FURNACE WALL CORROSION IN REDUCING-SULFIDIZING COMBUSTION GAS

The corrosion resistance of several candidate furnace-wall materials w as evaluated in a laboratory retort, simulating the reducing-sulfidizi ng combustion environment existing in PC-fired utility boilers burning coal substiochiometrically. These materials, including three alloys a nd nine coatings, were exposed to a mixed gas containing 400 ppm H2S, 100 ppm HCl, 900 ppm SO2, and other essential gaseous components at 80 0 degrees F (degrees C) for 1000 hours with and without the coverage o f a chloride-bearing ash deposit. The nine coatings consisted of three weld overlays, two diffusion coatings, and four thermal-spray coating s. The test results indicate that sulfidation was the primary mode of attack on unprotected low-Cr steels, such as SA213-T2. Furthermore, th e role of 900 ppm SO2 in the corrosion mechanism varied significantly with the Cr content in the alloys. A Cr concentration of approximately 9 wt% appeared to be a threshold, below which the scale surface favou red the catalytic conversion of SO2 to H2S, thus increasing the local H2S concentration and gas corrosivity. Above this threshold, the SO2 r eacted with Cr in the alloy preferentially to form Cr2O3, resulting in a reduced sulfidation attack. With relatively high Cr concentrations, the weld overlays generally performed well in the simulated reducing- sulfidizing combustion environment. Both diffusion coatings also exhib ited satisfactory resistance to the mixed gas under the test condition employed. However, the protectiveness of the thermal-spray coatings w as limited by their porous coating microstructures, which allowed loca l penetration of sulfur into the base metal. The HVOF process appeared to produce a denser coating than the are spray and thus provided bett er corrosion resistance. No effect was found from the addition of a sm all amount of HCl to the mixed gas and chlorides to the ash on the fur nace-wall corrosion. The probable corrosion mechanism is briefly discu ssed.