Corrosion of Specific Glassy Materials 287 Copyright © 2004 by Marcel Dekker, Inc. heavy crown glass by organic acid solutions. Yogyo Kyokai Shi 1978, 86 (5), 230–237. 6.64. Walters, H.V. Corrosion of a borosilicate glass by orthophosphoric acid. J. Am. Ceram. Soc 1983, 66 (8), 572– 574. 6.65. Metcalfe, A.G.; Schmitz, G.K. Mechanism of stress corrosion in E glass filaments. Glass Technol. 1972, 13 (1), 5–16. 6.66. Priest, D.K.; Levy, A.S. Effect of water content on corrosion of borosilicate glass. J. Am. Ceram. Soc. 1960, 43 (7), 356– 358. 6.67. Koch, G.H.; Syrett, B.C. Progress in EPRI research on materials for flue gas desulphurization systems. In Dewpoint Corrosion; Holmes, D.R., Ed.; Ellis Horwood Ltd: Chichester, UK, 1985; 105–124. 6.68. Velez, M.H.; Tuller, H.L.; Uhlmann, D.R. Chemical durability of lithium borate glasses. J. Non-Cryst. Solids 1982, 49 (1– 3), 351–362. 6.69. Conzone, C.D.; Brown, R.F.; Day, D.E.; Ehrhardt, G.J. In vitro and in vivo dissolution behavior of a dysprosium lithium borate glass designed for the radiation synovectomy treatment of rheumatoid arthritis . J. Biomed. Mater. Res. 2002, 60 (2), 260–268. 6.70. Day, D.E. Reactions of bioactive borate glasses with physiological liquids. Glass Res 2002–2003, 12 (1–2), 21–22. 6.71. Yoon, S.C. Lead release from glasses in contact with beverages; M.S. thesis, Rutgers University, New Brunswick, NJ, 1971. 6.72. Pohlman, H.J. Corrosion of lead-containing glazes by water and aqueous solutions. Glastech. Ber. 1974, 47 (12), 271–276. 6.73. Yoon, S.C. Mechanism for lead release from simple glasses, Univ. Microfilms Int. (Ann Arbor, Mich.) Order No. 73–27, 997; Diss. Abstr. Int. 1973, B34 (6) 2599. 6.74. Lehman, R.L.; Yoon, S.C.; McLaren, M.G.; Smyth, H.T. Mechanism of modifier release from lead-containing glasses in acid solution. Ceram. Bull. 1978, 57 (9), 802–805. 6.75. Krajewski, A.; Ravaglioli, A. Lead-ion stability in vitreous systems. J. Am. Ceram. Soc 1982, 65 (5), 265–269. 288 Chapter 6 Copyright © 2004 by Marcel Dekker, Inc. 6.76. Lead Glazes for Dinnerware, International Lead Zinc Research Organization Manual, Ceramics I, International Lead Zinc Research Organization and Lead Industries Association, New York, 1974. 6.77. Haghjoo, M.; McCauley, R.A. Solubility of lead from ternary and quaternary silicate frits. Ceram. Bull. 1983, 62 (11), 1256–1258. 6.78. Moore, H. The structure of glazes. Trans. Br. Ceram. Soc. 1956, 55, 589–600. 6.79. Clark, A.E.; Pantano, C.G.; Hench, L.L. Spectroscopic analysis of bioglass corrosion films. J. Am. Ceram. Soc. 1976, 59 (1–2), 37–39. 6.80. Hench, L.L. Surface modification of bioactive glasses and ceramics. In Corrosion of Glass Ceramics and Ceramic Superconductors; Clark, D.E., Zoitos, B.K., Eds.; Noyes Publications: Park Ridge, NJ, 1992; 298–314. 6.81. Minami, T.; Mackenzie, J.D. Thermal expansion and chemical durability of phosphate glasses. J. Am. Ceram. Soc. 1977, 60 (5–6), 232–235. 6.82. Reis, S.T.; Karabulut, M.; Day, D.E. Chemical durability and structure of zinc-iron phosphate glasses. J. Non-Cryst. Solids 2001, 292 (1–3), 150–157. 6.83. Hench, L.L. Bioactive glasses help heal, repair and build human tissue. Glass Res. 2002–2003, 12 (1–2), 18. 6.84. Hench, L.L.; Wilson, J. Introduction. In An Introduction to Bioceramics; Advanced Series in Ceramics; World Scientific Publishing Co. Ltd: Singapore, 1993; Vol. 1, 1–24. 6.85. Avent, A.G.; Carpenter, C.N.; Smith, J.D.; Healy, D.M.; Gilchrist, T. The dissolution of silver-sodium-calcium- phosphate glasses for the control of urinary tract infections. J. Non-Cryst. Solids 2003, 328, 31–39. 6.86. Murch, G.E., Ed.; Materials Science Forum, Halide Glasses I and II, Proceedings of the 3rd International Symposium on Halide Glasses, Rennes, France, Trans Tech Publications: Aedermannsdorf, Switzerland, 1985. 6.87. Ravaine, D.; Perera, G. Corrosion studies of various heavy- metal fluoride glasses in liquid water: application to fiuoride- Corrosion of Specific Glassy Materials 289 Copyright © 2004 by Marcel Dekker, Inc. ion-selective electrode. J. Am. Ceram. Soc. 1986, 69 (12), 852–857. 6.88. Doremus, R.H.; Bansal, N.P.; Bradner, T.; Murphy, D. Zirconium fluoride glass: surface crystals formed by reaction with water. J. Mater. Sci. Lett. 1984, 3 (6), 484–488. 