On the oxidation of alpha-Fe and epsilon-Fe2N1-z .1. Oxidation kinetics and microstructural evolution of the oxide and nitride layers

The oxidation of alpha-Fe and epsilon-Fe2N1-z at 573 K and 673 K in O-2 at 1 atm was investigated by thermogravimetrical analysis, X-ray diffraction, light-optical microscopy, scanning electron microscopy and electron probe X-ray microanalysis. Upon oxidation at 573 K and 673 K, on alpha-Fe initially alpha-Fe2O3 develops, whereas on epsilon-Fe2N1-z initially Fe3O4 develops. In an early stage of oxidation the oxidation rate of epsilon-Fe2N1-z appears to be much larger than of alpha-Fe. This can be attributed largely to an effective surface area available for oxygen uptake, which is much larger for epsilon-Fe2N1-z than for alpha-Fe due to the porous structure of epsilon-Fe2N1-z as prepared by gaseous nitriding of iran. The development of a magnetite layer in-between the hematite layer and the alpha-Fe substrate, at a later stage of oxidation, enhances layer-growth kinetics. After 100 min oxidation at 673 K the (parabolic) oxidation rates for alpha-Fe and epsilon-Fe2N1-z become about equal, indicating that on both substrates the oxide growth is controlled by the same rate limiting step which is attributed to short-circuit diffusion of iron cations. Oxidizing epsilon-Fe2N1-z increases the nitrogen concentration in the remaining epsilon-iron nitride, because the outward flux of iron cations, necessary for oxide growth, leads to an accumulation of nitrogen atoms left behind.