The role of hydroxide in the electrochemical impedance response of passive films in corrosion environments

A simple model has been developed (within the framework of the point defect model, PDM) assuming that the impedance response of passivated metals is dominated by the barrier layer, which consists of a partly hydrated and highly defective oxide layer. Simulations were carried out for cation vacancy conducting barrier layers (with p-type electronic character) and O2- vacancy conducting barrier layers (with n-type electronic character) including in both cases the possible contribution by transport of O H- vacancies. A very good agreement was obtained between calculated impedance diagrams and typical impedance spectra for passive films on nickel and titanium. For both types of films the impedance response is quite sensitive to variations in the formal rate constant k-3b involved in the generation of hydroxyl vacancies at the metal/film interface. Furthermore, in the case of films with p-type electronic character the shape of the Nyquist diagram is sensitive to variations in the ratio of the kinetics of hydroxyl vacancies generation to the kinetics of cation vacancies generation at the metal/film interface.