03471 METAL DUSTING OF NICKEL-BASE ALLOYS IN SIMULATED SYNGAS MIXTURES

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Product Number: 51300-03471-SG

ISBN: 03471 2003 CP

Author: Y. Nishiyama, N. Otsuka, T. Kudo, and O. Miyahara

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Thermodynamic equilibrium calculation, along with laboratory corrosion experiments, were conducted for H2, CO, CO2, and H2O gas mixtures to understand the metal dusting environments of syngas reformer plants. Carbon deposition on .metal. surface from the gas mixture was well represented by the activity of carbon ac determined from the equilibrium reaction of CO+H2=H2O+C. For gas mixtures of high CO content, the Boudouard reaction, 2CO=CO2+C, should be considered as well. The oxygen potential Po2 of the gas mixture, which affects the chemistry and thickness of the protective oxide scale, was given primarily from the dissociation reaction of H2O, H2O=H2+1/2O2, at temperatures below 850°C. Carbon activity ac of a syngas mixture in some practical reforming plants evaluated by the present study was approximately 2 to15 whereas the oxygen potential Po2 was in the order of 10-23 atm at 650°C. Preliminary laboratory corrosion tests of two conventional nickel-base alloys were conducted at 650°C for 200 h in various CO-H2-CO2-H2O gas mixtures. For gas mixtures of high ac and CO content, alloy 600 (75mass%Ni-15%Cr) specimens lost its weight due to metal dusting and deposition of coke was heavy on the surface of the specimen. For alloy 690 (60%Ni-30%Cr) specimen, metal dusting did not occur. Microscopic observation indicated that for the corroded specimens, inward diffusion of carbon presumably at cracks and flaws in the oxide scale was prominent. Carbon in the diffusion zone reacted with chromium to precipitate carbides in the matrix, followed by the formation of graphite plates aligned perpendicular to the surface. This was similar to the pearlite lamellar structure observed in Fe-C system. The metal lamella thinned with the growth of graphite plates, and small metal particles of Ni and Fe which may catalyze the coke deposition reaction were crumbled and detached from the lamella. Keywords: metal dusting, carburization, nickel base alloy, oxide, carbide, graphite, thermodynamics, reformer, syngas