New Rate Laws for the Growth and Reduction of Passive Films

A new rate law for the growth of anodic passive films on metal surfaces is derived from the point defect model (PDM). The model recognizes both the growth of the barrier oxide layer into the metal via the generation of oxygen vacancies at the metal/film interface and the dissolution of the barrier layer at the film/solution interface. The new rate law accounts for the existence of a steady state in film thickness, as well as for the transients in thickness and film growth current as the potential is stepped in the positive or negative direction from an initial steady state. The predictions of the PDM, with respect to the thickness of the barrier layer, are compared with those of the high field model. The latter cannot account for the existence of steady states or the decrease in barrier layer thickness on stepping the potential in the negative direction. The predicted transients in film thickness and growth current density are in good fidelity with the measured transients on tungsten in pH 1.5 phosphate buffer solution as the voltage is cycled between 10 V in reference to saturated calomel electrode and 6 Finally, the new rate law accounts for passive film thinning under negative potential step conditions as the pH is changed over the range of 1.5 to 5.1 and as the initial and final voltages are changed in a systematic manner, such that the voltage excursion is constant. © 2001 The Electrochemical Society. All rights reserved.