A General Thermodynamic Theory of the Potential of Passive Electrodes and Its Influence on Passive Corrosion

Abstract

A theory of the electrode potential of nonstoichiometric electron conductive metal oxides on a foreign metal is presented. When the oxygen ions of the oxide are in equilibrium with the electrolyte the oxide electrode potential is pH with the free enthalpy for the reaction as a function of the degree of oxidation n. The oxide electrode potential for the case of an equilibrium of the metal ions between the oxide and the electrolyte is also derived. The condition of equilibrium between the oxide and the electrolyte with dissolved z‐valent metal ions is given. The “equilibrium oxide” relative to its parent metal and the corresponding potential are discussed. The potential of a passive electrode depends only on the surface oxide in contact with the electrolyte and is independent of the construction of the passive layer.