Effect of Fluoride Media on the Stability of Anodic ZrO2 Films

Abstract

Stability of anodic ZrO2 films on zirconium electrodes in sodium fluoride media was investigated using open-circuit impedance and potential measurements. The study indicates that the anodic oxide film comprises two layers; the extent of foreign ion incorporation during film growth in 1.0 N H2SO4 is responsible for the difference in the physical nature of each of these layers. Furthermore, the extent of incorporation depends on both the initial formation CD and film thickness. The dissolution process follows a zero-order kinetic law. The results of potential measurements in 0.1 N NaF medium indicate that thin anodic ZrO2 films are more passive than thicker ones. The potential value drifts towards the more negative direction with increasing NaF concentration up to 0.1 N. At this concentration, the overall dissolution rate constant (k¯, in terms of cm2·μF−1·min−1) possesses its highest value. The presence of a critical concentration (0.1 N) and also a critical temperature for the maximum dissolution rate constants of the two layers composing the anodic ZrO2 films reveals that the thinning process is sensitive to the amount of absorbed fluoride ion at the oxide/electrolyte interface.