A model for surface charge-assisted barrier film growth on metals in acidic solutions based on ac impedance measurements

A kinetic model of the passivation of metals via formation of a dual layer comprising a barrier film and a salt overlayer is proposed. It is assumed that the barrier film grows through high field-assisted migration of defects. Ionic surface charges are formed both at the metal/barrier film and barrier film/ electrolyte interfaces. These charges can either enhance or retard the transport of defects during film growth resulting in the observation of a pseudoinductive or capacitive time constant in the impedance spectra. The dissolution of the barrier film at its interface with the electrolyte is assumed independent on the electrode potential. The model is able to reproduce the overall ac impedance response of several metals in their region of passivity. This response is a sum of the impedances of the barrier film/electrolyte interface, and the one of the barrier film itself. As representative systems, passive Bi and Sb in suphuric acid solutions are discussed. The main kinetic and structural characteristics of the metal/barrier film/electrolyte system are determined and some limitations of the model are outlined. Copyright (C) 1996 Elsevier Science Ltd.