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the rate. Corrosion reactivity is affected by the following items (not necessarily listed in the order of importance):

1.Heat transfer

2.Mass transfer

3.Diffusion-limited processes

4.Contact area

5.Mechanism

6.Surface-to-volume ratio

7.Temperature

8.Time

The following discussion will address some of these items and how they may be used to minimize the effects of corrosion by discussing various examples.

9.2 CRYSTALLINE MATERIALS—OXIDES

The most obvious method of providing better corrosion resistance is to change materials; however, this can be done only to a certain extent. There will be ultimately only one material that does the job best. Once this material has been found, additional corrosion resistance can be obtained only by property improvement or, in some cases, by altering the environment. Different parts of an industrial furnace generally involve variations in the corrosive environment, necessitating the use of different materials with the best properties for a particular location within the furnace. Furnace designers have thus for a long time used a technique called zoning to maximize overall service life by using different materials in different parts of the furnace.

9.2.1 Property Optimization

Since exposed surface area is a prime concern in corrosion, an obvious property to improve is the porosity. Much work has been done in finding ways to make polycrystalline materials

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less porous or denser. The most obvious is to fire the material during manufacture to a higher temperature. Other methods of densification have also been used. These involve various sintering or densification techniques: liquid-phase sintering, hot pressing, and others. If additives are used to cause liquid phase sintering, care must be exercised that not too much secondary phase forms, which might lower corrosion resistance, although porosity may be reduced.

Alterations in major component chemistry may aid in increasing corrosion resistance, but this is actually a form of finding a new or different material, especially if major changes are made.

The history of glass-contact refractories is a good example of corrosion resistance improvement in a polycrystalline material. Porous clay refractories were used originally. Changes in chemistry by adding more alumina were made first to provide a material less soluble in the glass. The first major improvement was the use of fusion-cast aluminosilicate refractories. These provided a material of essentially zero porosity. The next step was the incorporation of zirconia into the chemistry. Zirconia is less soluble than alumina or silica in most glasses. Because of the destructive polymorphic transformation of zirconia, a glassy phase had to be incorporated into these refractories. This glassy phase added a less corrosion-resistant secondary phase to the refractory. Thus the higher resistance of the zirconia was somewhat compromised by the lower resistance of the glassy phase. The final product, however, still had a corrosion resistance greater than the old product without any zirconia. Today, several grades of ZrO2 –Al2O3–SiO2 fusioncast refractories are available. Those with the highest amount of zirconia and the lowest amount of glassy phase have the greatest corrosion resistance.

As discussed in Chap. 2, Sec. 2.5.2 (Porosity and Surface

Area), thermal transpiration is the migration of a gas along a thermal gradient. As long as the pore size distribution is optimized, the transpiring gas will flow toward the hot face. This transpiring gas must be selected so that it will alter the

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reaction at the hot face in a beneficial way. One obvious way is to dilute the effects of a corroding gas. Although the author knows of no examples of the use of thermal transpiration of gases to minimize or eliminate corrosion, there is no reason why it should not work.

Another example from the glass industry is the development of furnace regenerator refractories through the optimization of materials made of fireclay by using higher purity raw materials and then increased firing temperatures. Changes in chemistry were then made by switching from the fireclay products to magnesia-based products. Again, improvements were made by using higher purity raw materials and then increased firing temperatures. Minor changes in chemistry were also made during the process of property improvement. Changes in processing involving prereaction of raw materials have also been done. The evolution of regenerator refractories for the flat glass industry up to the mid-1970s has been described by McCauley [9.1]. The latest development in regenerator refractories has been the use of fusion cast alumina- zirconia-silica cruciform products. These are in the shape of a cross and are stacked in interlocking columns. This represents not only a change in chemistry, but also a change in the shape of the product, both of which lead to better overall performance.

