Dissolution of Hydrous Chromium Oxide in Acid Solutions

It has been considered that the corrosion of passive metals and alloys is not caused by the active metal-dissolution but the dissolution of a nonporous passive oxide film in aqueous solution. The corrosion current density of iron(1)(2), chromium(3) and their alloys(4) is independent of the potential in all the range of passivity, although depending upon the pH of solution. Vetter(1) explained such a corrosion behavior by assuming that the dissolution rate of the passive oxide film is electrochemically controlled by the potential difference at the filmsolution interface, which is determined soley by the pH of solution. Recent studies of passivated stainless steels in acid solutions by means of X-ray photo electron spectroscopy(5) and Auger electron spectroscopy(6) have proved the high enrichment of chromium component in the passive oxide film, which seems to explain the high corrosion resistivity of stainless steels in acid solutions. The passive oxide film has an amorphous structure with a chemically bound water (7). Hydrous chromium oxide prepared by precipitation in aqueous solution according to the X-ray diffraction data(8), has also an amorphous structure. Therefore it is expected that the structure of hydrous chromium oxide is similar to that of the passive chromium oxide on stainless steels. In this study, hydrous chromium oxide prepared by precipitation is adopted as a model material of the passive oxide film on stainless steels and the dissolution of hydrous chromium oxide in acid solutions is investigated and the dissolution mechanism is discussed from the electrochemical point of view.