Kinetic theory 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. These diagrams plot equilibrium potential versus pH relationships for various reactions (e.g., Fe/Fe ₃O ₄, Fe/Fe ²⁺, Fe ₃O ₄/Fe ²⁺) to define regions of stability or predominance. 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. It is this kinetic theory for depassivation (loss of passivity) that is presented in this paper.