AN ELECTROCHEMICAL IMPEDANCE ANALYSIS OF PASSIVE FILMS ON NICKEL(111) IN PHOSPHATE BUFFER SOLUTIONS

Electrochemical impedance data for single crystal Ni(111) in phosphate buffer solutions of pH 9, 10, and 11 and at potentials spanning the passive range (0.1-0.5 V/sce) are interpreted in terms of a point defect model for the passive film in which the kinetics of generation and annihilation of vacancies at the metal/film and film/solution interfaces are taken into account. The best fit of the model to the experimental data was obtained by assuming that cation vacancies are mobile in the film. A single set of kinetic and transport parameters, a constant electric field strength, and a potential dependent double layer capacitance were sufficient to reproduce the experimental impedance at frequencies from 10 kHz to 1 mHz, at pH values from 9 to 11, and at applied voltages of 0.l and 0.5 V/sce. © 1990.