Microstructure and corrosion behavior of Ca/P coatings prepared on magnesium by plasma electrolytic oxidation

In order to enhance the corrosion resistance and biocompatibility of Mg based implants, porous MgO-CaP coatings were fabricated on commercial Mg by adding hydroxyapatite (HAp) particles in alkaline phosphate solution for plasma electrolytic oxidation (PEO) treatment. The effects of particle concentration on the coating morphology and composition were investigated under constant current PEO processing. Electrochemical impedance spectroscopy (EIS) and immersion test conducted in simulated body fluid (SBF) environment were applied to evaluate the corrosion performance of the coatings. The results showed that most HAp particles were reactively incorporated into the PEO coatings and the coating microstructure was significantly modified with increasing concentration of HA, which resulted in higher corrosion resistance in return. The consecutive growth of impedance during 72 h immersion resulted from the precipitation and deposition of CaP compound and corrosion product into the pores sealing the defects of PEO coatings. Overall, a higher concentration (20 g/L) of HAp particles addition would bring out a denser and protective PEO-HAp coating.