On the oxidation of γ′‐Fe4N1−x layers; redistribution of nitrogen

Abstract

AbstractIron nitride (γ‐Fe4N1−x) layers produced by gaseous nitriding of pure iron in an NH3/H2 mixture at 843 K were gaseously oxidized at 673 K in pure O2. During oxidation an ε‐Fe2N1‐y layer developed between a magnetite (Fe3O4) scale and the parent γ′‐nitride layer. Porosity was observed in the oxide‐adjacent part of the ε‐grains. Scanning Auger microscopical analysis and electron probe microanalysis showed (1) that the occurrence of the ε‐nitride layer is associated with a locally higher nitrogen content, (2) that no nitrogen is incorporated in the oxide layer and no oxygen is dissolved in the nitride layer and (3) that a nitrogen concentration‐depth gradient occurs which corresponds to a change of about 1 at.% N over the thickness of the ε‐layer. From a thermodynamical point of view, formation of N2 gas during the oxidation of γ′‐nitride is more likely to occur than formation of ε‐nitride; the latter reaction appears to be kinetically favoured. Quantitative analysis of the nitrogen redistribution suggests that some of the nitrogen atoms released by conversion of γ′‐nitride into magnetite are transformed into N2 which is present in the pores near the oxide/nitride interface.