The kinetics of the reduction of nanocrystalline γ′-iron nitride to α-iron in mixtures of nitrogen and hydrogen of different hydrogen concentration was studied. Kinetic measurements were carried out in a differential chemical reactor with a thermogravimetric system for measuring the weight of a sample and an analyzer of hydrogen at 400 °C and at atmospheric pressure. The occurrence of cyclic changes in the rate of chemical reaction in a solid phase has been observed. Such an oscillatory phenomenon was not observed so far in the reaction between gases and a solid with phase transition in a solid phase, which results in a solid products. The mechanism of the observed phenomenon is explained, taking into account processes occurring in the volume of the solid phase. The oscillatory rate changes of the described processes prove a nonmonotonic coverage degree of the surface with nitrogen, which is caused by a step change in the free enthalpy of segregation of nitrogen in the reduction of γ′-Fe4N nitride. On the basis of model calculations the changes of free enthalpy of segregation during reduction of nitrides have been specified, and modification of the Fowler–Guggenheim equation has been proposed.

The kinetics of nanocrystalline α-iron nitriding to γ′-iron nitride in an ammonia atmosphere was studied at 598–648 K and at atmospheric pressure. Oscillatory changes in nitriding reaction rates depending on nitrogen concentration in a solid sample were observed. This phenomenon was explained by a gradual change in the iron active surface coverage degree, with nitrogen resulting from a gradual change in the free enthalpy of nitrogen segregation. The α-Fe(N) nanocrystallites’ transformation into γ’-Fe4N went through six metastable FeNx states. The continuous function proposed by Fowler and Guggenheim was modified to a stepwise variable function.