On the effect of preoxidation on nitriding kinetics

Oxidation of ferritic surfaces prior to gaseous nitriding has beat reported to lead to improved compound layer thickness uniformity and enhanced nitri ding kinetics. The present work considers the nucleation and growth of a mo del compound layer on pure iron and using previous experimental and theoret ical results reported in the literature, puts forward two hypotheses to exp lain the effects of preoxidation on compound layer formation. It is propose d that the nucleation of iron nitrides is enhanced by rite presence of mt i t on oxide layer, and that the growth of nit ir on nitride layer proceeds f aster after preoxidation owing to a higher nitrogen content in the part of the compound layer closest to the surface.