The influence of electrophoretic deposition of HA on Mg-Zn-Zr alloy on its in-vitro degradation behaviour in the Ringer's solution

Abstract

Hydroxyapatite (HA) were synthesized via co-precipitation method using Ca(OH)2 and H3PO4 and have been electrophoretically deposited on an as-extruded Mg-6Zn-0.5Zr alloy. The synthesized HA powders were characterized and the over-time (24 h–150 h) degradation behaviour of HA coated Mg alloy has been compared with the bare alloy in the Ringer's solution. Potentiodynamic polarization tests showed that the HA coatings were quite robust till 48 h, started degrading significantly at −1.8 V vs Ag/AgCl after 72 h of immersion and were completely dissolved in 128 h. The 15 μm thick HA coating on ZK60 alloy reduced the anodic current density by one order of magnitude and resulted in more than five times lower uniform corrosion rate. Nyquist plots showed that due to the strong resistance to current flow at low frequency and the strong capacitance of charge double layer at high frequency exhibited by the HA coating, the depressed semicircles of the HA coated sample have larger diameters and heights than the ones for bare alloy. SEM results revealed that the coarse grains had thicker Mg(OH)2 build-up than the fine grains. In addition to the bulk Mg(OH)2, HA coated sample also exhibited nanosized Mg(OH)2 needles believed to be formed via a self-growth process.