ニッケルの不働態化機構―高級酸化物皮膜説―

抄録

It has been pointed out in our previous paper that the Flade potential of nickel is the equilibrium potential of the redox reaction of 3NiO+H₂O\ ightleftarrowsNi₃O₄+2H⁺+2e and that the passive film is composed of these two oxides. For the purpose of clarifying the role of this double oxide film on the passivation of nickel, the transition phenomena from the active to the passive state were investigated by means of an electrochemical technique, and a generalized theory of passivity was presented from the kinetic standpoint. It can be expected that passivation would result when the metal surface is covered with a compact film which has the character of self-repairing action. The defects of the double oxide film of NiO and Ni₃O₄ can be repaired by the electrode reaction of Ni²⁺+2NiO+2H₂O→Ni₃O₄+4H⁺+2e, because the rate of this reaction increases with increasing the concentration of nickel ion resulting from the active dissolution at the defects. Such a repairing electrode reaction cannot be kinetically expected on the single oxide film of NiO, so that no passivition results from the formation of this film. It is assumed in general for the passivation of metals in corrosive solution that the passive film should always be composed of oxides in which the valency of metal ions is higher than that of the metal ions dissolved from the active defects into the solution. The passivation of iron and chromium could also be explained by this higher-oxide-film theory.