Electrochemical and Optical Studies on the Passivation of Nickel

[摘要] The technique of combined-measurement of reflectance and ellipsometric parameters was used for studying the anodic film formed on nickel surface in basic solutions. An ellipsometer was automated for transient measurements by way of modulating the plane-polarized light with the Faraday effect. Surface film was formed electrochemically by applying a potential step from the reduction potential range to the passivation range on a polished, high-purity, polycrystalline nickel specimen. From that instant, the changes in the reflectance(r) and the ellipsometric parameters(¥Ä, ¥×) of the surface film were recorded by the automatic ellipsometer. Three exact simultaneous equations including these optical signals, ¥Ä,¥× and r were solved numerically with a computer in order to determine the optical properties, n, k, and the thickness, ¥ó, of the surface film. From the computed results which showed dependence on pH and time, it was found that passivation of nickel can be effectively attained by surface film thinner than 15¡Ê and this passivation film has a small optical absorption coefficient. It seemed that a high pH environment enhances the rate of passivation and is favorable for a denser structure of the surface film. The experimental evidence is in accordance with the hypothesis that the composition of the passive film can be approximated by Ni(OH)2 in the early stage of passivation and that as time passes the composition changes partially toward that of NiO through dehydration.

[学科分类] 化学（综合）