6.89. Simmons, C.J.; Simmons, J.H. Chemical durability of fluoride glasses: I. Reaction of fluorozirconate glasses with water. J. Am. Ceram. Soc. 1986, 69 (9), 661–669. 6.90. Gbogi, E.O.; Chung, K.H.; Moynihan, C.T.; Drexhage, M.G. Surface and bulk -OH infrared absorption in ZrF 4 -and HfF 4 - based glasses. J. Am. Ceram. Soc. 1981, 64 (3), C51-C53. 6.91. Robinson, M.; Drexhage, M.G. A phenomenological comparison of some heavy metal fluoride glasses in water environments. Mater. Res. Bull. 1983, 18, 1101–1112. 6.92. Simmons, C.J.; Azali, S.; Simmons, J.H. Chemical Durability Studies of Heavy Metal Fluoride Glasses. Extended Abstract # 47, 2nd International Symp. on Halide Glasses, Troy, NY; 1983. 6.93. Lin, F.C.; Ho, S M. Chemical durability of arsenic-sulfur- iodine glasses. J. Am. Ceram. Soc. 1963, 46 (1), 24–28. 291 7 Corrosion of Composites Materials The whole is most always better than the sum of the parts. ANONYMOUS 7.1 INTRODUCTION Although the term composite historically meant any product made from a combination of two (or more) materials, the modern meaning is less broad in scope. In general, a composite is manufactured in an attempt to obtain the best properties of two materials or at least to capture a specific property of each material that is potentially better in the composite. It is also possible for the composite to have a particular property that neither component exhibited individually. According to Holmes and Just [7.1], a true composite is where distinct materials are combined in a nonrandom manner to produce overall structural characteristics superior to those of the individual components. Although, in a very broad sense, products such as glazed Copyright © 2004 by Marcel Dekker, Inc. 292 Chapter 7 ceramic tile, enameled metal, and ceramic coated metal (e.g., thermal barrier coatings) could be considered composites, they will not be considered as such here. Only those materials where a substantial intermixing of the different materials exists on a microscopic scale will be considered composites. The concept of composite materials is not a new idea and is definitely not limited to ceramics. Nature has provided us with several excellent examples of composite materials. Wood is a composite of cellulose fibers contained in a matrix of lignin. Bone, another example, is composed of the protein collagen and the mineral apatite. In all these materials, the result is a product that is lighter and stronger than either of the components individually. Because of this, they can be used in more severe environments, e.g., space exploration. A list of the more desirable properties of a composite is given in Table 7.1. In a very broad sense, all engineering materials are composites of one kind or another. The matrix and the reinforcement, quite often fibrous, provide two different functions. The reinforcement is most often a discontinuous phase whether it be a fibrous material or a particulate material. It is important that the reinforcement be discontinuous, especially if it is a ceramic, so that cracks TABLE 7.1 Desirable Properties of Composites Copyright © 2004 by Marcel Dekker, Inc. Corrosion of Composites Materials 293 will not be able to propagate through it. The matrix must not damage the reinforcement and it must transmit any stresses to the reinforcement. Thus the adhesion of the matrix to the reinforcement is of prime importance for mechanical integrity and is the region of greatest importance related to corrosion. Since it is necessary to have weak interfaces to maximize toughness (i.e., resistance to crack propagation), the development of optimum fiber/matrix interfaces is quite difficult. To obtain these optimum characteristics, it is sometimes required to coat the reinforcement fibers with various materials to obtain the proper debonding, sliding, and/ or reaction characteristics. Fibers that do not debond do not enhance toughening and lead only to increased brittle fracture of the composite [7.2–7.7]. A recent development in composites is that of a nanosized second phase or reinforcement material. The second phase particles are generally less than 300 nm and are present in amounts equal to 1–30 vol.