Preparation of MgF₂ Coatings on AZ31 Mg Alloy in Micro-Arc Oxidation Process Based on the Solubility Product Rule

This work mainly explores whether the solubility product principle has a guiding role in regulating the composition of micro-arc oxidation (MAO) coatings. The MAO process was conducted on AZ31 Mg alloy in silicate electrolyte. Varying amounts of Potassium fluoride (KF) and Ammonium fluoride (NH₄F) were separately added to the basic electrolyte to regulate the OH^- and F^- contents in the electrolyte. The microstructure, phase composition and corrosion resistance of the MAO coatings prepared in different electrolytes were analyzed. Results showed that regardless of KF content, MgO was the main component for the MAO coatings obtained in electrolytes with KF. This was because the addition of KF not only elevated the F^- concentration in the electrolyte but also enhanced the OH^- concentration as a result of F^- hydrolysis. Based on the solubility product constants (Ksp) of MgO and MgF₂, a relatively lower concentration of Mg²⁺ was sufficient for the formation of MgO. Hence, Mg²⁺ consistently exhibited preferential reactivity with OH^-, leading to the formation of MgO. The findings of the study demonstrated that the presence of KF electrolyte resulted in an enhancement of conductivity and an increase in the concentration of OH^-. Conversely, the growth rate of the coating was observed to be low, and the coating-forming phases of the coating were identified as MgO and Mg₂SiO₄, and the coating had better corrosion resistance. NH₄F electrolyte with the increase in NH₄F concentration, conductivity decreases and then increases, OH^- concentration decreases, the growth rate of the coating is faster, the concentration of F^-/OH^- is higher, the coating-forming phase is transformed into MgF₂, and the corrosion resistance of the coating is reduced.