DECARBURIZATION KINETICS AS A PROBE FOR STUDYING CARBON-INDUCED AND SULFUR-MODIFIED SURFACE STRUCTURAL-CHANGES ON NICKEL AND NI-CR ALLOYS

Carburization and decarburization of polycrystalline Ni and Ni-Cr alloys ( < 10 wt % Cr) in CH4-H2 or H2 atmospheres, respectively, have been investigated at 1150 to 1400 K and 1 atm total pressure in a flow apparatus by applying the resistance-relaxation method. The kinetics and mechanisms of the surface-controlled reactions and the surface phenomena induced by carbon have been studied. During carburization of Ni and Ni-Cr alloys at high carbon activities, 0.75 < ac < 1, the formation of a graphitic carbon monolayer accompanied by surface restructuring occurs if the exposure time ta is long enough. Additions of H2S to the carburizing CH4-H2 mixture cause a shift of the critical carbon activity ac for carbon monolayer condensation to higher values. In comparison, the influence of Cr concentration xcr on ac is rather small. The fraction of the material's surface which is restructured and covered with a carbon monolayer has been determined as a function of T, ac, ta, xcr and pH/pH ratio by using the kinetic features of the decarburization of Ni and Ni-Cr alloys in a flowing H2 atmosphere as a probe. © 1990 Pergamon Press plc.