Surface reaction kinetics as a probe for studying carbon-induced surface structural changes on nickel and Ni-Cr alloys

Abstract

During carburization of polycrystalline Ni and Ni-Cr alloys (cCr≤13.1 wt.% Cr) in CH4-H2 gas mixtures at 1150 to 1400 K, the formation of a surface graphitic carbon monolayer with 2 C Atoms per 1 metal atom (accompanied by surface restructuring on faces other than (111) and probably (110) and (311)) occurs if the carbon activity ac exceeds a critical value (0.75 for Ni) and if the exposure time ta is long enough. The fraction of the materials surface which is covered with the carbon monolayer after a definite carburization treatment has been determined quantitatively as a function of ta, ac, temperature T, chromium concentration cCr and additions of H2S to the carburizing CH4-H2 mixture. This was done by decarburizing the materials in a H2 or H2-H2S atmosphere immediately after carburization and using the kinetic features of the decarburization process (which is controlled by surface reaction steps) as a probe. The cyclic carburization/decarburization experiments were carried out in a flow apparatus by using thin foils with 5 μm thickness and applying the resistance-relaxation method.