On the oxidation of γ′‐Fe₄N_1−x layers; redistribution of nitrogen

Iron nitride (γ‐Fe₄N_1−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‐Fe₂N_1‐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.