(+oxidation) Treatments

The oxidation of a-Fe and e-FezNl-z at 573 K and 673 Kin Oe at 1 atm was investigated by thermogravimetrical analysis, X-ray diffraction, light-optical microscopy, scanning electron microscopy and electron probe X-ray microana-lysis. Upon oxidation at 573 K and 673 K, on a-Fe initially a-Fez03 develops, whereas on e-FezN>z initially Fes04 develops. In an early stage of oxidation the oxidation rate of e-FezN~-z appears to be much larger than of a-Fe. This can be attributed largely to an effective surface area available for oxygen uptake, which is much larger for e-FezN~_ ~ than for a-Fe due to the porous structure of e-FezNl- ~ as prepared by gaseous nitriding of iron. The develop-ment of a magnetite layer in-between the hematite layer and the a-Fe substrate, at a later stage of oxidation, enhances layer-growth kinetics. After 100 min oxidation at 673 K the (parabolic) oxidation rates for a-Fe and e-FezN~-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 e-Fez Nl-z increases the nitrogen concentra