It was found that the thermal decomposition of iron nitrides was closely related with the formation of black layer by thermal analysis and XRD. The decomposition accompanied a phase transformational path such as epsilon-Fe3N --> gamma'-Fe4N --> alpha-Fe. The black layer consisted of alpha-Fe phases resulting from the decomposition of iron nitride. The activation energy of each reaction step was measured by DSC and the results were analyzed using a Kissinger plot. It was concluded that the self-diffusion of Fe was a rate-controlling step in the formation of the black layer during the discontinuous duplex process and its activation energy was confirmed to be 64.8 kcal/mol. (C) 2003 Elsevier Science B.V. All rights reserved.