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.