DEFINING THE PASSIVE STATE

The growth of passive film on tungsten in phosphate buffer solution ha s been described in terms of the Point Defect Model (PDM). The steady- state current and passive film thickness have been measured as a funct ion of voltage, with the him thickness being obtained from an analysis of capacitance and reflectance data. The observed data cannot be acco unted for by the High Field Model (HFM) in its classical form, but can be understood in terms of the PDM. Diagnostic criteria that have been derived from the PDM were used to identify the majority charge carrie rs in the passive film. The Point Defect Model was employed, together with Mott-Schottky analysis to explore the crystallographic defect str uctures of the passive films, whereas their electronic structures were studied using photoelectrochemical impedance spectroscopy (PEIS). The experimental results demonstrate that these structures are strongly c oupled with the vacancies acting as the dopants.