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Ca(OH)2 precipitated within the pores of the Ca(OH)2 being dissolved. The assumptions of Cussler and Featherstone were that all reactions in the solid were much faster than diffusion so that the reactions reached equilibrium, the diffusion coefficients of all species were equal, and the porous solid was present in excess. Although these assumptions may yield reasonable first approximations for simple systems, they generally do not hold true, especially for the more complex type systems often encountered.
Another effect of water has been reported in the literature in which the reaction with water resulted in the transformation of a metastable phase to the more stable form. This has been reported by Yoshimura et al. [2.65] for partially stabilized zirconia (PSZ) where the reaction with yttria causes the transformation of the metastable tetragonal zirconia to the stable monoclinic form. Similarly, the adsorption of water onto the surface of zirconia has been reported by Sato et al. [2.66] to cause this transformation. Yoshimura et al. concluded that if the reactivity of Y2O3 in YSZ was the same as in Y-PSZ, the transformation would not be caused by strain release but by the formation of nucleating defects caused by the chemisorption of water that forms stress concentration sites.
One of the more practical problems associated with service life of ceramics is the often observed degradation of mechanical properties attributed to attack by atmospheric water vapor. This is commonly called stress corrosion, is time-dependent, and is capable of decreasing both Young’s modulus and fracture strength [2.67]. For more information concerning property degradation caused by corrosion, see Chap. 8.
2.2.3 Glasses
Bulk Glasses
Probably the most abundant examples of glass corrosion are those caused by a liquid. Release of toxic species (such as PbO or radioactive waste) from various glass compositions has
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received worldwide interest during the past 20–30 years. Although glass is assumed by many to be inert to most liquids, it does slowly dissolve. In many cases, however, the species released are not harmful.
The corrosion resistance of glasses is predominately a function of structure, which is determined by the composition. Although some have related glass durability to the number of nonbridging oxygens, a function of composition, White [2.68] has suggested that glass durability is more closely related to the presence of specific depolymerized units. He arrived at this conclusion through the correlation of vibration spectra with the effective charge on bridging and nonbridging oxygens. In a study of the leaching behavior of some oxynitride glasses, Wald et al. [2.69] reported that the nitrogen-containing glasses exhibited a greater durability (i.e., silicon release) by at least a factor of 2 than either fused silica or quartz tested under identical conditions at 200°C in deionized water for 28 days. This they attributed to the increased amount of cross-linking of the silica network and the resultant reduction in hydrolysis.
Glasses can be soluble under a wide range of pH values from acids to bases, including water. Water-soluble sodium silicates form the basis of the soluble silicate industry that supplies products for the manufacture of cements, adhesives, cleansers, and flocculants. At the other extreme are glasses designed for maximum resistance to corrosion.
The mechanism of silicate glass corrosion by water involves competition between ion exchange and matrix dissolution [2.70] that are affected by glass composition and the possible formation of a protective interfacial layer. The characteristics of this interfacial layer control subsequent dissolution. Dealkalization of this layer, which generally causes further matrix dealkalization and dissolution, is dependent upon the ease of alkali diffusion through this layer, the physical properties of the layer (i.e., porosity, thickness, etc.), and the pH of the solution. The increase in pH of the solution caused by dealkalization causes increased silica dissolution. High initial
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reaction rates are quite often observed and are generally caused by an excessively large exposed surface area due to microcracks or generally rough surfaces. This excessive surface area can be eliminated by proper cleaning procedures.
Jantzen [2.71] has used a thermodynamic approach to the corrosion of glasses, especially applied to nuclear waste glass leachability. The earlier work of Newton and Paul [2.72] on a wide variety of glasses was expanded and then combined with that of Pourbaix [2.73] and Garrels and Christ [2.74] to describe the effects of natural aqueous environments. Using thermodynamic hydration equations, Newton and Paul predicted glass durability from composition. Jantzen showed that the kinetic contribution was primarily a function of the test conditions (SA/ V ratio,* time, and temperature). The major assumptions in Jantzen’s approach were that the total free energy of hydration of the glass was the sum of the free energies of hydration of the components and that the glass structure was a primary function of glass composition. The activity-pH diagrams of Pourbaix provided the needed correlation between free energy of hydration and ion concentration in solution. Thus Jantzen was able to determine glass durability from glass composition by use of a pH-adjusted free energy of hydration term for several hundred compositions of nuclear waste glasses, manmade glasses, and natural glasses. The more negative the pH-adjusted free energy of hydration term, the less durable the glass.
