Abstract
Galvanostatic reduction of Fe2O3 and Fe3O4 sputter-deposited thin films was investigated as a model for reduction of the passive film on iron. 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 borate buffer, the reduction of Fe2O3 takes 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 to Fe3O4). This is followed by a second reductive dissolution step which 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 remaining 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 mechanism is similar to that in borate buffer, except that chemical dissolution of the surface layer of Fe3O4 competes with reductive dissolution. In 0.1 M NaOH, no dissolution takes place; instead, there is a solid-state reaction in which the oxides are reduced to a solid Fe(Ii) film, possibly interpretations of the cathodic reduction treatment on a natural passive films on iron can be clarified. 29 refs., 11 figs.