CORROSION OF SILICON-BASED CERAMICS IN COMBUSTION ENVIRONMENTS

Silicon-based ceramics and composites are prime candidates for heat engine and heat exchanger structural components. In such applications these materials are exposed to combustion gases and deposit-forming corrodents. In this paper combustion environments are defined for various applications. These environments lead to five main types of corrosive degradation: passive oxidation, deposit-induced corrosion, active oxidation, scale/substrate interactions, and scale volatility. Each of these is discussed in detail. The key issues in oxidation mechanisms of high-purity silicon carbide (SiC) and silicon nitride (Si3N4) in pure oxygen are discussed. The complicating factors due to the actual combustion environment and commercial materials are discussed. These discussions include secondary elements in the ceramics; additional oxidants, such as water and carbon dioxide (CO2); combustion environment impurities; long-term oxidation effects; and thermal cycling. Active oxidation is expected in a limited number of combustion situations, and the active-to-passive transition is discussed. At high temperatures the limiting factors are scale melting, scale volatility, and scale/substrate interactions. Deposit-induced corrosion is discussed, primarily for sodium sulfate (Na2SO4), but also for vanadate and oxide-slag deposits as well. In applying ceramics in combustion environments it is essential to be aware of these corrosion routes and how they affect the performance of a component.