Oxidation and Volatilization of Silica Formers in Water Vapor

Abstract

At high temperatures, SiC and Si 3 N 4 react with water vapor to form a SiO 2 scale. SiO 2 scales also react with water vapor to form a volatile Si(OH) 4 species. These simultaneous reactions, one forming SiO 2 and the other removing SiO 2 , are described by paralinear kinetics. A steady state, in which these reactions occur at the same rate, is eventually achieved. After steady state is achieved, the oxide found on the surface is a constant thickness, and recession of the underlying material occurs at a linear rate. The steady‐state oxide thickness, the time to achieve steady state, and the steady‐state recession rate can be described in terms of the rate constants for the oxidation and volatilization reactions. In addition, the oxide thickness, the time to achieve steady state, and the recession rate also can be determined from parameters that describe a water‐vapor‐containing environment. Accordingly, maps have been developed to show these steady‐state conditions as a function of reaction rate constants, pressure, and gas velocity. These maps can be used to predict the behavior of SiO 2 formers in water‐vapor‐containing environments, such as combustion environments. Finally, these maps are used to explore the limits of the paralinear oxidation model for SiC and Si 3 N 4 .