CARBON-INDUCED SURFACE STRUCTURAL AND COMPOSITIONAL CHANGES ON NICKEL AND NI-CR ALLOYS AS STUDIED BY AN ANALYSIS OF SURFACE-REACTION KINETICS

Carburization of polycrystalline foils (5-mu-m thickness) of Ni and low-chromium Ni-Cr alloys (c(Cr) less-than-or-equal-to 13.1wt%Cr) in CH4-H-2 gas mixtures at 1150 to 1400 K and high carbon activities a(C) above a a critical value a(C,cr) (> 0.75 for Ni) leads to the formation of a surface graphite monolayer with an atomic ratio of 2 carbon atoms per 1 metal atom. This process is accompanied by surface restructuring if the crystallite orientations are different from (111) (and probably (110) and (311)) and proceeds in dependence on the time t(a) of sample exposure to the carburizing atmosphere. The fractional area of the surface covered with the carbon monolayer has been determined by decarburizing the samples in H-2 immediately after the carburization treatment and analyzing the surface-reaction controlled decarburization kinetics as a function of t(a), a(C), temperature, alloy composition and H2S additions. Moreover, the rate equation for decarburization of the materials in H-2 was extended to apply also to decarburization via a surface which is partially covered with the carbon monolayer. The cyclic carburization/decarburization experiments were carried out in a flow-apparatus by applying the resistance-relaxation method.