Iron oxidation under the influence of phosphate thin films

Iron phosphate thin films have been obtained under controlled conditions by chemical conversion deposition. The films were formed on polycrystalline alpha-iron by immersion in acetone, as an organic solvent. After deposition, the films were investigated by x-ray photoelectron spectroscopy (XPS) and their influence on the thermal oxidation of alpha-iron was studied by means of in situ low incidence x-ray diffraction of synchrotron radiation. The study reveals interesting features related to the structure of both the phosphate and thermal oxide films. The XPS data suggest the iron phosphate to be constituted of long chains of phosphate groups PO43-; these groups being interconnected by Fe2+ and Fe3+ cations. X-ray diffraction measurements have shown a significant modification of the oxidation behavior of alpha-iron at 400 degreesC and atmospheric pressure, which is derived from the presence of the thin film: alpha-Fe2O3 formation is clearly enhanced to the detriment of Fe3O4, compared to the oxidation of pure iron in which Fe3O4 is the dominant phase. As the growth rate of the alpha-Fe2O3 layer is significantly reduced compared to that of Fe3O4, thus the phosphated layers may find niche applications in low-to-moderate temperature environments. (C) 2003 American Institute of Physics.