Kinetics of the alpha-Fe2O3 formation on Fe3O4 between 400 degree C and 570 degree C studied by means of X-ray diffraction with grazing incidence

The formation of alpha-Fe2O3 on Fe3O4 during the oxidation of iron below 570 deg. C occurs very slowly and the scales are so thin that they cannot be detected by microscopy. X-ray diffraction with grazing incidence, equipped with a high temperature device, allows an 'in situ' study of the alpha-Fe2O3 formation even the initial reactions. The formation of alpha-Fe2O3 was studied in isothermal experiments at 400, 450, 500 and 550 deg. C. alpha-Fe2O3 could already be detected at the beginning of the experiments. The difference method, a numerical treatment for kinetic evaluation, yields curves showing the growth of the alpha-Fe2O3 layer. Fitting the parabolic time law to these curves yields the rate constants.