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Chapter 1 |
1.1EXERCISES, QUESTIONS, AND PROBLEMS
1.List the different fields that an engineer might want to study to be proficient in evaluating corrosion problems.
2.List all the various courses taught in a standard ceramic engineering curriculum that might contain information related to corrosion.
3.Discuss whether it is more important to know whether a material will corrode or how long it will last.
4.Discuss several applications where the corrosion is a beneficial factor rather than a problem. Find several applications not listed in the text.
5.Develop an equation that represents all the various factors involved in determining the total cost of a particular corrosion problem. Pick a particular product and try to find as much information as possible concerning the actual costs.
6.Discuss the relationship of erosion to corrosion and how it can be a major actor in the overall process of corrosion.
Copyright © 2004 by Marcel Dekker, Inc.
2
Fundamentals
Everything should be made as simple as possible, but not simpler.
ALBERT EINSTEIN
2.1 INTRODUCTION
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 either 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
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Copyright © 2004 by Marcel Dekker, Inc.
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Chapter 2 |
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 allencompassing 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.
Corrosion, being an interfacial process, requires a thorough understanding of the surface structure of the material being corroded. Thus the study of single crystals is the best method
Copyright © 2004 by Marcel Dekker, Inc.