Theory of Passive Film Stability

Some of the physico-electrochemical properties of passive oxide films that form on reactive metal and alloy surfaces, and which protect the underlying metals from reaction with corrosive environments, are reviewed within the context of the Point Defect Model (PDM). This model yields analytical expressions for the steady state current and film thickness and for the transients in these properties that can be used to predict deterministically the accumulation of general corrosion damage to metal surfaces, provided that the evolutionary path to the future state is continuous and can be specified. The conditions under which passivity may exist are defined in terms of Phase Space Analysis (PSA) of the PDM and it is found that passivity is invariably a meta-stable phenomenon. PSA leads to the development of Kinetic Stability Diagrams (KSDs) as alternatives to the equilibrium thermodynamic Pourbaix diagrams that are now used to describe the conditions under which passivity is observed.