Magnesium Silicate Coatings Were Prepared by Micro-Arc Oxidation on the Surface of Magnesium Alloys Through the Synergistic Effect of SiO32−/F−

To enhance the long-term corrosion resistance of micro-arc oxidation coatings on magnesium alloys, this study regulated the ionic composition of the electrolyte based on the solubility product rule. In the silicate system, a micro-arc oxidation coating mainly composed of magnesium silicate was successfully prepared on AZ31A magnesium alloy by synergistically optimizing the ratio of SiO₃²^-/F^-. The results show that the addition of KF significantly promotes coating growth, with the thickness increasing from 19.32 μm to a maximum of 46.86 μm. As the Na₂SiO₃ concentration increases, the main phase of the coating changes from MgO to Mg₂SiO₄. Electrochemical tests indicate that the coating prepared with 30 g/L Na₂SiO₃ and KF addition exhibits the best corrosion resistance, demonstrating the lowest corrosion current density of 3.89 × 10^-⁹ A·cm^-², which is approximately four orders of magnitude lower than that of the uncoated substrate. However, when the Na₂SiO₃ concentration is too high, the corrosion resistance decreases due to increased pore size and defects, confirming a non-monotonic relationship between silicate concentration and coating performance.