380 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. Methods to Minimize Corrosion 381 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 SO 4 - 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: (9.2) 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 ester- type 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. (9. ) 1 382 Chapter 9 alike. The major difference is that for the monuments, one is concerned with their final appearance, whereas with ceramics, in general, that is not the case. 9.3 CRYSTALLINE MATERIALS—NONOXIDES 9.3.1 Property Improvement Most of the items discussed earlier can also be applied to these materials. The one property improvement that should be discussed a little further is that of porosity. For example, Si 3 N 4 is predominantly covalent and does not densify on heating as do conventional ionic ceramics. In applications such as turbine blades, a theoretically dense material is desired. Only through special densification procedures can theoretically dense materials be obtained. In the past, this could be accomplished for Si 3 N 4 only through hot pressing with large amounts (up to 10 wt.%) of additives at very high temperatures and pressures. SiC, in contrast, could be prepared in the fully dense state with only a few percent of additives. Newer techniques have recently been developed using gas pressure sintering and much lower amounts of additives that allow the production of materials that are fully dense. The additives in these processes cause a liquid phase to form at high temperatures, and therefore densification can proceed through liquid-phase sintering. This liquid either crystallizes or forms a glass phase upon cooling. Much work has been done in attempting to obtain either crystalline phases with higher melting points or glassy compositions with higher viscosities to improve the high- temperature properties. The densification processes using lower amounts of additives (generally <2 wt.%) help to maximize the high-temperature properties. Improved corrosion resistance of porous materials can be obtained by impregnating with either a material of the same composition as the bulk or with a material that, in the case of SiC or Si 3 N 4 , is later exposed to a carbiding or nitriding treatment. Other pore-filling materials can also be used, such Copyright © 2004 by Marcel Dekker, Inc. Methods to Minimize Corrosion 383 as nitrates or oxychlorides. Decomposition reactions then produce pore-filling oxides. Impregnation with organosilicon compounds will yield SiC as the pore filler. Corrosion resistance can sometimes be improved by changing the processing method. Chemical vapor deposition (CVD) is one of the most attractive methods to produce high purity dense materials because the sintering process is not required if a bulk material can be obtained directly from the raw vapors or gases. Microstructures of CVD products are strongly dependent upon the deposition temperature and total gas pressure. Chemical vapor deposition can produce materials with no grain boundary phases but which are highly oriented. It is a well-known fact that CVD materials contain residual internal stresses. At present, the effects of these stresses upon high-temperature strength and corrosion are not well known. Preoxidation under some conditions can form a protective oxide layer that will minimize or possibly eliminate continued corrosion [9.7]. In addition, impurities present, generally in the form of sintering aids, may migrate toward the surface and become part of the protective oxide layer. This layer can then be removed resulting in a purer material with subsequent improvement in mechanical properties. The development of nitride-based materials today has progressed to the point of studying materials in Si a M b O c N d systems, where M has been confined mostly to trivalent cations. Most work has been in systems where M=Al, Y, and/or Be. These materials form secondary grain boundary phases which are highly oxidation-resistant and thus provide a better material than conventional Si 3 N 4 materials. Cemented carbide cutting tools made from WC wear rapidly due to local welding of the tool to the steel piece being cut. To overcome this welding, additions of TiC were made to the WC to form a TiO 2 surface layer that protected the tool from rapid wear. WO 3 also formed, but it was volatile and produced no protective layer. In addition, small amounts of TaC and NbC were added to increase the overall oxidation resistance by Copyright © 2004 by Marcel Dekker, Inc. 384 Chapter 9 increasing the melting temperature of the carbide solution formed. 