Cathodic reactions of electrolytic hydroxyapatite coating on pure titanium

The cathodic reactions of electrolytic HA (hydroxyapatite) coating on pure titanium in the mixed solution of Ca(NO3)2·4H2O and NH4H2PO4 were investigated by the cathodic polarization tests in Ca(NO3)2·4H2O, NH4H2PO4 and HCl aqueous solution. The deposited films at various applied current densities were characterized by XRD analysis and SEM observations. XRD analysis revealed Ca(HPO3H)2·H2O at a −0.7 V, CaHPO4·2H2O (DCPD) and Ca10(PO4)6(OH)2 (HA) at −1.25 V. SEM observations showed hydrogen bubble effect on deposition of HA at −1.55 V. The cathodic reactions in the mixed solution were verified into four regions: (1) O2+2H2O+4e−→4OH− (−0.1 to −0.3 V); (2) H2PO4−+H2O+2e−→H2PO3−+2OH− (−0.3 to −1.1 V); (3) H2PO4−+e−→HPO42−+12H2 and H2PO4−+2e−→PO43−+H2 (−1.1 to −1.5 V); (4) 2H2O+2e−→H2+2OH− (−1.5 to −3.0 V). A pure HA deposition film can be obtained after annealed at 300 °C for 4 h.