HIGH-RESOLUTION IN-SITU XANES INVESTIGATION OF THE NATURE OF THE PASSIVE FILM ON IRON IN A PH 8.4 BORATE BUFFER

The nature of the passive film grown on iron in a pH 8.4 borate buffer at a high potential [+0.4 V (MSE)] was investigated with high resolution in situ XANES (x-ray absorption near-edge spectroscopy). The pre-edge peak at the base of the Fe K edge showed no evidence of the splitting associated with structures such as alpha-Fe2O3, alpha-FeOOH, and gamma-FeOOH which contain iron that is entirely octahedrally coordinated to oxygen. The single pre-edge peak is consistent with the proposed gamma-Fe2O3/Fe3O4 structure(in which one-third of the iron ions are tetrahedrally coordinated) or with a disordered structure with distorted coordination polyhedra.