ELECTROCHEMISTRY OF TITANIUM IN MOLTEN 2ALCL(3)-NACL

The electrochemistry of titanium has been examined in 2AlCl(3)-NaCl electrolyte. Titanium may be oxidized to yield Ti(II), Ti(III), and Ti(IV) complexes. The divalent species may be used to electrodeposit Al-Ti alloys, while the trivalent species is sparingly soluble. Cyclic voltammetry on a tungsten electrode in solutions with varying Ti(II) concentration has been used to examine the kinetics of the precipitation reaction associated with Ti(LII). The induction time required for precipitation is dependent upon the bulk concentration of Ti(II), in a manner similar to that reported for homogeneous precipitation from aqueous solutions. At higher Ti(II) concentrations and slower sweep rates the electrode is passivated by the Ti(III) precipitation. Slow sweep rate voltammetry suggests that the i-E characteristics of the passivation reaction are dominated by the resistance associated with the precipitate film. The film blocks the electrode preventing oxidation of Ti(II) to Ti(IV). A parallel study of the dissolution kinetics of titanium metal reveals similar passivation phenomena due to Ti(III) precipitation. However, the passive film on titanium is somewhat conductive unlike that associated with the precipitated film formed on a tungsten electrode. This distinction presumably results from the formation of a compact passive film at the interface between the precipitated film and the titanium substrate. At more oxidizing potentials the protective nature of the passive film breaks down with the generation of Ti(IV). A comparison between the Ti(II) concentration determined by voltammetry and that anticipated from dissolution of titanium metal reveals a deviation from Faraday's law at high Ti(II) concentrations. This discrepancy is resolved by adopting a previously postulated model involving the formation of oligomers of Ti(II).