%. These new composites unfortunately have been called nanocomposites. Before going into the specifics of corrosion of composite materials, a few words must be said about those materials that have been called cermets. Historically, the term cermet was derived to cover those materials composed of cobalt-bonded tungsten carbide and used as cutting tools. Since cermets contain both ceramics and metals, some confusion has existed in the literature as to an exact meaning. The term, however, has been used to cover a broad list of materials. It appears that the ceramic community confines cermets to essentially cutting tool materials, whatever the matrix or reinforcement, whereas the metals community confines cermets to only those materials with a metal matrix. Since the broader concept of composites includes those materials called cermets, only the term composite will be used in the discussion below. The actual corrosion of composite materials quite often begins with reaction of the reinforcement material and especially with any interface material (called the interphase) used to coat the reinforcement for debonding. One property Copyright © 2004 by Marcel Dekker, Inc. 294 Chapter 7 that exacerbates this is a mismatch in thermal expansion coefficients between the reinforcement and the matrix, leading to microcracks. These microcracks allow the ingress of corrosive gases (e.g., oxygen). Courtright [7.8] has given the value of 10 -12 g O 2 / cm sec for the limit of oxygen ingress that causes nonoxide fiber deterioration. Microcracks are also quite often a product of sample preparation techniques, and thus great care must be used in cutting and grinding/polishing samples for testing. If the composite is cut or machined, any exposed fiber reinforcement will be susceptible to attack by the environment. Because of this inherent problem, protective coatings are often applied to the exterior surfaces. Actually, the whole corrosion process of composite materials is not unlike that of other polyphase ceramic materials where the grain boundary phase is the first to corrode. A complete understanding of all the phases that make up the microstructure of the composite must also be known for an accurate interpretation of any corrosion. For example, Munson and Jenkins [7.9] reported that their samples were actually attacked internally by molten metal from a small amount of free aluminum present as a residue during the manufacture of Dimox™* (a melt-infiltrated alumina). Actually, a large amount of the literature on composites is concerned with an evaluation of the internal reactions that take place among the various reinforcement, interphase, and matrix materials. The time-dependent loss of strength due to the corrosive nature of moist environments at room temperature is a major concern for composites containing glass or glass-ceramics as either the matrix or the reinforcement [7.10]. As temperatures are increased, the concern shifts toward oxidation problems associated with nonoxide materials. See the discussions in Properties and Corrosion, for more details of oxidation and its effects upon the properties of nonoxides. * DIMOX™ (directed metal oxidation) is the name given to composites manufactured by a process developed by Lanxide Corp., Newark, DE in 1986. Copyright © 2004 by Marcel Dekker, Inc. Chap. 5, Sec. 5.2.2, Nitrides and Carbides, and Chapter 8, Corrosion of Composites Materials 295 With the advancement of the development of composites, there is an increasing number of acronyms with which one must contend. To aid the reader, a list is given in Table 7.2 of the most common acronyms. 7.2 REINFORCEMENT 7.2.1 Fibers Various types of materials have been used as the fibrous reinforcement. These include various glasses, metals, oxides, TABLE 7.2 Acronyms Used in the Discussion of Composites Copyright © 2004 by Marcel Dekker, Inc. 296 Chapter 7 nitrides, and carbides either in the amorphous or crystalline state. The surface chemistry and morphology of fibers is very important in determining their adherence to the matrix. Fiber internal structure and morphology determines the mechanical strength. A tremendous amount of literature is available that discusses the degradation of mechanical properties as temperatures are increased in various atmospheres; however, there is very little interpretation of any corrosion