Sulfidation properties of Cr23C6 at 1073 K in H2S-H2 atmospheres of 103.5−10−6 Pa sulfur pressure
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.
Key wordsChromium carbides Cr23C6 Cr3C2 Cr7C3 sulfidation kinetics sulfide scales
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