Behavior and Nature of Thin Oxide Films On Some Metals in Gaseous Media And in Electrolyte Solutions

Two schools of thought have existed regarding the passivation mechanism of metals. One considers the film formed on the metal’s surface to be the principal protective device. The other ascribes the metal’s passivation to adsorption layers of oxygen or oxygen-containing compounds. The author believes that neither the surface films nor the adsorption layers predominate in the passivation process, but that both act simultaneously to provide metal protection. The passivation mechanisms of chromium, titanium and a stainless steel were studied. An optical polarization method was used for determining thicknesses of very thin oxide films. Electrochemical (galvanostatic and potentiostatic) methods were employed to follow film behavior in neutral and acid electrolyte solutions. Dissolution of chromium and of the stainless steel studied, in the passive state (transpassive region), apparently proceeds through cyclic formation of a film which becomes soluble to some extent as it grows thicker. Partial dissolution then occurs, followed by new formation of a less-thick film.