Characterization of Oxide Films by Electrochemical Impedance

Abstract

When performing global and local electrochemical impedance measurements with metals or alloys covered by an oxide layer, CPE behavior ascribed to a 3-D normal distribution of an oxide property is often seen in the high-frequency range. Impedance data obtained with Nb anodically polarized in acid fluoride medium (thick oxide layer) and Fe­17Cr stainless steel immersed at OCP in aerated mildly acidic solution (thin oxide layer) were analyzed using the Young model that assumes an exponential decay of the conductivity within the oxide layer. The corresponding Young impedance directly provides the oxide film capacitance from which the oxide film thickness can be estimated. For both thick and thin oxide layers, the Young expression accounts well for the CPE behavior, and the values obtained for the thickness are in good agreement with independent measurements. However, in the case of thin oxide layers, the deduced conductivity profile is not fully satisfactory and new models must be tested.