Species may be leached from a glass as a result of ion exchange with protons from solution, or silica may be leached as the siloxane bonds of the matrix are attacked by hydroxyl ions from the solution. The former mechanism is predominant at low pH, whereas the latter is predominant at high pH. Hench and Clark [2.75] categorized leached glass surfaces into five groups. These groupings are listed in Table 2.5. In Types I, II,
* SA/V ratio is the ratio of the surface area of the sample to volume of the corroding liquid.
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TABLE 2.5 Leached Glass Surface Types
Source: Ref. 2.75.
and III, the surface reaction layer that forms has a low solubility if composed of metal hydrates or hydrated silicates. Quite often, these layers are protective, essentially stopping further attack.
Many consider the pH of the solvent to be the most important parameter that affects glass durability [2.76]. At pH <5, ion exchange is the predominant mechanism, and at pH >9, matrix dissolution is predominant. Between pH 5 and 9, the corrosion is a minimum. This is represented schematically in Fig. 2.9. Thus dissolution is rapid when the metal-oxygen bond is extensively coordinated to hydrogen or hydroxyl ions and is a minimum under neutral conditions.
Since the driving force for dissolution of silicate glasses is the hydrogen ion activity, the loss of hydrogen ions during dissolution causes a continuous decrease in the dissolution rate. The change of the pH of the solution can drift into the basic region causing breakdown of the silica matrix with subsequent increasing dissolution rates. To determine accurate rate constants, the experimenter must hold the pH constant by use of buffer solutions or an automatic titration system. Silica solubility increases significantly above about pH=9.
In systems containing ions of variable valence, one must consider the redox potential, Eh, of the system since the solubility of ions is dependent upon their valence state. A general rule of thumb is that higher valence states are more soluble.
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FIGURE 2.9 Effect of pH upon glass dissolution.
Hogenson and Healy [2.77] developed the following equation:
(2.25)
where: |
|
|
W |
= |
weight loss |
a |
= |
experimentally determined coefficient |
b1 |
= |
experimentally determined coefficient |
b2 |
= |
experimentally determined coefficient |
φ= time
T = temperature
for describing the effects of time and temperature upon the acid (10% HCl) corrosion of silicate glasses. This equation, since it relates total multicomponent weight loss to time and temperature assuming a uniform surface corrosion, does not take into account the mechanism of dissolution, but instead determines the total
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overall corrosion. This is probably sufficient for practical problems but does not allow one to study mechanisms.
Budd [2.78] has described the corrosion of glass by either an electrophilic or a nucleophilic mechanism, or both. The surface of the glass has electron-rich and electron-deficient regions exposed. Various agents attack these regions at different rates. Exposed negatively charged nonbridging oxygens are attacked by H+ (or H3O+), whereas exposed network silicon atoms are attacked by O2, OH-, and F-.
Budd and Frackiewicz [2.79] found that by crushing glass under various solutions, an equilibrium pH value was reached after sufficient surface area was exposed. The value of this equilibrium pH was a function of the glass composition, and it was suggested that it was related to the oxygen ion activity of the glass. When foreign ions were present, the amount of surface required to reach an equilibrium pH was greater.
The rate of hydrolysis of a glass surface is one of the major factors that delineates the field of commercial glasses. The rate of hydrolysis is of great importance because it determines the service life of a glass with respect to weathering or corrosion and also because it influences the mechanical properties. Glass fracture is aided by hydrolysis. The rate of hydrolysis of alkalisilicate glasses of the same molar ratios proceeds in the order Rb>Cs>K>Na>Li.
The mechanism of corrosion of fluorozirconate glasses is substantially different from that of silicate-based glasses [2.80]. The fluorozirconate glass corrodes by matrix dissolution, with the components going into solution as fluorides, without first hydrolyzing as in the silicates. These glasses are also characterized by the formation of a nonprotective porous hydrated interfacial layer. Compounds highly insoluble in water remain in the porous layer. The formation of a hydroxylated zirconia fluoride complex in solution causes the pH of the solution to decrease considerably increasing the solubility of zirconia fluoride, thus increasing the overall dissolution rate by orders of magnitude.
The properties of the leached layers that build up can dramatically affect the dissolution rate since the silanol groups
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