mechanisms that may be involved. Although many composites are classified as continuous-fiber-reinforced, some composites contain fibers that are actually not continuous but of a high aspect ratio (i.e., length-to-width). The actual matrix material will determine the aspect ratio required to obtain a certain set of properties. Thus the term “high aspect ratio” is a relative term. Boron fibers can generally be heated in air to temperatures of about 500°C without major strength deterioration. Above 500°C, the oxide that formed at lower temperatures becomes fluid increasing the oxidation rate and drastically reducing the strength [7.11]. Galasso [7.11] discussed the benefits of coating boron fibers with either SiC or by nitriding the surface. The SiC coating was more protective than the nitride with strength retention even after 1000 hr at 600°C in air. Boron carbide (B 4 C) is stable to 1090°C in an oxidizing atmosphere, whereas boron nitride is stable to only 850°C. Carbon or graphite fibers have been used since the early 1970s as reinforcement for composites. Strength loss due to oxidation occurs at temperatures above 500°C in air. An interesting structural feature of carbon fibers is that they have a relatively large negative axial thermal expansion coefficient. Glass fibers generally are used as reinforcement for composites that are to be used at low temperatures (i.e., <500°C) due to the softening of glasses at elevated temperatures. These composites are generally of the polymer matrix type and are used for marine or at least moist environments. It is well known that glass is attacked by moist environments with the specific mechanism dependent upon Copyright © 2004 by Marcel Dekker, Inc. Corrosion of Composites Materials 297 Schmitz [7.12] that borosilicate glass fibers when exposed to moist ambient environments developed surface tensile stresses caused by exchange of alkali for hydrogen sufficient to cause failure. A large portion of the CMC today contains SiC fiber reinforcement. This is mainly due to the excellent properties of SiC—low reactivity to many matrix materials, its strength at elevated temperatures, and its oxidation resistance. It is this latter property (i.e., oxidation resistance) that generally causes deterioration in these materials. SiC will oxidize readily when heated to temperatures greater than 1000°C. As discussed in oxygen, active corrosion takes place with the formation of gaseous products of CO and SiO. At higher partial pressures, passive oxidation occurs with the formation of CO and SiO 2 that may be protective if cracks do not form. The formation of cracks is dependent upon the heat treatment and whether the oxide layer is crystalline or amorphous. These reactions generally result in the decrease of fiber strength. Nicalon™ fiber*, being formed by the pyrolysis of organometallics, actually contains some remnant oxygen (~9%) and carbon (~11%) that will affect the subsequent oxidation of the fiber. Two different grades of Nicalon™ fiber have been examined by various investigators [7.13–7.15]. Clark et al. [7.13] reported these fibers to exhibit weight losses of 13% and 33% after being treated in argon at 1400°C. Both grades of fiber gained weight (on the order of 2–3%) when treated in flowing wet air at 1000°C, 1200°C, and 1400°C. As-received Nicalon™ fibers have protective sizing (i.e., polyvinyl acetate) on their surfaces. When heated in air, this sizing will burn off at temperatures between 250°C and 500°C. At temperatures above about 1250°C, the SiC x O y amorphous phase contained in these fibers decomposed to SiO and CO [7.16]. * Nicalon™, Nippon Carbon Co., Tokyo, Japan. Copyright © 2004 by Marcel Dekker, Inc. the pH (see Chap. 6). It has been shown by Metcalfe and Chap. 5, Silicon Carbide, page 223, at low partial pressures of [...]