Valence and core photoemission of the films formed electrochemically on nickel in sulfuric acid

Abstract

The nature of the anodic films formed on nickel in 0.5 mol dm^–3 sulfuric acid after polarization in the passive and transpassive state has been investigated by X-ray photoelectron spectroscopy. It is found that the film thickness (10–25 Å) grows linearly with potential in the passive region. Film thickness drops at the beginning of the transpassive region, due to the further oxidation of NiO to NiO₂H (which may be soluble in the electrolyte). The results from anaerobic cell and ex situ experiments were compared in order to study the effect of atmospheric conditions on the electrochemical treatment. Experiments performed in the anaerobic cell show strong sulfate adsorption and the trapping of sulfate ions in the passive film. No significant sulfate adsorption is found in the ex situ experiments, presumably because the air-formed oxide layer prevents these ions from being trapped in the film.