Influence of pH on Corrosion and Passivation of Nickel

Abstract

Abstract Anodic and cathodic polarizations of nickel have been studied using a galvanostatic technique with low current densities and an atmosphere of hydrogen. Polarization experiments have been conducted using 0.1 N sodium hydroxide, 1 N sodium carbonate and 0.1 N sodium borate solutions. It has been shown that passivation is caused by the formation of a monomolecular layer of Ni(OH)2 in alkaline solutions and by 1–2 molecular layers in borate solution. On further oxidation, the Ni(OH)2 becomes oxidized to Ni2O3 and finally to a nonstoichiometric oxide corresponding to NiO1.8. This final oxidation state is unstable and decomposes as soon as the electrode is made cathodic. It has been shown that current density has a critical effect on polarization characteristics in the borate solution. No evidence was found for the existence of the intermediate oxide Ni3O4.