Hydrothermal Growth of Nano-Hydroxyapatite Coating Formed on Fluorinated Magnesium Alloy and Its Corrosion Behaviour

In order to explore the hydrothermal growth mechanism of hydroxyapatite (HA) coating on fluorinated magnesium alloy, the changes of morphology, composition and crystal phase of the calcium phosphate coating during the hydrothermal treatment were studied. And the change of electrochemical impedance spectroscopy (EIS) of the coating specimen of different hydrothermal treatment time was discussed to further understanding the change of the coating structure. The results demonstrated that calcium phosphate could rapidly nucleate on fluorinated AZ31 magnesium alloy. The crystal phase of calcium phosphate coating was mainly octacalcium phosphate (OCP) at the early stage of hydrothermal treatment. Then the content of OCP decreased and the content of HA increased with hydrothermal time. The coating consisted of only HA after hydrothermal treatment for about 4h. The HA coating composed of rod-like crystals exhibited an obvious double layer structure. The rod-like crystals of inner layer arranged into dense bundles and the rod-like crystals of outer layer arranged into loose chrysanthemum-like clusters. Fluoride conversion layer acted as an intermediate transition layer to connect magnesium alloy and HA coating into a whole. The results of immersion test in simulated body fluid demonstrated that HA crystals dissolved slowly. No peeling occurred of HA coating during the 12 days' immersion. Pitting corrosion was still the mainly corrosion mode of magnesium alloy substrate due to the electrolyte infiltration during the immersion.