... CAS=calcia-aluminosilicate, and BaMAS=bariamagnesia-aluminosilicate Copyright © 2004 by Marcel Dekker, Inc Corrosion of Composites Materials 305 react with the SiC reinforcement fibers forming particles of NbC on the surface of the fibers A very thin carbon-rich layer formed between the SiC fibers and the NbC that contributed to the excellent toughness and crack deflection of these composites The reactions... diffuse from low partial pressures of oxygen to high partial pressures A third innermost layer was composed of porous cordierite None of the scale layers contained SiC At a temperature of 1450°C, the nonprotective scale was essentially one porous layer composed of cordierite and small grains of spinel Panda and Seydel [7.56] found that a spinel (prepared from hydrated magnesium nitrate and aluminum hydroxide)... form as the * The Pilling and Bedworth coefficient is defined as the ratio of the volume of silica formed to the volume of the ceramic consumed See N.G Pilling and R.E Bedworth, J Int Met., 29, 529 (1923) Copyright © 2004 by Marcel Dekker, Inc Corrosion of Composites Materials 315 temperature is raised above 1400°C The severity of the reaction is dependent upon the amount of atomic hydrogen present... matrix-graphite fiber composites The seawater corrosion of SiC/Al was found to be more resistant than graphite/Al by Aylor and Kain [7.80] This was attributed to a lack of a galvanic driving force between the SiC and the aluminum matrix, although both composites exhibited similar mechanisms of corrosion essentially pitting of the metal matrix around the reinforcement material The reactions of hydrogen and SiC reinforced... the effective pO2 of the glass that was the driving force for the reaction Long time exposure in air eventually oxidized the carbon and the SiC fibers resulting in deterioration of the composite Glass-ceramics being materials composed of several phases are considered composites by many investigators and by all definitions they should be These are materials that are formed as a glass and then either heated... corrosion at 1300°C for samples of SiC exposed to pure hydrogen The effects of weight loss and corrosion were noted at times as low as 50 hr After 500 hr at 110 0°C and 1300°C, the room temperature MOR decreased by one-third Carbon-Carbon Composites Carbon-carbon (i.e., carbon fiber reinforcement and carbon matrix) composites are probably the only materials that possess a combination of high strength/weight... among the remaining constituents and SiC His calculations indicated that the formation of Cr3C2, CrSi2, SiO2, and BaC2 was probable and that SiO formation was not Upon heat treatment of composite samples at 115 0°C and 1350°C, Mendelson found that Cr3C2, CrSi2, and SiO2 did indeed form and that barium diffused into the fibers at the higher temperature causing embrittlement and degraded strengths Mendelson... 1000°C and 110 0°C The oxidized layer contained cordierite along with mullite, zirconia, and alumina Copyright © 2004 by Marcel Dekker, Inc Corrosion of Composites Materials 307 at 1200°C and 1300°C At 1400°C, mullite and zircon were detected along with a viscous amorphous phase The silica oxidation product apparently reacted with the free alumina and zirconia present to form additional mullite and zircon... surface of the fibers inhibited the formation of Al4C3; however, it allowed improvements of only short duration Although there is not much on corrosion, the book by Taya and Arsenault [7.78] contains a lot of information about the properties and behavior of MMC Galvanic corrosion (see Chap 2, page 25 for a discussion on galvanic corrosion) between the matrix metal and the reinforcement fibers can occur,... (7.9) and (7.10) below] These reactions are all temperatureand oxygen partial pressure-dependent as discussed in Chaps 2 and 5 This causes embrittlement and loss of toughness (7.7) (7.8) (7.9) (7.10) The latest preference for SiC/SiC composite is one with fibers of improved microstructure and chemistry called Sylramic™* Copyright © 2004 by Marcel Dekker, Inc 312 Chapter 7 incorporated into a matrix of . 1976, 59 (1–2), 37–39. 6.80. Hench, L.L. Surface modification of bioactive glasses and ceramics. In Corrosion of Glass Ceramics and Ceramic Superconductors; Clark, D.E., Zoitos, B.K., Eds.; Noyes Publications:. intermixing of the different materials exists on a microscopic scale will be considered composites. The concept of composite materials is not a new idea and is definitely not limited to ceramics excellent examples of composite materials. Wood is a composite of cellulose fibers contained in a matrix of lignin. Bone, another example, is composed of the protein collagen and the mineral apatite.