The ability of a surface charge approach to describe barrier film growth on tungsten in acidic solutions

A renewed version of a surface charge approach to describe the impedance response of anodic film growth on passive metals in acidic solutions is presented. It is based on the chemistry of the Point Defect Model, the fact that oxygen vacancies are the main charge carriers in a range of anodic oxides and the suggestion of a constant field strength in the bulk of the barrier layer. Generalized transport equations valid for any field strength are employed to the oxygen and metal vacancy motion and the two limiting cases of high and low fields are discussed. A negative surface charge due to accumulation of metal vacancies near the film/solution interface accelerates the oxygen vacancy transport, thus explaining the pseudo-inductive behaviour of the metal/film/electrolyte system under small amplitude ac perturbation. Using the W/WO3/electrolyte system as a model, the basic parameters characterizing film growth are determined on the basis of the potential dependence of ac impedance spectra. (C) 1997 Elsevier Science Ltd.