Early Stages of Passive Film Formation on Chromium Studied by Three‐Parameter Ellipsometry

• Published in: Journal of the Electrochemical Society Vol. 144; no. 5; pp. 1581 - 1585
• Main Authors: Kim, Changwook, Paik, Woon‐kie
• Format: Journal Article
• Language: English
• Published: Pennington, NJ Electrochemical Society 01.05.1997
• Subjects: Applied sciences; Corrosion; Corrosion environments; Exact sciences and technology; Metals. Metallurgy; Optical constant; Surface layer; Refraction index; Citrate; Buffer solution; Layer thickness; Passivity; Ellipsometry; Chromium; pH; Anodic polarization; Electrode potential; Passivation
• DOI: 10.1149/1.1837644
• ISSN: 0013-4651

Chromium is an important component of numerous corrosion-resistant alloys, whose effective resistance against corrosion is due to the existence of a passive film that is enriched in chromium. Therefore, the mechanism of passive film formation on chromium and the properties and thickness of the film have been the subject of numerous studies. The studies were made by various methods including electrochemical methods, Auger spectroscopy, surface-enhanced Raman spectroscopy, and x-ray photoelectron spectroscopy (XPS) in conjunction with electrochemical experiments. The usual (two-parameter) ellipsometry has also been used to study the passive film on chromium. In this study, passive film formation on a chromium surface was studied by three-parameter ellipsometry as the film was formed by anodic polarization in citrate buffer solutions. The thickness and the optical constants of the film were obtained from automatic measurements as functions of time of anodic polarization, from subsecond to 60 s range. The thickness of the passive film increased rapidly within a few seconds, during which both the real and imaginary parts of the complex refractive index of the film changed. These film properties approached near stationary-state values after about 20 s. The stationary- state film thickness increased linearly with the potential of the chromium electrode and depended on pH of the electrolyte, whereas the optical constants did not show significant dependence on these factors. The passive film appeared to be a homogeneous single near-dielectric film, possibly of a composition close to Cr2O3. No indication of a transformation of the film was observed during the early stages of passivation. 6 figs., tab. of thickness and refractive index of films, 17 refs.