Comparison of calcium phosphate coatings on AZ31 and fluoride-treated AZ31 alloy prepared by hydrothermal method and their electrochemical corrosion behaviour

HA (hydroxyapatite) coating was deposited on AZ31 and the fluoride-treated AZ31 magnesium by hydrothermal treatments in EDTA-Ca and KH2PO4 aqueous solution to improve the corrosion resistance and biocompatibility of magnesium alloys. Crystal phase, morphology and composition of the coatings were investigated by XRD, SEM and EDS, respectively. The electrochemical measurements and immersion tests were carried out in Hank's solution to examine and compare the corrosion behaviours of the coated specimens. HA coating formed on AZ31 alloy consisted of flake-like crystallites, while the coating on fluoride-treated AZ31 magnesium consisted of needle-like crystallites. The HA coatings have two layers. The inner layer is dense and fine-grained, while the outer layer is loose and well-crystallized. Fluoride conversion film with high resistance and low capacitance is more effective than HA coating on delaying the degradation of magnesium alloy. HA coating further increased the open circuit potential and decreased the corrosion current densities of MgF2 coated AZ31, but the Rp of HA/MgF2 coated specimen is a little lower than that of MgF2 coated specimen due to a slight dissolution of fluoride conversion layer during the hydrothermal treatment process. Reducing the dissolution or keeping enough thickness of fluoride conversion film during the hydrothermal treatment could improve the quality of the coating and further postpone the degradation of magnesium alloy.