A structural investigation of the passive film on iron and iron/chromium alloys

Abstract

The Electrochemical Polarisation, Photocurrent Spectroscopy and Extended X-ray Absorption Fine Structure (EXAFS) techniques have been used to study the passive film on pure iron and iron alloy samples containing up to 25% chromium. The material used in this work was prepared both as bulk and thin films. The bulk samples were passivated electro- chemically at various anodic potentials whereas the film ones were either fully converted into passive films by simple immersion in various solutions for one week or electrochemically at various anodic potentials. The Fe and Fe/Cr film samples used in the electrochemical passi­vation were deposited onto gold substrate and those passivated by immersion were deposited directly onto mylar. Polarisation curves for both the bulk and film materials were recorded. They suggest that the electrochemical behaviour of the two materials is similar. The wavelength and potential dependence of the photocurrent spectra were also recorded for the bulk and film samples of Fe and Fe/Cr alloys. The data were analysed to obtain the effective optical band gaps and flat band potentials of the passive films respectively. These results also show that the two materials are similar. Furthermore, the photocurrent data suggest that the passive film on Fe/Cr alloys consists of Fe(III) and Cr(III) phases. The fluorescence EXAFS above the Fe and Cr K-absorption edges of the passive film on Fe and Fe/Cr alloy films has been recorded both in-situ and en- situ. The spectra obtained in these studies were analysed to obtain average Fe-O and Fe-Fe separations as well as Cr-0 and Cr-Cr ones. These results together with a detailed examination of the XANES suggest that the passive film on iron in the absence of chromium is best described as a disordered 7—FeOOH-like structure and that on Fe/Cr alloys as well as on pure Fe passivated in chro­mate solution contains two simultaneous phases of disordered 7-FeOOHand Cr(OH)j. The structure of the chromium containing phase of the passive film was found to be independent of the source of chromium whether it was present in the alloy or it was incorporated from the passivating solution. The EXAFS data show also that the structure of the passive film is insensitive to drying. The results obtained in these studies have enabled the determination of the structure of the passive film on iron and iron/chromium alloys and the rôle played by chromium in improving the corrosion resistance of iron and its alloys has been identified. These conclusions are discussed in the light of other structural investigations of the passive film