Sulfidation of coal gasifier heat exchanger alloys

Three steels, viz., INCOLOY/sup */ 800H, Fecralloy, /sup +/ and AISI 310, were exposed to a simulated low Btu coal gasifier product gas at 450/sup 0/C. Sulfidation occurred in each case and the reaction was studied as a function of thermal cycles, the presence of coal fines, time, and the HCl content of the gas. Duplex scales consisting of an outer layer of an Fe/Ni sulfide and an inner chromium-rich sulfide formed on most of the specimens. The effect of coal fines and HCl gas separately accelerated corrosion, but when present together in short-term tests these deleterious effects were substantially reduced. Sulfidation of Fecralloy was observed only when thermal cycles were imposed, and only very slight resistance was conferred by preoxidizing this steel. Once rapid sulfidation initiated, rates of attack were very similar for the three steels studies. The results obtained have been discussed in terms of the likely mechanisms of the corrosion process, and extrapolation of the metal loss data to design lifetimes is considered in terms of the test procedure and likely operating conditions.