A part of the concept of improvement through chemistry changes is that of improving resistance to corrosion of the bonding phases. Bonding phases normally have a lower melting point and lower corrosion resistance than does the bulk of the material. The development of high alumina refractories is a good example of improvement based on the bonding phase. The best conventional high alumina refractories are bonded by mullite or by alumina itself. To change this bond to a more corrosion-resistant material compatible with alumina, knowledge of phase equilibria played an important role. Alumina forms a complete series of crystalline solutions with chromia, with the intermediate compositions having melting points between the two end members. Thus a bonding phase

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formed by adding chromia to alumina would be a solution of chromia in alumina with a higher melting point than the bulk alumina and thus a higher corrosion resistance. In addition to the more resistant bonding phase, these materials exhibit a much higher hot modulus of rupture (more than twice mullite or alumina-bonded alumina). Nothing is ever gained, however, without the expense of some other property. In this case, the crystalline-solution-bonded alumina has a slightly lower thermal shock resistance than does the mullite-bonded alumina. Owing to the excellent resistance of these materials to iron oxide and acid slags, they have found applications in the steel industry.

The development of tar-bonded and tar-impregnated basic refractories to withstand the environment of the basic oxygen process of making steel is yet another example of a way to improve the corrosion resistance of a material. Tar-bonded products are manufactured by adding tar to the refractory grain before pressing into shape. In this way, each and every grain is coated with tar. When the material is heated during service, the volatiles burn off, leaving carbon behind to fill the pores. An impregnated product is manufactured by impregnating a finished brick with hot tar. This product, once in service, will similarly end up with carbon in the pores. Impregnated products do not have as uniform a carbon distribution as do the bonded types. Newer products incorporate graphite into the raw material mix. The carbon that remains within the refractory increases the corrosion resistance to molten iron and slags by physically filling the pores, by providing a nonwetting surface, and by aiding in keeping iron in the reduced state, which then does not react with the oxides of the refractory. Any oxygen that diffuses into the interior of the refractory causes carbon oxidation that slightly increases the pore pressure and thus minimizes slag and metal penetration. A thin layer on the hot face (1–2 mm) does lose its carbon to oxidation and various slag components penetrate and react within this layer. This corrosion, however, is much slower than with a product that contains no carbon.

An additional improvement upon the carbon-containing

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magnesia refractories has been the incorporation of magnesium metal, as reported by Brezny and Semler [9.2]. Upon magnesium volatilization and diffusion toward the hot face, oxidation and precipitation enhance the formation of the dense magnesia-rich zone that forms behind the hot face and thus minimizes slag and metal penetration in addition to oxygen diffusion to the interior of the refractory (see Chap. 5, Sec. 5.3.2 for a discussion on the formation of this magnesia-rich dense zone).

Others [9.3] have tried to take advantage of the nonwetting enhancement caused by antiwetting additives; however, their success was questionable.

The automotive industry in their efforts to develop a gas turbine engine has conducted a considerable amount of research on low expansion lithium aluminosilicates (LAS) and magnesium aluminosilicates (MAS) for a rotary wheel heat exchanger. The LAS materials are based upon solid solutions of the high-temperature polymorphs of two different compounds—eucryptite (Li2O·Al2O3·SiO2) and spodumene (Li2O· Al2O3·4SiO2). Both of these materials have an upper use temperature of about 1200°C. Both have a very low thermal expansion (eucryptite being slightly negative) which gives them excellent thermal shock resistance. These materials, however, suffer from corrosion problems when used in dirty environments. To overcome these corrosion problems, an aluminosilicate (AS) material was developed by the acid leaching of lithium from LAS prior to application. This material had acceptable thermal expansion, although not as low as LAS, but did not distort or crack as much.

The development of direct bonded basic refractories is an example where the chemistry was changed to take advantage of the formation of multiple phases and the effects of surface energy upon penetrating liquids. See Chap. 2, Sec. 2.5.3 for a discussion of the surface energy effects of multiphase systems. The direct bonded basic refractory contains magnesia and spinel crystalline phases along with a grain boundary phase that is partially amorphous and partially small spinel crystals. At

Copyright © 2004 by Marcel Dekker, Inc.