中性溶液中のニッケルの不働態化挙動

Abstract

The anodic passivity of nickel in neutral solutions has been investigated by means of potentiostatic oxidation, galvanostatic reduction and chemical analysis techniques, and the thickness of anodic oxide film and the effect of solution pH and of anion species on the passivation behaviour are examined. It is shown that the oxide film formed by anodic oxidation is difficult to be removed by cathodic reduction in neutral solution. To obtain a bare surface, cathodic reduction has to be carried out in sulphuric acid for oxidized surfaces and either in acid or in neutral solution for electropolished surfaces.In the passivity region, where NiO film is assumed to be formed, nickel dissolution is still observed during oxidation and the amount of nickel dissolved is dependent on the oxidation potential, the solution pH, and the anion species present. The film thickness estimated from coulometry in the pH range 5.5 to 9.5, however, is not practically affected by the solution pH and the anion species but increases linearly with rise of the potential from 6 to 16 Å.Above the potential of passivity region, the secondary passivation occurs where nickel dissolution is not detected. It is also shown that the secondary passivation is due to the formation of a higher valence nickel oxide, probably Ni2O3, which may be electrodeposited from dissolved Ni2+ ion in solution. The thickness, estimated from the potential decay curve of galvanostatic reduction, increases rapidly with rise of the potential and then decreases through a maximum. Further rise of potential results again in the dissolution of nickel independent of the solution pH and the anion species.The growth kinetics of both the passivation film and the secondary passivation film is concluded to obey the inverse logarithmic rate law.