IN-SITU X-RAY-ABSORPTION NEAR-EDGE SPECTROSCOPIC STUDY OF THE CATHODIC REDUCTION OF ARTIFICIAL IRON-OXIDE PASSIVE FILMS

Galvanostatic reduction of Fe2O3 and Fe3O4 sputter-deposited thin film s was investigated as a model for reduction of the passive film on iro n. X-ray absorption near-edge spectroscopy was used to perform in situ measurements of the valence state and dissolution (thinning rate) of the films during reduction. In berate buffer, the reduction of Fe2O3 t akes place in two stages. The first reduction stage, a consecutive two -step reaction, is a gradual lowering of the average valence in the Fe -oxide (conversion of Fe2O3 into an oxide with a stoichiometry close t o Fe3O2). This is followed by a second reductive dissolution step whic h can be approximated by a reaction scheme: Fe2O3 --> Fe3O4 --> Fe2+ ( aq). In the second stage, after complete conversion of the Fe2O3, the charge passed is used completely for reductive dissolution of the rema ining Fe3O4 layer, leading to a corresponding increase in the thinning rate. Reduction of Fe3O4 sputter-deposited films is identical to the second stage of Fe2O3 reduction. In 0.1 M H2SO4, the reduction mechani sm is similar to that in berate buffer, except that chemical dissoluti on of the surface layer of Fe3O4 competes with reductive dissolution. In 0.1 M NaOH, no dissolution takes place; instead, there is a solid-s tate reaction in which the oxides are reduced to a solid Fe(II) film, possibly Fe(OH)(2). From comparison of these findings with literature data on natural passive films on iron, various controversial interpret ations of the cathodic reduction treatment on a natural passive films on iron can be clarified.