Mechanism of electrolytic ZrO2 coating on commercial pure titanium

Through cathodic polarization tests of commercial pure titanium electrode in H2O, HCl, NaNO3, ZrO(NO3)2 and a mixture of HCl and NaNO3 aqueous solutions, respectively, the mechanism of electrolytic deposition of ZrO2 on commercial pure Ti has been investigated. It is found that the cathodic polarization curves in 0.0125 M ZrO(NO3)2 should be divided into three stages: the first, the reduction of dissolved H+ (2H+ + 2e− → H2), the second, the diffusion limit of H+ in the first stage (concentration polarization), and the third, the reduction of H2O (2H2O + 2e− → H2 + 2OH−). The electrolytic deposition experiments show the highest Faradaic coating efficiency of 26% at the current density 1 mA cm−2 which is at the transition stage between the diffusion limit current density (iL) of H+ (2nd stage) and the onset H2O reduction, since H+ ions are depleted and thus much more OH− ions are provided to form Zr(OH)4 gel on the cathodic surface. In the third stage, many H2 bubbles are formed and the H2 bubbles latter push the formed Zr(OH)4 away form the cathodic surface. A good adhesion to Ti substrate is obtained after coated specimens annealed at ≤973 K, and the crystallization of the coating to form tetragonal ZrO2 can occur at 723 K.