ION SELECTIVITY OF PRECIPITATE FILMS AFFECTING PASSIVATION AND CORROSION OF METALS

When a metal corrodes in aqueous solutions, its surface is frequently covered by a precipitate film of corrosion products of hydrated metal oxides or other insoluble salts. In this paper, the effects of solution pH and of adsorption of CrO//4**2** minus , MoO//4**2** minus , and WO//4**2** minus ions on the ion selectivity of hydrated iron (III) oxide membranes are studied by means of membrane potential and polarization measurements. Furthermore, a molecular mechanism of metal passivation is presented in which the bipolar fixed charges consisting of an anion selective hydrated oxide film on the metal and a cation selective layer in contact with the solution play an important role.