CORROSION BEHAVIOR OF HIGH-SILICON ALLOYS IN CARBON-BEARING AND HIGH-SULFUR ATMOSPHERES

On the basis of alloy 800 H and alloy AC-66, materials introduced in the 1960s and the late 1980s, respectively, two new alloys, Cronifer III-TM (alloy III-TM) and Nicrofer 45-TM (alloy 45-TM), were developed to resist high sulfur and carbon-bearing environments. Carburization behaviour was examined in the temperature range 750-1100 degrees C for more than 1000 h in CH4/H-2 (a(C) = 0.8). The silicon-containing alloys Cronifer III-TM and Nicrofer 45-TM showed a mass gain similar to or even lower than that of the nickel-base alloy 625. Data on sulfidation resistance up to 650 degrees C obtained in 10% SO2/N-2 indicated a mass loss of less than 5 gm(-2) after 1000 h for all alloys tested (800 H, AC-66, 625, III-TM, 45-TM). At 750 degrees C alloy 800 H suffered breakaway corrosion and alloy 625 severe spallation. Alloys AC-66, III-TM and 45-TM showed superior corrosion resistance. Tests performed in a simulated coal gasification environment (1% H2S, 4% CO, 3% H2O, balance H-2) showed negligible mass gain for the alloys at 550 degrees C. Results at 650 degrees C remained satisfactory after 1000h for alloys 800 H, AC-66, III-TM and 45-TM, with weight gains of less than 35 gm(-2). Metallographic examination of the alloys, however, indicated local corrosion attack, which was lowest for 45-TM at 550 degrees C and lowest for AC-66 at 650 degrees C. Alloy 800 H showed the worst behaviour at all temperatures. Exposure tests in a simulated waste incineration atmosphere (2.5 gm(-3) HCl, 1.3 gm(-3) SO2, 9% O-2, balance N-2) in the temperature range 550-850 degrees C showed alloy 45-TM to have the best performance over the whole temperature range.