Furnace wall corrosion in reducing—sulfidizing combustion gas

Abstract

The corrosion resistance of several candidate furnace-wall materials was evaluated in a laboratory retort, simulating the reducing—sulfidizing combustion environment existing in PC-fired utility boilers burning coal substiochiometrically. These materials, including three alloys and nine coatings, were exposed to a mixed gas containing 400 ppm H2S, 100 ppm HC1, 900 ppm SO2, and other essential gaseous components at 800°F (°C) for 1000 hours with and without the coverage of a chloride-bearing ash deposit. The nine coatings consisted of three weld overlays, two diffusion coatings, and four thermal-spray coatings. The test results indicate that sulfidation was the primary mode of attack on unprotected low—Cr steels, such as SA213—T2. Furthermore, the 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 favoured the catalytic conversion of SO2 to H2S, thus increasing the local H2S concentration and gas corrosivity. Above this threshold, the SO2 reacted 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 exhibited satisfactory resistance to the mixed gas under the test condition employed. However, the protectiveness of the thermal-spray coatings was limited by their porous coating microstructures, which allowed local penetration of sulfur into the base metal. The HVOF process appeared to produce a denser coating than the arc spray and thus provided better corrosion resistance. No effect was found from the addition of a small amount of HC1 to the mixed gas and chlorides to the ash on the furnace-wall corrosion. The probable corrosion mechanism is briefly discussed.