On the effect of preoxidation on nitriding kinetics

Oxidation of ferritic surfaces prior to gaseous nitriding has been reported to lead to improved compound layer thickness uniformity and enhanced nitriding kinetics. The present work considers the nucleation and growth of a model compound layer on pure iron and, using previous experimental and theoretical results reported in the literature, puts forward two hypotheses to explain the effects of preoxidation on compound layer formation. It is proposed that the nucleation of iron nitrides is enhanced by the presence of an iron oxide layer, and that the growth of an iron nitride layer proceeds faster after preoxidation owing to a higher nitrogen content in the part of the compound layer closest to the surface.