On the Oxidation of α-Fe and ε-Fe2N1-z:I. Oxidation Kinetics and Microstructural Evolution of the Oxide and Nitride Layers

The oxidation of α-Fe and ε-Fe2N1-z at 573 K and 673 K in O2 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 α-Fe initially α-Fe2O3 develops, whereas on ε-Fe2N1-z initially Fe3O4 develops. In an early stage of oxidation the oxidation rate of ε-Fe2N1-z appears to be much larger than of α-Fe. This can be attributed largely to an effective surface area available for oxygen uptake, which is much larger for ε-Fe2N1-z than for α-Fe due to the porous structure of ε-Fe2N1-z as prepared by gaseous nitriding of iron. The development of a magnetite layer in-between the hematite layer and the α-Fe substrate, at a later stage of oxidation, enhances layer-growth kinetics. After 100 min oxidation at 673 K the (parabolic) oxidation rates for α-Fe and ε-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 ε-Fe2N1-z increases the nitrogen concentration in the remaining ε-iron nitride, because the outward flux of iron cations, necessary for oxide growth, leads to an accumulation of nitrogen atoms left behind