Abstract
The formation and corrosion protectiveness of calcium phosphate coating layers depend on the coating conditions and the type of magnesium (Mg) substrate. Therefore, this study aimed to investigate the formation of the calcium phosphate coating layers on Mg-6mass% Zn-0.5mass%Zr (ZK60) alloy over a wide pH range of pH 6.5 to 10.2. Immersion tests with Mg2+ ion quantification were performed to study the degradation behavior of ZK60 coated with different types of calcium phosphate. It was found that the morphology and types of the calcium phosphate coating layers were controlled by pH conditions. At pH 6.5, an octacalcium phosphate (OCP) single phase layer was formed. At pH 7.0, an OCP + hydroxyapatite (HAp) mixture layer was formed. A HAp single layer and a HAp layer with an intermediate Mg(OH)2 layer were formed at pH 7.8 and 10.2, respectively. The coating layers provided good corrosion protection but at different levels. The specimen coated at pH 7.8 possessed the best corrosion resistance. On this specimen, biocompatible hydroxyapatite (HAp) was deposited as a corrosion product after the immersion. The results suggest that the specimen coated at pH 7.8 serves as a suitable orthopedic implant.
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