Kinetics of nitrogenation of α-irons containing chromium,molybdenum,and silicon in ammonia–hydrogen mixtures

Abstract

The effects of chromium, molybdenum, and silicon as alloying elements on the nitrogenation and denitrogenation of α-iron are presented. Nitrogenation was carried out at 507 and 537°C in NH₃–H₂ mixtures. The rate-determining reaction steps were studied using the resistance relaxation method of thin iron foils. The rate-determining reaction step of the nitrogenation at the surface of both unalloyed and alloyed α-iron as well as at the surface of γ′-iron. nitride at lower hydrogen pressures is NH_3(ad) = NH_2(ad) + H_(ad), and at higher hydrogen pressures is NH_2(ad) = NH_(ad) + H_(ad). The nitrogenation rate at the surface of α-iron was found to be increased by chromium and molybdenum and retarded by silicon alloying. The rate of nitrogenation was also found to be of about the same magnitude at the surface of both α-iron and γ′-iron nitride.