Water Effects on Corrosion Behavior of Structural Ceramics
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MRS BULLETIN/OCTOBER 1994
geological repository.2 Though during the last decade significant progress has been made in understanding corrosion effects of water in ceramic materials, the information has never been comprehensively reviewed. Recent books2"5 on corrosion of ceramics contain only very limited information on water-induced corrosion. Ceramic materials have some specific features that make them different from other materials such as metals and polymers. Since they are brittle, have very low fracture toughness, and their critical flaw size is much smaller than that in metals, they are more sensitive to environmentallyinduced strength degradation than metals and alloys. From this point of view, ceramics have much in common with glasses. However, unlike glass, the majority of structural ceramics have a complicated crystal structure and microstructure. Particularly, they contain sintering or stabilizing addi-
Figure 1. Factors determining the corrosion of ceramics. SC = stress corrosion, SCG = subcritical crack growth.
tives, and/or reinforcing particles. Si3N4, SiC, A12O3, and some other ceramics, contain grain-boundary layers (Figure 3) which often determine the corrosion behavior. Thus, the corrosion resistance of ceramics depends not only on composition, but primarily on microstructure. Another important factor is the influence of water on the protective properties of the oxide scales on the surface of non-oxide ceramics. Ceramics based on Si3N4 and SiC are used to fabricate components of engines, heat exchangers, and equipment for petrochemical processing and coal gasification.6 Since these materials are thermodynamically unstable in oxidizing environments,7 they owe their life to a passivating silica layer, which is formed on their surface during oxidation. Structural ceramics based on A12O3 and ZrO 2 possess high strength, wear, and corrosion resistance, but they are generally used at lower temperatures compared with non-oxide ceramics.6 Therefore, stability at moderate temperatures is of main importance for oxides. Generally speaking, the corrosion behavior of all materials can be divided into three large groups: (1) electrochemical corrosion, (2) chemical corrosion, and (3) mechanochemical corrosion. The science of electrochemical corrosion is well developed for metals, but its application to ceramics has been limited. This is because ceramics are mostly insulators or semiconductors, and therefore have little tendency to give up electrons. For ceramics, transport of ions1 may be more important. Though electrochemical corrosion techniques offer certain advantages in the study of corrosion, until now only their applications to B4C3 and SiC3l8 have been reported. Chemical corrosion of ceramics is the most studied case of corrosion in aqueous environments. As schematically shown in Figure 3, it includes generally several steps: (1) reaction (dissolution) of grain-boundary phases; (2) water transport along grainboundaries into the bulk of ceramics, which is often accompanied by phase changes in the grain-
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