An Ellipsometric Determination of the Mechanism of Passivity of Nickel

Abstract

The capacity of nickel in acid solution have been followed by means of ellipsometry under conditions of potentiostatic control of the material at various points in the passivation current- potential diagrams and also with automated equipment which allows the recording of the ellipsometer reading under transient conditions. It is found possible to evaluate the refractive index, thickness, and light abosrption of the film at all parts of the process. The film begins to form at the first inflection on the current-potential diagram, but after the current reaches its maximum point, it does not change further in thickness. When the current begins to descend, there is a parallel rapid increase in the electronic conductivity of the film. Passivation arises from a critical increase in semiconduction of the film on the surface. The increase in electronic conductivity reduces the field available for the encouragement of anodic dissolution and transport through the film. Hence, dissolution ceases, i.e., passivity, occurs