Effect of temperature and gas composition on The corrosion behaviour of silicon nitrides exposed to sulphidizing environments of low oxygen potential

Abstract

AbstractLittle is known about the behaviour of advanced silicon nitride materials in reducing atmospheres (e.g. in coal gasification or in petrochemical plant). In such atmospheres with low oxygen potential and containing sulphur volatile silicon sulphides are formed in addition to the volatile silicon oxide, so that corrosion would be accelerated. In order to graphically represent these conditions a “volatility map” is presented which shows the conditions in such atmospheres at 1200 and 1300°C; the map shows the partial pressures of SiO and SiS (in equilibrium with Si or SiO2) vs oxygen and sulphur partial pressures. The map shows various corrosion regions: when SiO2 is not stable and high SiS partial pressures are predicted, corrosion may be catastrophic; when SiO2 is stable, the glassy surface layer initially prevents evaporation of SiS which, however, is gradually transformed into SiO, and high SiS partial pressures may result in internal sulphidation.