IN-SITU ELLIPSOMETRIC CHARACTERIZATION OF FILMS FORMED BY CATHODIC REDUCTION OF CHROMATE

In the chlorate process, it is known that a small addition of chromate to the electrolyte increases the current efficiency at the cathode by hindering the electrochemical reduction of intermediately formed hypo chlorite. This paper presents results from in situ ellipsometric studi es of the formation of the chromium(III) hydroxide film in an attempt to further elucidate the consequences to the cathode surface. The resu lts obtained are in good agreement with findings from conventional ele ctrochemical and surface analytical techniques. The thickness of the c hromium(III) hydroxide layer obtained by cathodic reduction on evapora ted gold electrodes is less than 4 nm after 2 h of film growth at -1.0 and -0.6 V rel. SCE. At -1.0 V, the initial film growth is rate limit ed by the nucleation process. With 0.15 mM chromate, the film attains a maximum thickness within 2 h of growth. When the chromate concentrat ion is 15 mM, the rate of film growth rapidly decreases, but continuou s growth is still in progress after 2 h of exposure. The film grows ho mogeneously and there are neither structural rearrangements within the film during build-up, nor changes in the mechanism of film growth wit h time. Studies of deposition and dissolution of the film indicate tha t the mechanistic path is the same in both directions. Copyright (C) 1 996 Elsevier Science Ltd