In situ observation of the oxidation and reduction processes on Fe-Cr alloys

Abstract

We present atomic force microscopy (AFM) observations on the micron and submicron scale of the surface morphology evolution of Fe-Cr alloys during oxidation-reduction cycles. Industrial Fe-17%Cr, which contains impurity inclusions, exhibits a specific type of corrosion behaviour during the initial stages of oxidation. The inclusions, which are rich in silicon, undergo a very rapid oxidation process which leads to a large increase in their volume. This in turn leads in many cases to the ejection of the enlarged precipitates from the sample, leaving a pitted surface. Purer material is free from this type of attack. Apart from the precipitate behaviour, the oxidation-reduction cycle appears to proceed in most cases similarly in the pure and industrial Fe-17%Cr alloys. Both also contain Cr-rich particles which behave differently from the rest of the sample. Fe-5%Cr and pure Fe samples do not exhibit enhanced activity compared to the 17% samples either during oxidation or reduction. Indeed, in the case of the pure Fe sample it is often quite difficult to observe any changes at all. For these mechanically polished surfaces, which are similar to those used in most electrochemical studies and encountered in real corrosion problems, the AFM observations at the scale of a few microns illustrate that chemical heterogeneities on this scale have more influence than polishing scratches, but that other heterogeneous effects sometimes exist which do not appear to be connected with either property. The usefulness of in situ observations carried out on this scale is underlined.