Corrosion resistance and biocompatibility of calcium-containing coatings developed in near-neutral solutions containing phytic acid and phosphoric acid on AZ31B alloy

Calcium phosphate (Ca-P) coatings were fabricated by micro-arc oxidation (MAO) on AZ31B magnesium alloy in near-neutral pH solutions (pH 7.6–7.9) and the influences of EDTA-CaNa2, phytic acid (IP6), phosphoric acid (PA), and treatment time on coating properties were investigated by an orthogonal experiment. The results show that coating corrosion resistance is synergistically determined by coating characteristics with coating thickness playing a particularly important role. EDTA-CaNa2 acts as a corrosive agent of magnesium alloys and the increased concentration increases calcium content but decreases corrosion resistance of MAO coatings. As a strong chelating agent, IP6 can promote EDTA-CaNa2 solubility and therefore increases the calcium content more effectively than PA does. Compared with PA, IP6 more effectively improves corrosion resistance mainly by increasing coating thickness and bonding strength between coating and substrate, although the uniformity of anodic coatings becomes worse due to the greater interfacial tension of the IP6 solution than that of the PA solution. During MAO, P and F compete with each other to enter into anodic coatings. F amount in MAO coatings is closely related to the coating biocompatibility. MAO coatings developed in the solution composed of both IP6 and PA achieve low F content (3.49 at%) and good biocompatibility, while those fabricated in solutions containing only IP6 or PA achieve high F content (higher than 19.00 at%) and exhibit high toxicity.