Effect of sodium on reaction of nitrogen with type AISI 316 stainless steel

Abstract

Specimens of solution-treated AISI 316 stainless steel have been nitrided in pure nitrogen over the temperature range 873–923 K in the presence of sodium and over the temperature range 1047–1099 K in the absence of sodium. The kinetics of the reaction were followed by measuring pressure changes in a small volume system and the rate, in both cases, was found to be first order with respect to nitrogen pressure over the major part of the reaction. The rate-determining step appeared to be the dissociative adsorption of nitrogen at the steel surface. Sodium reduced the apparent activation energy of the reaction from 313 kJ mol⁻¹ (at 1047–1099 K) to 78 kJ mol⁻¹ (at 873–923 K) and increased the rate of reaction, probably by removing oxygen which poisons the nitriding reaction from the surface of the steel. Steel surfaces exposed to nitrogen in the presence of sodium vapour formed CrN, while those exposed in the presence of liquid sodium formed both CrN and Cr₂N. In the absence of sodium a mixed Fe–Cr nitride with the formula (Cr_1·885Fe_0·115)N was identified as the only reaction product at 1099 K. At 1047 K CrN and Cr₂N (probably with some iron substitution) were formed.