<I>In situ</I> Analysis of Passive and Transpassive Films on Nickel by Modulated UV-Visible Reflection Spectroscopy

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Modulated reflection spectra have been measured on Ni at passive and transpassive potentials in 0.5 kmol·m−3 H2SO4 (pH 0.3), 1 kmo·m−3 Na2SO4 (pH 2.0–12.0), 0.1 kmol·m−3 NaOH (pH 13.1), and 1 kmol·m−3 NaOH (pH 13.7). By comparing the spectra with those of several reference Ni oxides and oxyhydroxides, constituent substances of passive and transpassive films formed on Ni in these solutions have been identified.<BR>It was found that the composition of the films changed remarkably with solution pH and electrode potential: In acid solutions of pH ≤4.0, the primary passive film was mainly composed of NiO, and the transpassive film of a nonstoichiometric Ni(II) oxide, i.e., NiOx. The value of x in NiOx increased with increasing potential. The change in the film composition in neutral solutions having pH between 6.0 and 9.0 was very similar to that in acid solutions, except that a thin layer of γ-NiOOH·2NiO2 was formed on the surface of NiOx film. This occurred at transpassivation and oxygen evolution potentials through the anodic oxidation of dissolved Ni2+ ions. In alkaline solutions of pH ≥10.0, the passive films formed at lower potentials were composed of Ni(OH)2 and NiO. The outer Ni(OH)2 layer took an α-form in solutions of pH ≤12 and a β-form in those of pH ≥13. These α- and β-Ni(OH)2 in the films were oxidized respectively to γ-NiOOH·NiO2 and β-NiOOH with increasing potential.

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