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to determine the fundamentals of corrosion mechanisms. It is not always possible, however, to obtain single crystals of sufficient size for appropriate measurements. Although the crystal surface characteristics determine short-term corrosion behavior, they may not be as important for long-term corrosion. Single crystals do lend themselves to evaluation of the effects that various dopants and defects (e.g., dislocations) have upon dissolution kinetics.

When attempting to understand the corrosion of a ceramic, it is a good idea to remember some of the fundamental concepts of chemistry that are too often forgotten. The following are just a few concepts that go a long way in helping one to understand corrosion:

1.A ceramic with acidic character tends to be attacked by an environment with a basic character and vice versa.

2.The vapor pressure of covalent materials is generally greater than that of ionic materials and therefore tend to vaporize or sublime more quickly.

3.Ionic materials tend to be soluble in polar solvents*(e.g., salt in water) and covalent materials tend to be soluble in nonpolar solvents (e.g., SiC in hexane).

4.The solubility of solids in liquids generally increases with increasing temperature.

2.2CORROSION BY LIQUIDS

2.2.1 Introduction

The solubility of materials in liquids can be obtained from phase diagrams, which give the saturation composition at a given temperature. Unfortunately, for many practical systems, phase diagrams are either very complex or nonexistent. Many data are available, however, for twoand three-component

* Polar solvents have a high dielectric constant, whereas nonpolar solvents have a low dielectric constant. At 20°, the dielectric constant of water is 80 and that of hexane is 1.874.

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systems, and these should be consulted before attempting to evaluate the corrosion of a specific material* [2.1]. The corrosion of a single pure compound by a liquid can be evaluated by use of the Gibbs Phase Rule. For example, the system of a binary oxide AxByOz corroded by a liquid MaOb contains three components, where a solid and liquid are in equilibrium at some fixed temperature and pressure. This system has only 1 degree of freedom. Thus if the concentration of one dissolved component is changed, the concentrations of the others must also change. A good discussion of the use of phase diagrams in dissolution studies is that by Cooper [2.2].

The corrosion of a solid crystalline material by a liquid can occur through the formation of an interface or reaction product formed between the solid crystalline material and the solvent. This reaction product, being less soluble than the bulk solid, may or may not form an attached surface layer. This type of mechanism has been called indirect dissolution, incongruent dissolution, or heterogeneous dissolution by various investigators. There are many examples of this reported in the literature. In another form, the solid crystalline material dissolves directly into the liquid either by dissociation or by reaction with the solvent. This type of mechanism is called direct dissolution, congruent dissolution, or homogeneous dissolution. The term selective dissolution is also found in the literature, but is used to imply that only a portion of the species in the solid are dissolved whether or not an interface is formed. The saturation solution concentrations of the crystalline species in the liquid along with the diffusion coefficients of the species involved all determine whether one mechanism will exist or the other. The most abundant species and their concentrations in the liquid must be known for one to determine the degree of saturation. This, in turn, will determine whether or not the solid will dissolve. The corrosion rate-limiting step in the indirect type may be the

* A software package called Thermo-Calc is available for calculating phase diagrams from the Dept. of Materials Science & Engineering, KTH, S-100 44 Stockholm, Sweden.

Copyright © 2004 by Marcel Dekker, Inc.

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chemical reaction that forms the interfacial layer, diffusion through this interfacial layer, or diffusion through the solvent. When one is involved in the study of single crystals, dissolution of the various crystallographic planes may be different. If the dissolution rate is the same for all planes, it is called isotropic dissolution. If the dissolution rate varies among the various planes, it is called anisotropic dissolution. It is easy to understand why dissolution may be different for different planes due to the differences in density of atoms, charges, and/or bonding; however, isotropic dissolution is not as easy to understand, even for a single cation-anion combination. In the case of a cubic crystal such as NaCl where the crystal faces exposed to dissolution are all of the same family, then isotropic dissolution is easy to understand.

Examination of the appropriate phase diagram will aid one in determining whether or not a particular combination of liquid and solid will form an interface. An example is given in Fig. 2.1 that shows a portion of the CaO–Al2O3–SiO2 diagram. If the 1265°C eutectic composition were selected as the liquid and melted in a crucible of Al2O3 at 1500°C, the dissolution would be indirect and an interface of CaAl12O19 would form. As the melt temperature increases, the dissolution type changes to direct above 1700°C, with the eutectic melt being in equilibrium with the alumina. A slightly different situation exists if mullite were selected as the crucible material. At 1400°C, the dissolution is indirect with the interface being anorthite. As the temperature increases, the dissolution remains indirect but the interface changes to alumina above 1500°C. As can be seen from these examples, the interface is determined by the intersection of the isotherm of interest and the construction line joining the liquid and solid compositions.

The wetting characteristics between a ceramic and a liquid are very important in determining the degree of contact that is developed. Although the investigation of Kramer and Osborne [2.3] was performed in an attempt to understand better the parameters involved in the processing of glass-ceramic-to-metal seals, their data exhibited the importance of atmospheric effects

Copyright © 2004 by Marcel Dekker, Inc.

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FIGURE 2.1 A portion of the CaO–Al2O3–SiO2 phase diagram [2.1]. The heavy lines between 1265°C eutectic and either alumina or mullite are construction lines. The lighter dashed lines are isotherms. The solid lines are liquidus field boundary curves.

upon the degree of contact between a ceramic and a liquid. The better the contact, the more aggressive the attack can be. Kramer and Osborne studied the atmospheric effects upon the wetting of a glass-ceramic by Ni-based allovs. They found that helium lowered the contact angle more than argon or an argon/ hydrogen mixture. They also found that contact angles were inversely dependent upon the atmospheric dew point.

Noyes and Whitney [2.4], in their classic work of the dissolution of lead chloride in boric acid and water, speculated that the rate of corrosion of a solute by a solvent was controlled by the diffusion rate of atoms away from the solute surface. Nernst [2.5] postulated that a thin layer of solvent adjacent to the solute became rapidly saturated and remained saturated during the dissolution process and that beyond a certain distance, the concentration was that of the bulk solution. The

Copyright © 2004 by Marcel Dekker, Inc.