In Situ X‐Ray Absorption Near‐Edge Spectroscopic Study of the Cathodic Reduction of Artificial Iron Oxide Passive Films

Abstract

Galvanostatic reduction of and 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 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 into an oxide with a stoichiometry close to ). This is followed by a second reductive dissolution step which can be approximated by a reaction scheme: (aq). In the second stage, after complete conversion of the , the charge passed is used completely for reductive dissolution of the remaining layer, leading to a corresponding increase in the thinning rate. Reduction of sputter‐deposited films is identical to the second stage of reduction. In , the reduction mechanism is similar to that in borate buffer, except that chemical dissolution of the surface layer of 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 . 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.