 386.30 × 103 Ω cm2, respectively, were obtained for a 6 g/L K2ZrF6 concentration.