METAL DUSTING OF NICKEL-BASE ALLOYS IN SIMULATED SYNGAS MIXTURES

摘要

Thermodynamic equilibrium calculation, along with laboratory corrosion experiments, were conducted for H_2, CO, CO_2, and H_2O gas mixtures to understand the metal dusting environments of syngas reformer plants. Carbon deposition on inmetallo surface from the gas mixture was well represented by the activity of carbon ac determined from the equilibrium reaction of CO+H_2=H_2O+C. For gas mixtures of high CO content, the Boudouard reaction, 2CO=CO_2+C, should be considered as well. The oxygen potential Po_2 of the gas mixture, which affects the chemistry and thickness of the protective oxide scale, was given primarily from the dissociation reaction of H_2O, H_2O=H_2+1/2O_2, at temperatures below 850 deg C. Carbon activity a_c of a syngas mixture in some practical reforming plants evaluated by the present study was approximately 2 to 15 whereas the oxygen potential Po_2 was in the order of 10~(-23) atm at 650 deg C. Preliminary laboratory corrosion tests of two conventional nickel-base alloys were conducted at 650 deg C for 200 h in various CO-H_2-CO_2-H_2O gas mixtures. For gas mixtures of high a_c and CO content, alloy 600 (75mass percent Ni-15 percent 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 percent Ni-30 percent 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.