Influence of sodium fluoride in acidic electrolytic solution on the structure and properties of plasma electrolytic oxidation coating on AZ91D magnesium alloy

By adopting the strategy of generating chelating agents during plasma electrolytic oxidation (PEO), PEO coatings were prepared in acidic eletrolytic solution by constant current mode. The effect of NaF concentration on the coating and the growth and regulation mechanism of fluoride in acidic eletrolytic solution by constant current mode were explored. The composition, structure, corrosion resistance, and formation mechanism of the coating were investigated. The results showed that the addition of NaF into the acidic electrolyte solution caused the pore diameter decreased, while the coating thickness and compactness were increased. The increase of NaF concentration in the acid electrolyte increases the MgF 2 content and affects the adhesion of the coating to the substrate, which was attributed to the changes in the spark discharge characteristics. Subsequently, the self-sealing of the coating and the amount of MgF 2 composition in the coating will affect the corrosion resistance of the coating. The PEO coating fabricated in the acidic electrolyte solution containing 7 g/L NaF displayed small pore size, stronger bonding, and thicker coating, which resulted in superior corrosion resistance, the lowest corrosion tendency and enhance the polarization resistance.