Mechanism of Depassivation

For more than forty years, the conditions for the existence of the passive state, and hence for the existence of our metals-based civilization, which is based upon the reactive metals (Al, Cr, Fe, Ni, etc), have been described in terms of equilibrium thermodynamics in the form of Pourbaix diagrams. However, Pourbaix diagrams provide an equilibrium view of passivity, whereas passive films are non-equilibrium structures, whose existence depends upon an appropriate relationship between the rate of formation and the rate of destruction. Accordingly, a more accurate and realistic description of the phenomenon of passivity must be found in the field of electrochemical kinetics. This paper presents a kinetic theory for depassivation (loss of passivity), in terms of the relative rates of film growth at the metal/barrier layer interface at zero barrier layer thickness and destruction of the film at the film/solution interface.