Abstract
The kinetics of growth of the primary and of the secondary passivating films have been examined for an Fe‐24% Cr alloy in 
over a potential range of several hundred millivolts above the activation potential. From these data it has been possible to evaluate the effectiveness of an adsorbed film and of an oxide film in causing passivity. It is concluded that the type of film that gives rise to passivity, as well as the mechanism of anodic dissolution during the passivation process, is potential dependent: At the activation potential an adsorbed film is responsible for passivity by inhibiting the direct, activated transfer of ions from the metal surface into solution; at high potentials an oxide film acts as the passivating agent with the current limited by the rate of diffusion of ions through the film; in a 200 mV intermediate region the adsorbed film is responsible for initiating passivity, but the steady‐state film is an oxide. In addition the data indicate that the rate of growth of both the primary and the secondary films is potential dependent; at sufficiently low potentials the rate constant for primary film formation is greater than that for secondary film formation, while at higher potentials the rate constant for secondary film formation becomes greater.