Effects of Sodium Fluoride on Structure and Corrosion Resistance of Plasma Electrolytic Oxidation Ceramic Coatings on Magnesium Alloys Available to Purchase

F⁻ is used widely as an additive in the electrolyte solutions during the plasma electrolytic oxidation (PEO) of Mg alloys. The goal of this work was to investigate effects of sodium fluoride (NaF) on the structure and corrosion resistance of the ceramic coatings containing Ca and P on AZ91D (UNS M11916) Mg alloy using the PEO technique. The phase composition, morphology, and the element distribution of the coatings were studied using x-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The corrosion resistance of the coatings in the simulated body fluid (SBF) was examined using electrochemical impedance spectroscopy (EIS) and the polarizing curve methods. The results show that the coating was composed mainly of magnesium oxide (MgO), which was increased with a dosage of NaF. Ca and P also existed in the coating. The coating had a two-layer structure: the outer layer was loose and porous and the inner layer was comparatively dense. There were many microholes on the coating surface. Increasing the dosage of NaF, the thickness and roughness of the coating were increased. The amount of Mg increased gradually from the outer layer to the inner layer, the amount of O changed less, and the amount of F and Ca in the outer layer was more than that in the inner layer. EIS and the polarizing curve tests showed the coatings improved the corrosion resistance of the AZ91D Mg alloy in SBF solution by nearly two orders of magnitude; the coating prepared without NaF had the best corrosion resistance; the doping of NaF was not liable to improve the corrosion resistance of the coating.