Comparison of internal nitridation reactions in ammonia and in nitrogen

Abstract

Nine commercial high-temperature alloys were reacted with NH₃ and with N₂-5%H₂ at temperatures of 1000–1200°C. In all cases extensive internal nitridation developed according to parabolic kinetics. Reaction with N₂ produced precipitates of Cr₂N plus, at 1000°C, some external CrN, in accordance with thermodynamic prediction. However NH₃ produced an external scale of CrN and a near-surface zone of internal CrN as well as a deeper zone of Cr₂N at 1100 and 1200°C. The CrN phase is metastable, and results from catalytic dissociation of NH₃, which produces a high effective nitrogen activity. This high activity also leads to faster internal-precipitation reactions, whereas reaction rates in nitrogen are in reasonable agreement with Wagner's model of rate control by inwardly diffusing nitrogen at its equilibrium solubility. Precipitate morphologies are complex, reflecting the importance of the energy barrier to homogeneous nucleation.