Abstract
In the present work surface analytical experiments (XPS and AES) on the passive film on iron have been carried out, in order to refine or verify models previously suggested for reaction mechanisms of passive films on iron. Of a special interest is to elucidate the influence of solution chemistry (especially a comparison of borate and phosphate buffer, pH 8.4) on the nature of the passive film on Fe, and the mechanism of the reduction of the passive film. The nature of the passive film (chemical composition and thickness) depends on the buffer solution used for passivation, even at an identical pH value. In the passive film formed in phosphate buffer, pH 8.4, a significant amount of phosphates is found in the outer part of the film. Boron species, however, are not significantly incorporated in the passive film formed in borate buffer. The mechanism of the reduction of the passive film depends strongly on the electrolyte composition. In borate buffer, cathodic polarization leads to reductive dissolution of the passive film whereas in phosphate buffer the passive film is converted into metallic iron without dissolution. This conversion of the passive film into metallic iron proceeds via a formation of an intermediate Fe (II) phosphate layer. The function of the phosphates therefore is to capture the iron (II) species formed in the reduction reaction (very low solubility of Fe (II) phosphates) and therefore to hinder reductive dissolution. The conversion of the passive film into metallic iron via the intermediate phosphate film seems to proceed laterally inhomogeneously over the sample surface. This suggests that the reaction starts at certain active sites of the surface, and then the reaction front spreads over the sample surface.