9.3.2 External Methods of Improvement One method of minimizing corrosion not widely practiced is that of coating the ceramic with a layer of more resistant material. Probably the best method to coat a ceramic is by a layer of CVD [9.8] or plasma-sprayed material of the same composition as the substrate [9.9]. Chemical vapor deposition, in general, provides a better coating than plasma-sprayed coatings since it is difficult to form pore-free coatings with uniform thickness using plasma spraying. This provides a well- attached, pure, nonporous layer that has a good thermal expansion match with the substrate. Coating conditions can be varied to produce layers of amorphous material covered by crystalline material of the same composition. This sometimes provides a more complex diffusion path that minimizes oxidation. Although plasma or flame spraying can be used to deposit most materials, control of the spraying parameters confines the coating to mainly oxides. Other methods investigated have been cathode sputtering [9.8,9.10], glow-discharge cathode sputtering, electron beam evaporation, and detonation deposition. These methods are not necessarily confined to the coating of nonoxides; oxides can also be coated. Wittmer and Temuri [9.11] in their work on oxidation of carbon-carbon composites have described a method of protection by coating first with a well-adhering solid oxygen barrier and then coating with a glass-forming material to seal any cracks that may develop from thermal expansion mismatch. The carbon-carbon composite coating system used for the Space Shuttle nose cap is composed of an inner SiC layer covered by a silicate glaze. This is probably the most successful example of the use of oxygen barrier coatings to protect carbon-carbon composites [9.12]. Copyright © 2004 by Marcel Dekker, Inc. Methods to Minimize Corrosion 385 9.4 GLASSY MATERIALS 9.4.1 Property Optimization The development of more resistant glasses has been predominantly through optimization of compositions. Historically, small amounts of alumina have been added to the basic soda-lime-silicate composition to improve durability. In general, lowering the alkali content increases the durability. This, however, has practical limits based upon melting temperatures, viscosities, softening points, and working ranges. Borosilicate glasses are, in general, more are less resistant to alkali solutions than they are to acid solutions. of varying compositions. One technique of composition variation to improve durability that has not received much attention is that of incorporation of nitrogen into the glass structure. Frischat and Sebastian [9.13] have shown that soda-lime-silica glasses containing 1.1 wt.% nitrogen exhibited considerable improvement toward leaching by water at 60°C over compositions containing no nitrogen. This improvement was attributed to a denser structure for the nitrogen-containing glass. Small changes in the chemistry of the glass can cause a significant change in the dissolution mechanism as shown by Lehman and Greenhut [9.14]. They reported that 1 mol% P 2 O 5 addition to a lead silicate glass caused the formation of lead phosphosilicate crystals on the glass surface when exposed to 1% acetic acid at 22°C. They attributed the reduction in dissolution to the reduction of the apparent average interdiffusion coefficient of lead by a factor of 11.3. This is an example of changing the material chemistry to form an interface reaction product that reduces the diffusion rate of the species being leached. 9.4.2 External Methods of Improvement The development of coating technology has provided a means to improve corrosion resistance, abrasion resistance, and Copyright © 2004 by Marcel Dekker, Inc. resistant than soda-lime silicate glasses. In general, silicate glasses Table 6.1 of Chap. 6 lists the corrosion resistance of many glasses 386 Chapter 9 strength. Combinations of coatings applied while the glass is hot and after it has cooled have been developed that form a permanent bond to the glass. These coatings are not removed by cooking or washing. The most commonly used metallic hot-end coatings are tin and titanium. As the piece goes through the annealing lehr, the metal oxidizes, forming a highly protective ceramic coating. Tin is easier to work with since a thicker coating can be applied before problems of iridescence occur. These hot-end metallic coatings give the glass a high glass-to-glass sliding friction and thus a cold-end coating must be applied over these metallic coatings. The cold-end coatings usually have a polyethylene or fatty acid base. Another type of coating is one that reacts with the surface of the glass to form a surface layer that is more corrosion- resistant than the bulk composition. Chemically inert containers are needed to contain various beverages and pharmaceuticals. To provide increased corrosion resistance, these containers are coated internally to tie up the leachable