Oxide Film Thickening on Titanium in Aqueous Solutions in Relation to Anion Type and Concentration

Abstract

The open circuit potentials of the Ti electrode were followed as a function of time in strongly aerated solutions of Cl⁻, Br⁻, I⁻, NO₂ ⁻, NO₃ ⁻, H₂PO₄ ⁻, SO₄ ²⁻, CrO₄ ²⁻ and WO₄ ²⁻ at different concentrations until steady-state values were attained. In CrO₄ ²⁻ solutions the potential tended at first to move towards more negative values, then changed to more positive ones. This indicates the instability of the air-formed oxide film, and its transformation into a thin, more compact layer, probably incorporating Cr. In solutions of the other anions, on the other hand, the potential shifted immediately towards more positive values, indicating film thickening and repair. Film thickening on Ti in solutions of the various anions followed a logarithmic growth law. Plots of the potential of the Ti electrode as a function of immersion time were generally formed of two linear segments; probable reasons for this behaviour are suggested.

The rates of oxide thickening were computed for all solutions. In Br⁻, I⁻, H₂PO₄ ⁻, SO₄ ²⁻, WO₄ ²⁻ and CrO₄ ²⁻ solutions the rate of film thickening’ was independent of concentration. With oxidising anions it tended to increase with decrease of their oxidising properties, e.g., CrO₄ ²⁻, WO₄ ²⁻, SO₄ ²⁻ or H₂PO₄ ⁻4 (the last two not being oxidising agents). Film thickening in NO₂ ⁻ solutions decreased with increase in concentration, and in NO₃ ⁻ and Cl⁻ solutions it increased with increase in salt concentration.