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On the anodic passivation of metals, two different processes, (A) precipitation from a super-saturated solution and (B) direct reaction between a metal and solution, have been previously considered for the anodic formation of passive oxide film. Both of these mechanisms were criticized from the kinetic point of view to conclude that both are negative for the formation of a compact pore-free oxide film. Then, in order to clarify the passivation process, many electrochemical investigations were made on the active-passive transition of nickel in sulfuric acid solutions. It was concluded that the passive film is formed in accordance with the following successive reaction, The critical potential beyond which such an oxide formation takes place is calculated to be equal to the Flade potential, if the concentration of NiOH+ ion in the vicinity of the surface has a saturated value corresponding to the solubility product of NiO. A generalized theory for the passivation of metals in corrosive solution can be proposed. The compact pore-free passivating film can be formed only by an anodic reaction of soluble metal ion on the surface of metals, and so the passive film having an ability to protect the metals against dissolution in corrosive solutions must be composed with an oxide film of higher valency than the ionic valency of the soluble metal ion in active dissolution. (Received June 12, 1961)

A Mechanism of the Anodic Passivation of Nickel in Acid Solution &ndash;Higher Valence Oxide Film Theory&ndash;

A Mechanism of the Anodic Passivation of Nickel in Acid Solution –Higher Valence Oxide Film Theory–