Sulfidation properties of Ni-20Cr and Ni-13.5Co-20Cr alloys at 873 K under low sulfur pressures in H{sub 2}S-H{sub 2} atmospheres
The sulfidation properties of Ni-20Cr and Ni-13.5Co-20Cr alloys as well as a nickel-base superalloy AISI685 were investigated at 873 K in H{sub 2}S-H{sub 2}gas mixtures with sulfur partial pressures of 10{sup {minus}4}, 10{sup {minus}5.5}, and 10{sup {minus}7} Pa by mass-gain measurements, electron-probe microanalysis (EPMA), and X-ray diffraction (XRD) analysis. Sulfidation obeyed the parabolic rate law, and the parabolic rate constants decreased in the order Ni-20Cr, Ni-13.5Co-20Cr, and AISI685 at each sulfur pressure. With decreasing sulfur pressure, the rate constants first decreased slowing and then rapidly at a 10{sup {minus}7} Pa sulfur pressure. At both 10{sup {minus}4} and 10{sup {minus}5.5} Pa sulfur pressures, Ni-20Cr formed a surface scale with a duplex structure of inner (Cr{sub 3}S{sub 4}) and outer (Ni{sub 3}S{sub 2}) layers, while Ni-13.5Co-20Cr formed a triplex structure of inner (Cr{sub 3}S{sub 4}), intermediate (Ni,Co)Cr{sub 2}S{sub 4}, and outer [Ni{sub 3}S{sub 2}+(Co,Ni){sub 9}S{sub 8}] layers. At the 10{sup {minus}7} Pa sulfur pressure, which is lower than the dissociation pressure of Ni{sub 3}S{sub 2}, both Ni-20Cr and Ni-13.5Co-20Cr formed a surface scale of Cr{sub 3}S{sub 4} covered by a thin NiCr{sub 2}S{sub 4} layer, accompanied by copious internal sulfidation of Cr{sub 3}S{sub 4} and/ or CrS. On AISI685 there was a surface scale of (Cr,Ti){sub 3}S{sub 4} accompanied by the usual internal sulfidation. It is discussed that diffusion of cations in the inner Cr{sub 3}S{sub 4} layer is the rate-determining step for the growth of the multilayer structures. At the 10{sup {minus}7} Pa sulfur pressure, diffusion of Cr and S contribute to form a thin surface scale and internal sulfidation, respectively.
- Research Organization:
- Hokkaido Univ., Sapporo (JP)
- OSTI ID:
- 20050535
- Journal Information:
- Oxidation of Metals, Vol. 53, Issue 5-6; Other Information: PBD: Jun 2000; ISSN 0030-770X
- Country of Publication:
- United States
- Language:
- English
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