High Temperature Corrosion of Silicon Based Ceramics in Environments Containing Halogens and Alkali Halides

Abstract Silicon carbide and other silicon-based ceramics obtain their oxidation resistance by the formation of protective silica films in oxygen containing environments. In the presence of chlorine or alkali chlorides, this film may not form, or may be unprotective, so that accelerated oxidation occurs. Chlorine and alkali halides may contribute to accelerated oxidation via three possible mechanisms. Active oxidation occurs at relatively low temperatures and high ratios of chlorine to oxygen, and is characterized by simultaneous formation of silicon chlorides and non-protective oxides. Silica film disruption occurs at higher temperatures when volatile chlorides form at the interface between SiC and the SiO2 scale, and cause bubbles to form in the film. Alkali fluxing occurs in the presence of alkali chloride vapors. In alkali fluxing, a low melting glass is formed by the reaction between the alkali vapor species and silica rather than by reactions with chlorine itself.