ARTIFICIAL CR-OXIDE AND FE-OXIDE PASSIVE LAYERS PREPARED BY SPUTTER-DEPOSITION

The electrochemical behavior of synthetic Cr- and FE-oxide films with well known chemical composition and structure, prepared by sputter dep osition, has been studied. Photoelectrochemistry and Mott-Schottky ana lysis show that the parameters during the sputter deposition process i nfluence the stoichiometry of Cr- and Fe-oxides. The bandgap energy of Cr-oxides is independent of the solution pH, thus the values of the b andgap energy lower than those found for ideal Cr2O3 are due mainly to nonstoichiometry. The defect structure influences the electrochemical reactivity of the Cr-oxide. The photocurrent behavior of Fe3+-oxides is strongly determined by the presence of Fe2+ in the oxide, which can be introduced by a partial electrochemical reduction of the Fe-oxides . Chloride ions did not show an effect on the electrochemical reactivi ty of stoichiometric Fe2O3. Mott-Schottky analysis shows changes in th e capacitance behavior if chloride is added to the solution; this sugg ests that a specific adsorption of chloride takes place on the surface . A comparison of the stability of the synthetic oxides and natural pa ssive films may lead to new insights into the role of stoichiometry an d defects on the breakdown of passive films.