Cathodic Behaviour of TiO₂ in Molten CaCl₂-based Electrolytes

The cathodic behaviour of thermally formed TiO₂ layers on commercial purity titanium substrates was studied using linear sweep and cyclic voltammetry in molten CaCl₂-CaO electrolytes over the temperature range 973 - 1273 K. Five electrochemical processes were observed at potentials positive of the potential corresponding to the formation of Ca° at unit activity. Two initial processes, occurring at potentials 300 mV negative of the TiO₂ open-circuit potential, were attributed to the reduction of TiO₂ to Ti₃O₅, and subsequent reduction of Ti₃O₅ to Ti₂O₃. The concurrent increase in local oxide concentration exceeded the solubility product and caused the formation of a passivating calcium titanate surface layer. At potentials approximately 500 mV positive of the potential for unit activity Ca° formation, three cathodic processes were observed; the reduction of CaTiO₃ occurring at potentials positive of the reduction of Ti₂O₃ to TiO, and, finally, the reduction of TiO to metallic titanium with dissolved oxygen. The removal of dissolved oxygen was found to occur at potentials near to that for unit activity Ca° formation. Material was characterized before and after cyclic voltammetry using scanning electron microscopy, X-ray energy dispersive spectroscopy and X-ray diffraction.