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 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 → Fe₂O₃ → 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 Fe(OH)₂. From comparison of these findings with literature data on natural passive films on iron, various controversial interpretations of the cathodic reduction treatment on a natural passive films on iron can be clarified.