An EXAFS structural study of the passive films formed on Fe-Cr alloys
Comparatively little is known about the nature of the passive films formed upon the technologically important iron-chromium alloys. It is known however, that they are both very thin (50A) and possibly amorphous: as such they are difficult to characterize by conventional diffraction methods. To avoid the problems associated with the diffraction techniques, the EXAFS technique has been used to characterize the passive film. Iron-chromium alloys containing up to 29% atomic chromium were passivated in 0.1M NaNO2, 0.05M K2CrO4 and 2.0M H2S04. X-ray absorption measurements were then made upon the passivated alloys, using both total electron yield and fluorescence yield to determine the X-ray absorption. Analysis of the extended X-ray absorption fine structure has revealed that an increase in structural disorder of the passive film may be correlated with an increase in the chromium content of the alloy. It has been shown that chromium contained in the passive film is associated with the disorder in the film Fe-O and Fe-Fe bond lengths compatible with the known oxides and hydroxides of iron have been obtained, but the uncertainties in the bond length determination prevent any structure being put forward for the passive film.