Ellipso-reflectometry applied to the study of passivating films on nickel

Abstract

A method was established to derive the optical properties of a substrate and of a thin film on its surface by measuring the intensities at three directions of polarization after reflection of incident linearly polarized light. This method of ellipso-reflectometry was used to study the passivity of nickel in acid sodium sulphate solutions at pH 0.9 and pH 1.6. The optical constants of active nickel exhibited a systematic dependence on the angle of incidence and a statistical scatter related to surface roughness. Measurements with passive nickel at different angles of incidence did not provide evidence for an anisotropy of the optical constants parallel and perpendicular to the surface. The optical constants and the thickness of the passivating film depend on the electrode potential in the same way at pH 1.6, pH 0.9, and pH 0.1. The refractive index and the thickness change with the potential non-monotonously. At EHSS=1.05 V vs. the hydrogen electrode in the same solution, a maximum of the refractive index and a minimum of the thickness is observed, while the extinction index grows monotonously with the potential. At any constant potential, the refractive and the extinction indices both decrease with the pH value, and the thickness stays nearly constant. It is concluded that the water content of the passivating film rises with the pH value. During activation at negative potentials, the fast processes of reduction and transformation of the expanding film into nickel hydroxide precede the slow currentless dissolution. The first process becomes faster at more negative potentials. The dissolution rate is independent of the activation potential, is always smaller than the steady-state corrosion rate at any potential at which the film was formed prior to activation, and decreases with the pH value in the same way as the steady-state corrosion rate.