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service temperatures, the silicate amorphous phase becomes molten and will penetrate between like grains more easily than between unlike grains. Thus the presence of multiple phases keeps the liquid from penetrating between all the grains and causing failure of the refractory. Therefore the possibility exists of creating more corrosion-resistant materials by incorporating a small amount of a second phase that would form along grain boundaries and act to retard the penetration of a corroding liquid.

Since corrosion of ceramics quite often involves the diffusion of various cations and anions through an interfacial reaction layer, changes to the chemistry that would either provide a layer through which diffusion is more difficult or provide species that would form a reaction layer immune to continued corrosion should be investigated. This would undoubtedly involve considerable research into the diffusion of various cations and anions through various materials. Only then will it be possible to tailor a composition to provide minimum corrosion.

9.2.2 External Methods of Improvement

In Chap. 2 on Fundamentals, the importance of temperature was stressed several times. Various techniques have been used to lower the temperature of the interface or hot face of the material (lower hot face temperatures mean less corrosion). Many applications of a ceramic material subject the material to a thermal gradient. By altering the material or providing a means to increase the heat flow through the material, the hotface temperature can be lowered significantly, or more accurately, the slope of the thermal gradient is increased as shown in Fig. 2.3. One means of doing this is by forcibly cooling the cold face. This provides faster heat removal and thus lowers the hot-face temperature. Most industrial furnaces use some means of forced cooling on the cold face by cooling-air systems or water-cooled piping. In a few cases, water has actually been sprayed onto the cold face of the refractory using the heat of

Copyright © 2004 by Marcel Dekker, Inc.

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Chapter 9

vaporization of the water to extract heat from the refractory. If the thermal gradient through the material becomes too steep, failure may occur (this depends upon the thermal expansion characteristics of the material).

Another method that has been used to lower the hot-face temperature is to place metal plates either within individual bricks or between them. A large portion of the heat is thus conducted through the metal plate. A similar technique has been used by manufacturing a product containing oriented graphite particles. The steel industry has used many of these techniques in their blast furnaces. The most common technique today is the use of water-cooled internal metal plates (or boxes).

Another way to take advantage of increased cooling is initially to use a thinner material. This will automatically cause a thinner reaction layer to form on the surface. In general, glass furnace basin wall linings should not be greater than 10– 12 in. thick. Anything greater than about 12 in. does not normally increase overall life but adds an economic penalty in refractory cost per campaign. The thickness at the flux-line generally is 9 in. so that effective air cooling can be used. In fact, most linings could probably be less than 10 in.; however, the thermal-mechanical environment will determine the ultimate thickness that should be used.

If a refractory lining is insulated, a greater portion of the refractory will be at a higher temperature and corrosion will proceed at a faster rate. In these cases, a balance must be obtained between service life and energy conservation. Because of the potential for increased corrosion of insulated linings, the properties of the lining material must be carefully evaluated before insulation is installed. In many cases, the engineer may want to upgrade the lining material if it is to be insulated.

Historically, it has been recommended that flue gas temperatures be 20–30°C higher than the dew point [9.4]. However, because of thermal efficiencies and the related cost, this has been lowered to 5–10°C higher than the dew point. Once condensation has occurred, reevaporation of the water or other volatile can concentrate corrosive species causing a

Copyright © 2004 by Marcel Dekker, Inc.

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more severe corrosion problem; thus condensates should be removed as rapidly as possible.

The addition of redox couples in photoelectrochemical corrosion of electronically conductive materials in acids acts on the environment to minimize corrosion. An example is the addition of cobalt as the redox couple to scavenge SO4- that is formed by the reaction of a positive hole with the sulfate ion [9.5]. The positive hole is photogenerated in the valence band of an illuminated titania semiconductor. The reactions listed below act to minimize corrosion:

(9.1)

(9.2)

In the preservation of national monuments, engineers have tried various materials to fill the porous limestones, marbles, etc. to slow corrosion. Not only are the fillers used to eliminate open porosity, but also they are used to consolidate or strengthen friable portions of the structure. The most important parameter of the fillers is that they too must be corrosion-resistant. Many materials have been used to provide this filler/strengthening characteristic from waxes to acrylic polymers to silicic estertype products [9.6]. According to Amoroso and Fassina [9.6], these materials must have the following basic characteristics:

1.Control the diffusion of water

2.Protect against atmospheric pollutants

3.Possess a low coefficient of thermal expansion

4.Be inert toward the parent structure

5.Not modify the original appearance

Although the preservation of national monuments around the world has been in progress for over 100 years, it has been only recently that true advances have been made in their preservation. At first, it would seem that the preservation of monuments is not very much like the slowing or elimination of corrosion of ceramics; however, the two areas are very much

Copyright © 2004 by Marcel Dekker, Inc.