Precise potential control and measurement have permitted the determination of the primary activation potential, Ea, of an Fe‐24% Cr alloy in to within ± 0.5 mv and the determination of the kinetics of passivating film growth at this potential, as well as at other potentials. Small changes in potential give rise to large differences in film thickness. From the time dependence of the anodic current and the film thickness during passivation, a theory for the primary passivation process is proposed based on different rates of dissolution and different degrees of passivation at various types of surface sites, e.g., kinks, ledges, and terraces; these differences arise from distinct activation energies for anodic dissolution and distinct adsorption energies associated with each type of site. The steady‐state film thickness at the activation potential, as measured by cathodic reduction, is and corresponds to between one and three monolayers of film depending on the assumed model of a monolayer and the reduction product. Finally it is shown that secondary film formation can occur at the activation potential after sufficiently long times of passivation.