Sulfidation properties of Cr23C6 at 1073 K in H2S-H2 atmospheres of 103.5?10?6 Pa sulfur pressure

Abstract

The sulfidation behavior of chromium carbide, Cr23C6, was investigated in H2S-H2 gas mixtures over a sulfur partial pressure range of 103.5−10−6 Pa at 1073 K using thermogravimetry, optical and scanning electron microscopy, X-ray diffraction analysis, and electron-probe microanalysis. The kinetics were rapid for short time periods and followed a linear rate law at low sulfur pressures, whereas sulfidation tends to obey a parabolic rate law at high pressures. Sulfidation rates decreased with increasing carbon content in the carbide. Surface morphologies could be divided into three groups: (I) at high sulfur pressures, petal-like. crystals (Cr2S3); (II) at intermediate pressures, a twinlike structure (Cr3S4); (III) and at low pressures, a flat surface with numerous hexagonal pits (Cr1−xS). The scale consisted of two distinct layers: an external scale with a single or multilayer structure and an inner scale with a mixture of Cr1−xS, Cr3C2, and Cr7C3. These higher carbides, Cr3C2 and Cr7C3, may be formed by the sulfidation-carburization of Cr23C6. Pt-marker experiments indicated that the external scale grew by chromium diffusion and that sulfur migration played an important role in the growth of the inner scale.