DISSOLUTION OF THIN IRON-OXIDE FILMS USED AS MODELS FOR IRON PASSIVE FILMS STUDIED BY IN-SITU X-RAY-ABSORPTION NEAR-EDGE SPECTROSCOPY

This paper reports results from x-ray absorption near-edge spectroscop y (XANES) studies during polarization of thin sputter-deposited iron o xide films in acidic solutions. The dissolution rate of iron oxides in acidic solutions was found to be strongly increased by the presence o f Fe2+ in the oxide. During anodic polarization in acidic solutions, i t is found that dissolution is accelerated by chloride anions in compa rison with sulfates. In HCl solutions of increasing concentration, not only does the pH decrease, but also the increasing chloride concentra tion accelerates dissolution. On the other hand, the dissolution rate in sulfuric acid does not depend on the sulfate (bisulfate) concentrat ion. During anodic polarization, the dissolution rate is fairly indepe ndent of the potential, except at very high anodic potentials, and the XANES spectra reveal no changes in the average oxide valence during a nodic polarization. Thus the dissolution that takes place is mostly ch emical rather than electrochemical. During cathodic polarization, the dissolution rate is independent of the anion in the electrolyte. The f indings are interpreted in terms of the negative surface charge of n-t ype oxides at potentials lower than the flatband potential, retarding anion adsorption on the surface. Hence it is suggested that the detrim ental role of chloride anions on the stability of iron oxide films is due to a surface complexation effect. The findings and their relevance to the stability of natural passive films on iron surfaces are discus sed.