Kinetic Stability Diagrams

Our reactive metals-based civilization is possible only because of the formation of thin oxide (passive) films on surfaces that effectively separate the reactive metals from the corrosive environments. Previously, the conditions under which passivity occurred were defined in terms of equilibrium potential-pH relationships in the form of Pourbaix diagrams, which delineated regions of stability, passivity, and immunity. However, recent work has demonstrated that a passive film is never at equilibrium, but instead exists in the meta- stable state, if it exists at all. Thus, the limits of stability must be defined in terms of kinetic factors and not in terms of equilibrium principles. This has been done in the present study by phase space analysis of the rate law for film growth afforded by the Point Defect Model to yield Kinetic Stability Diagrams, which are proposed as kinetic alternatives to the equilibrium thermodynamic Pourbaix diagrams.