ABSTRACT
Corrosion of ceramics can take place by any one or a combination of mechanisms. Various models have been proposed to describe these mechanisms, several of which will be discussed below. In general the environment will attack a ceramic, forming a reaction product. The reaction product may be solid, liquid, gas, or any combination of these. This reaction product may remain attached to the ceramic or it may be fugitive, in the case when gaseous species make up the reaction product, or it may be a combination of both. When the reaction product remains as a solid, quite often it forms a protective layer toward further corrosion. At other times, for example if the reaction product is a combination of solid and liquid, this reaction layer may be removed through the process of erosion.* Thus to analyze corrosion one must have some idea of the type of processes that are in operation. When the reaction product remains as an intact interfacial layer, analysis is relatively easy. When gaseous species are formed, the consumption of the ceramic manifests itself as a weight loss. An understanding of the mechanism, however, requires analysis of the evolved gases. Many times the interface formed is very porous and/or friable requiring special care in preparing samples for analysis. Because of the various processes that may take place during corrosion, there is no one general model that can explain all cases of corrosion. In addition, a single ceramic material will react differently to different environments and thus there is no single explanation for the corrosion of a particular material for all environments. It is also true that the manufacturing history of a ceramic material will affect its performance. This may manifest itself, for example, as a low-corrosion-resistant grain boundary phase or a pore size distribution that greatly increases the exposed surface area to corrosion. Thus it should be obvious that a simple all-encompassing general theory of corrosion of ceramics does not exist and, because of the nature of corrosion
and ceramics, will most likely never exist. There does, however, appear to be a common thread connecting all the various studies that have been reported. That is, corrosion is dependent upon the structural characteristics of the material. The more compact or tightly bonded materials corrode less whether they are glasses or crystalline materials. Thus it appears that if a general theory is to be developed, a comprehensive investigation of single crystals and some structurally well-characterized glasses should be investigated.
