In situ structural studies of the passive film on iron and iron/chromium alloys using X-ray absorption spectroscopy

The structure of the passive film that protects iron and iron/chromium alloys from corrosion has been investigated in situ in an aqueous environment using X-ray absorption spectroscopy, with fluorescence yield detection. From an analysis of the EXAFS and XANES it has been shown that the film formed on iron consists of FeO6 octahedra, linked together by sharing edges to form what are probably sheets, or chains. When the iron is alloyed with chromium, or when it is exposed to a solution containing chromate ions, the passive film becomes more disordered and the distance between neighbouring iron atoms increases slightly. More significantly, from the passivation point of view, it has been shown that the chromium is incorporated into the passive layer as a phase essentially identical to Cr(OH)3, i.e. CrO6 octahedra linked by hydrogen bonds into an amorphous three-dimensional phase. It is this phase that appears to give rise to the enhanced corrosion resistance that chromium imparts to iron alloys.