Mechanism of film growth and passivation of iron as indicated by transient ellipsometry

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Abstract
A method has been devised which utilizes intensity measurements at different offset angles of the analyser to obtain Δ and ψ. The method is valid for examining changes in Δ and ψ, so long as (a) the ψ changes are small compared with the Δ changes; (b) the electrode surface can be obtained in a sufficiently reproducible state for three successive measurements under differing optical conditions. In examinations of the growth of thin films on metals in the electrochemical situation, the rate of dissolution of the metal in the early stage of film formation may be much greater than that of film formation. The surface of the electrode may roughen, and this may give rise to significant changes of measured Δ, which are then misinterpreted in terms of film formation. A model for this situation is proposed. If the metal surface has sites in which the dissolution rate [graphic omitted]c× 10 times that for the rest of the surface, and if their concentration is high (e.g., 1012 cm–2), the model predicts significant changes of Δ due to roughening could arise from dissolution of the metal for minutes at current densities in the micro-ampere region. Qualitative evidence is also in favour of this hypothesis. An initial phase oxide, probably Fe(OH)2 grows before the current-potential peak in the passivation of iron (pH – 8.4; borate buffer). It grows initially by a 1-d mechanism, followed by a mechanism involving place exchange of Fe and O. The phase oxide thickness at the current-potential peak is about 1 monolayer. At the current-potential peak, Fe(OH)2 is converted to a Fe2O3. The conversion continues without significant change in oxide thickness for 0.2–0.4 V + ve to the peak. The cause of passivation is the sealing off of the Fe surface by the phase-oxide, Fe(OH)2. The “passive layer” which grows at higher anodic potentials is Fe2O3. The present work is not consistent with a dissolution-precipitation mechanism for the mechanism of the initial growth of the phase-oxide, because changes in Δ are observed from 0.01 s. It is also not consistent with the importance of adsorbed O as the principal observable entity on the surface before the current peak. In spite of the importance of blocking of the half-crystal position in growth, the coincidence of the attainment of the monolayer of the phase-oxide along with the peak of the current-potential relation is inconsistent with initial kink site blocking as a major passivating mechanism.
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