components. Internal treatment with a fluoride gas provides a new surface that is more corrosion-resistant than the original and is more economical than the older sulfur treatment. Although not a true coating technique, the manufacturers of flat glass have, for many years, treated the surface of their glass with SO 2 gas just prior to the glass being annealed to increase the weatherability of their products. This surface treatment allows the sodium in the surface layers to react with the SO 2 forming sodium sulfate. The sulfate deposit that forms on the surface due to this reaction is then washed off prior to inspection and packing. The first step in weathering is then diminished due to the low alkali content of the surface. It has been shown by Harvey and Litke [9.15] that matrix dissolution of an aluminosilicate glass apparently does not occur if the leaching solution is saturated first with solution products of the same glass composition. This technique is an example of how dissolution can be minimized by decreasing the driving force for corrosion by lowering the concentration Copyright © 2004 by Marcel Dekker, Inc. Methods to Minimize Corrosion 387 gradient between the material and leachant, thus minimizing or eliminating the diffusion of cations and anions across the interfacial boundary. Using a different approach to minimize dissolution of a predominantly soda-borosilicate glass, Buckwalter and Pederson [9.16] have shown that the sorption of metal ions onto the glass surface and/or the buffering of the leachate solution caused by the corrosion of metal containers significantly lowered the rate of aqueous corrosion. REFERENCES 9.1. McCauley, R.A. Evolution of flat glass furnace regenerators. Glass Ind. 1978, 59 (10), 26–28, 34. 9.2. Brezny, R.; Semler, C.E. Oxidation and diffusion in selected pitch-bonded magnesia refractories. J. Am. Ceram. Soc. 1984, 67 (7), 480–483. 9.3. Siljan, O J.; Rian, G.; Pettersen, D.T.; Solheim, A.; Schøning, C. Refractories for molten aluminum contact Part I: thermodynamics and kinetics. Refract. Appl. News 2002, 7 (6), 17–25. 9.4. Meadowcroft, D.B.; Cox, W.M. Dewpoint corrosion: mechanisms and solutions. In Dewpoint Corrosion; Holmes, D.R., Ed.; Ellis Horwood Ltd.: Chichester, UK, 1985. Chapter 2. 9.5. Harris, L.A.; Cross, D.R.; Gerstner, M.E. Corrosion suppression on rutile anodes by high energy redox reactions. J. Electrochem. Soc. 1977, 124 (6), 839–844. 9.6. Amoroso, G.G.; Fassina, V. Stone Decay and Conservation; Elsevier: Amsterdam, 1983; 453 pp. 9.7. Lange, F.F.; Davis, B.I.; Metcalf, M.G. Strengthening of polyphase Si 3 N 4 materials through oxidation. J. Mater. Sci. 1983, 18 (5), 1497–1505. 9.8. Davies, G.B.; Holmes, T.M.; Gregory, O.J. Hot corrosion behavior of coated covalent ceramics. Adv. Ceram. Mater. 1988, 3 (6), 542–547. 9.9. Gogotsi, Yu.G.; Lavrenko, V.A. Corrosion protection and development of corrosion-resistant ceramics. Corrosion of Copyright © 2004 by Marcel Dekker, Inc. 388 Chapter 9 High-Performance Ceramics; Springer-Verlag: Berlin, 1992; 151–162. Chp. 7. 9.10. Gregory, O.J.; Richman, M.H. Thermal oxidation of sputter- coated reaction-bonded silicon nitride. J. Am. Ceram. Soc. 1984, 67 (5), 335–340. 9.11. Wittmer, D.E.; Temuri, M.Z. Thermochemical studies in selected metal-carbon-oxygen systems. J. Am. Ceram. Soc. 1991, 74 (5), 973–982. 9.12. Strife, J.R. Fundamentals of protective coating strategies for carbon-carbon composites. In Damage and Oxidation Protection in High Temperature Composites; Haritos, G.K., Ochoa, O.O., Eds.; ASME: New York, 1991; Vol. 1, 121–127. 9.13. Frischat, G.H.; Sebastian, K. Leach resistance of nitrogen- containing Na 2 O–CaO–SiO 2 glasses. J. Am. Ceram. Soc. 1985, 68 (11), C305-C307. 9.14. Lehman, R.L.; Greenhut, V.A. Surface crystal formation during acid corrosion of phosphate-doped lead silicate glass. J. Am. Ceram. Soc. 1982, 65 (9), 410–414. 9.15. Harvey, K.B.; Litke, C.D. Model for leaching behavior of aluminosilicate glasses developed as matrices for immobilizing high-level wastes. J. Am. Ceram. Soc. 1984, 67 (8), 553–556. 9.16. Buckwalter, C.Q.; Pederson, L.R. Inhibition of nuclear waste glass leaching by chemisorption. J. Am. Ceram. Soc. 1982, 65 (9), 431–436. Copyright © 2004 by Marcel Dekker, Inc. 389 Glossary Cor·rode, v.t. to eat into or wear away gradually, as by rusting or by the action of chemicals. WEBSTER’S NEW WORLD DICTIONARY Alteration The change or modification of a material through interaction with its environment, generally by the formation of a new phase. This reaction need not be deleterious. Atmospheric Corrosion The degradation of materials by natural atmospheric environments. Atmospheric corrosion is a term often used by the metallurgist, whereas an equivalent term used by ceramists is weathering. Copyright © 2004 by Marcel Dekker, Inc. [...]