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

The electrochemistry of titanium has been examined in 2AlCl(3)-NaCl el ectrolyte. Titanium may be oxidized to yield Ti(II), Ti(III), and Ti(I V) complexes. The divalent species may be used to electrodeposit Al-Ti alloys, while the trivalent species is sparingly soluble. Cyclic volt ammetry on a tungsten electrode in solutions with varying Ti(II) conce ntration has been used to examine the kinetics of the precipitation re action associated with Ti(LII). The induction time required for precip itation is dependent upon the bulk concentration of Ti(II), in a manne r 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 r eaction are dominated by the resistance associated with the precipitat e film. The film blocks the electrode preventing oxidation of Ti(II) t o Ti(IV). A parallel study of the dissolution kinetics of titanium met al reveals similar passivation phenomena due to Ti(III) precipitation. However, the passive film on titanium is somewhat conductive unlike t hat associated with the precipitated film formed on a tungsten electro de. This distinction presumably results from the formation of a compac t 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 comp arison between the Ti(II) concentration determined by voltammetry and that anticipated from dissolution of titanium metal reveals a deviatio n from Faraday's law at high Ti(II) concentrations. This discrepancy i s resolved by adopting a previously postulated model involving the for mation of oligomers of Ti(II).