Chemical Degradation

Abstract

This chapter reviews the thermodynamic approach to chemical degradation and degradation mechanism. The principles of equilibrium thermodynamics do not provide any explicit information about the rates or mechanisms of chemical reactions. Silicon nitride is a ceramic with high-temperature applications in such areas as crucibles and gas turbines. A study of its thermodynamic properties reveals that it is stable only under conditions of moderate partial pressure of nitrogen and very low partial pressure of oxygen. The surface of a solid undergoing attack by a gas or aqueous solution may be altered in either a uniform or nonuniform manner. In general, uniform corrosion is only observed for a homogeneous single-phase substance such as glass, whereas nonuniform reaction is more common in multiphase mixtures. The dissolution rate is controlled by the diffusion of chemical species out of the matrix of the solid into the gaseous or aqueous environment. It is found that this mechanism results in progressive growth of an amorphous leached layer through which the solid components diffuse in order to reach the solvent.