... limestone, marble, and glass; and is essentially the attack by water vapor, CO2, and SO2 Copyright © 2004 by Marcel Dekker, Inc Epilog The literature and data available on the corrosion of ceramics indicate that corrosion occurs by either one of several possible mechanisms or a combination of these mechanisms Many similarities exist between the corrosion of crystalline and glassy ceramics, although... literature of the corrosion of ceramics related to properties such as expansion, hardness, or softening point Only through a thorough understanding of all the parameters involved can the engineer make an intelligent selection of the material that will best resist corrosion for a particular application Only through intelligent materials selection can the cost of corrosion be minimized Since the application of. .. of the corrosion processes that occur in ceramics, it is a step in the direction of simplifying and unifying the whole area All of the data and discussion about corrosion point towards the need for more in-depth diffusion and solubility studies of the various species in the different corroding media encountered in practice Corrosion, being an interfacial process, requires a thorough understanding of. .. Biodurability The ability of a ceramic to withstand the action of a biological environment Condensation Corrosion Equivalent to dew point corrosion Corrosion The chemical interaction of a ceramic with its environment, generally producing a deleterious effect This chemical reaction can, in some cases, be put to beneficial use Dealkalization The corrosion of a ceramic through the selective solution of the alkalies... removal of alkalies from glasses Degradation A general decrease or lowering of the quality of a ceramic; often through corrosive action Dewpoint Corrosion The deterioration of a solid ceramic material caused by the condensation of a corrosive liquid from a saturated gas when the temperature is lowered below the point (the dew point) where the liquid will condense A form of atmospheric corrosion and equivalent... addition, various types of defects (e.g., vacancies, dislocations, etc.) could be incorporated into the lattice during production of the single crystals A large amount of published data on the corrosion of crystalline and glassy ceramics points toward the fact that more compact structures are more durable In the study of glasses, references are made to corrosion being a function of glass structures, which... rate of corrosion The mechanism of corrosion, however, may change as temperature is increased due to crystallization of amorphous reaction layers, polymorphic transitions, melting of crystalline layers, vaporization of various species in the layer, cracking, etc One method of minimizing corrosion that requires more emphasis appears to be the various coating methods These could be used to advantage in composites... term Hot Corrosion is nonspecific and could apply to any type of corrosion at an elevated temperature Intergranular or Grain Boundary Corrosion The corrosion through any mechanism that takes place preferentially along grain boundaries or between grains Leaching To remove through dissolution a portion of a ceramic material Leaching: Selective Removes one species in preference to another The use of the... photo-dissolution Stress Corrosion Corrosion by any mechanism that is enhanced by the presence of either a residual or applied stress Thermo-Oxidative Stability The resistance to oxidation at elevated temperatures Generally used in the discussion of composite materials Weathering This term describes the atmospheric effects upon materials of the construction industry, mostly structural clay products, sandstone, limestone,... also occur in other media Galvanic Corrosion The corrosion that takes place when two chemically dissimilar ceramics are in contact with one another, both of which are in contact with the same electrolyte Reaction occurs only when current flows in an external circuit A type of electrochemical corrosion Hot corrosion Normally used to designate high temperature oxidation of ceramics in contact with molten . Glossary Biodurability The ability of a ceramic to withstand the action of a biological environment. Condensation Corrosion Equivalent to dew point corrosion. Corrosion The chemical interaction of a ceramic with its environment,. liquid will condense. A form of atmospheric corrosion and equivalent to condensation corrosion. Dissolution Corrosion The corrosion of a ceramic through the solution of its various components into. decrease or lowering of the quality of a ceramic; often through corrosive action. Dewpoint Corrosion The deterioration of a solid ceramic material caused by the condensation of a corrosive liquid