CORROSION OF SIC MATERIALS IN N2-H2-CO GASEOUS ENVIRONMENTS .1. THERMODYNAMICS AND KINETICS OF REACTIONS

Two sintered SiC materials and a siliconized SiC material were exposed to two gas compositions in the N2-H2-CO system at 1000-degrees-1350-degrees-C, 133 Pa and 98.7 kPa for 8-1000 h. The two gases were a nitrogen-based gas which contained 98.2% N2,1.2% H-2, and 0.6% CO; and an endothermic gas which contained 37.8% N2, 41.1% H-2, and 21.1% CO by volume. Thermodynamics of reactions were modeled using the program SOLGASMIX. Kinetic studies were based on weight change measurements obtained from vacuum TGA and long-term tube-furnace experiments. In the case of a siliconized SiC, reactions were complex, owing to its two-phase composition and to the presence of impurities. Under many conditions, silicon was rapidly etched from the surface of the siliconized SiC during a short initial period due to the formation of SiO and carbon or SiO and beta-SiC. In the case of sintered materials (and the siliconized material during the later stages of corrosion), etching of SiC was observed under many of the conditions studied, due to the apparent formation of SiO, HCN, and/or carbon. Corrosion in an endothermic gas was typically more severe than in a nitrogen-based gas, due to its higher p(CO).