Anodic oxidation of tungsten in sulphuric acid solution—Influence of hydrofluoric acid addition

The anodic oxidation of tungsten has been studied in 1 M H2SO4 solutions containing 0–0.25 M HF. The active-to-passive transition is observed in the potential range 0.4–1.0 V vs. SCE, indicating coupling between film formation and metal dissolution reactions. Steady-state currents measured in the passivation and passivity ranges increase significantly with increasing fluoride concentration, indicating enhanced dissolution of the oxide film. The electrochemical impedance response is dominated by the processes in the barrier sublayer and at its interface with the electrolyte. The presence of a pseudo-inductive loop in the impedance spectra at intermediate frequencies indicates point defect interaction during the film growth and dissolution processes. A kinetic model including the recombination reaction between positively and negatively charged point defects at the film/solution interface as well as an elaborate kinetic scheme for the tungsten dissolution through the film mediated by cation vacancies is proposed. It is found to reproduce satisfactorily the steady-state currents and the impedance spectra as depending on potential in the range 0.4–5 V. Such a model for the conduction mechanism in the barrier sublayer is believed to be an essential part of a modelling approach to the formation of a nanoporous overlayer on tungsten in fluoride-containing solutions.