AbstractThe strikingly large initial effects observed in passivation phenomena are attributed to a temporary change associated with sorption of a suitable specie onto the clean metal in an amount corresponding to about a monolayer. It is agreed that permanent passivity stems from a thin, insoluble film of a compound of the metal but it is suggested that it forms by migration of the metal ion into the array displayed by the sorbed species. It is postulated that the more nearly a complete sorbed monolayer forms before metal ion migration starts the more likely is the formation of a defect‐free, fissure‐free compound film. The thickness of such films should be self‐limiting kinetically and should provide long‐lived passivity because they offer minimum opportunity for meeting of the reactants. In the condition offering optimum protection they are probably amorphous with a minimal number of grain boundaries. Further thickening, by temperature increase, by large applied potential, or by increased reactivity of the environment, should lead to crystallization and increased porosity by the introduction of more grain boundary paths, actual fissures, and increased lattice permeability. This results in loss of passivity unless the coherent sorption‐initiated film can be regenerated under the external scale. The effect of solution species either on activation or on passivation are associated with permeation of the “non‐porous” film or with adsorption competition which either aids or interferes with regeneration of the sorption