Corrosion of Ceramics

Ceramics are favorite materials for applications under severe conditions [1]. 

The reason for their extreme resistance can be explained as follows. Ceramics are compounds between metallic and non-metallic elements. Corrosion products are also ceramics. Hence, ceramic may be thought of as having already been corroded [2]. Contrary to the corrosion of metals, corrosion of ceramics, if at all they corrode, involves chemical dissolution. Metals corrode by electrochemical processes. Another form of corrosion is oxidation. Oxidation takes place in an oxidizing environment. In this respect, the oxidation resistance of materials is to be considered. When we consider ceramics for application in an oxidizing environment, the nonoxide ceramics may not be that resistant. Members of the ceramic spectrum of borides, carbides, nitrides, silicides, and so on tend to get oxidized when exposed to an oxidizing atmosphere. Herein, oxide ceramics are the most stable. In general, ceramics are more stable than metals in terms of their oxidation. 

The excellent corrosion resistance of ceramics can be gauged from the following. Household cutlery, pottery, century-old vases, and so on are made of ceramics. Corrosion resistance arises due to their chemical stability and the high covalent bonding. Often hydrofluoric acid (HF), one of the strongest chemicals, is required to etch the microstructure of engineering ceramics. 

Ceramics are generally electrically insulating. They have few charge carriers. Hence, electrochemical corrosion is negligible. They corrode mainly by acid-base type of reactions. Ceramics undergoes dissolution in acids and bases. The same composition can exist as crystalline or amorphous ceramics. There is a difference in corrosion behavior between the two forms. Also, there are glass-ceramics, in which both the phases coexist. 

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McCauley, R. A., Corrosion of Ceramics, Marcel Dekker, New York, 1995. 

Opila EJ, Jacobson NS (2000) Corrosion of ceramics. In Schiitze M (ed) Corrosion and Environmental Degradation. Wiley-VCH, Weinheim, p 329... 

The Use of Phase Diagrams to Predict Alkali Oxide Corrosion of Ceramics... 

Generally alkali corrosion of ceramics involves the salts or oxides of sodium and/or potassium. These alkalis are corrosive in the solid, liquid,... 

Greenberg, S. R. Poeppel, R. et al., "The Corrosion of Ceramic Refractories Exposed to Synthetic Coal Slags By Means of the Rotating-Cylinder Technique An Interim Report", U. S. Department of Energy ANL/FE-83-31, (September, 1984). Streeter, R. C. Diehl, E. K. Schobert, H. H. Preprints Am. Chem. Soc. Div. Fuel Chem. 1983, 28(4), 174. 

Ceramics are generally considered to be inert materials that do not undergo corrosion. In fact, however, corrosion of ceramics is generally an important cost factor in metals production and in most other technologies that use them. While the corrosion of metals is an oxidative process, the corrosion of ceramics can be oxidative, reductive, or not involve any electron transfer and still be controlled by the electrochemical nature of the material and environment. 

The aqueous corrosion of ceramics may involve a charge-transfer or electrochemical dissolution process. However, in many cases, dissolution or corrosion may take place with no charge transfer yet may be determined by one or more electrochemical factors such as absorbed surface charge or electronic band bending at the surface of narrow-band-gap semiconducting ceramics. The aqueous corrosion of ceramics is important in a number of areas. One of the most important is the stability of passive oxide films on metals. The stability of ceramics is a critical aspect in some aqueous photoelectrochemical applications (12), an example being the photoelectrolytic decomposition of water. Structural, nonoxide ceramics such as SiC or Si3N4 are unstable in both aqueous acid and alkaline environments the latter is virtually unstudied, however. 

As ceramics receive wider application in electrochemical systems and increased use as high-technology structural or electronic materials, their corrosion behavior will become important and possibly design-limiting. At present, mechanisms controlling the corrosion of ceramics are not well understood, and the available data base is extremely small. 

Blachere R, Pettit FS (1989) High temperature corrosion of ceramics. Noyes, Park Ridge, NJ Brandmp J, Immergut EH, Gmlke EA, Abe A (eds) (1999) Polymer handbook, 4th edn. Wiley, New York... 

For the corrosion of ceramics by ordinary liquid media, the testing is usually done by immersion tests the sample is placed in a heated retort or autoclave inside a stirred excess of corrosive medium. Common test conditions are at the boiling point of the medium (e.g. 10% H2SO4, 30% NaOH) for a week. 

J. R. Blachere and F. S. Pettit, High Temperature Corrosion of Ceramics, Noyes Data Corporation, Park Ridge, NJ, 1989, pp. 188. 

N. S. Jacobson, E. J. Opila, and K. N. Lee, Oxidation and corrosion of ceramics and ceramic matrix conqjosites, Curr. Opin. Solid State Mater. Sci., 5 301-309 (2001). 

Table 6.6. Corrosion of Ceramics in Combustion Reactors under Cyclic Exposure to H2SO4 and HCI Vapors and NO, at Temperatures up to SOO C for 900 h... 

V.A. Izhevskyi, L.A. Genova, A.H.A. Bressiani, and J.C. Bressiani, Liquid Phase Sintered SiC. Processing and Transformation Controlled Microstructure Tailoring, MatRes., 3(4), 131-38 (2000). V.A. Lavrenko, D.J. Baxter, A.D. Panasyuk, and M. Desmanion-Brut, High-Temperature Corrosion of AIN-Based Composite Ceramic in Air and in Combustion Products of Commercial Fuel. 1. Corrosion of Ceramic Composites in the AIN-SiC System in Air and in Combustion Products of Kerosene and Diesel Fuel, Powder Metallurgy and Meta Ceramics, 43(3-4), 179-86 (2004). 

Ronald A., Corrosion of Ceramic and Composite Materials McCauley CRC press, 2004. 

As with metals the corrosion of ceramics can take place by one or a combination of mechanisms. In general, a corrodent will attack a ceramic and form a corrosion product. Whether the reaction product is a gas or a solid will determine if the product remains on the surface or is fugitive. Reaction products may be gas, liquid, solid, or any combination thereof. If the reaction product formed is a solid it may form a protective layer against additional corrosion. When the reaction product is a combination of a solid and a liquid, the reaction layer may be removed. 

The fundamental concepts of chemistry can assist in the understanding of the corrosion of ceramics. A ceramic having a basic character tends to be attacked by an environment with an acidic character, whereas a ceramic having an acidic character tends to be attacked by an environment basic in character. Also, ionic materials tend to be soluble in polar solvents whereas covalent materials tend to be soluble in nonpolar solvents, and the vapor pressure of covalent materials is generally greater than that of ionic materials and therefore covalents tend to vaporize or sublime more quickly. 

The general principles of galvanic corrosion of metals apply to the galvanic corrosion of ceramics. Basic requirements for galvanic corrosion to proceed are as follows ... 

As can be seen from the foregoing, ceramics are susceptible to the same general forms of corrosion as metals, although in some cases the mechanisms may be different. Atmospheric corrosion of ceramics may take place through any or all of these forms, depending on the specific ceramic and the contents of the atmosphere. 

The same factors that promote atmospheric corrosion of metals, time of wetness, atmospheric types, initial exposure conditions, sheltering, wind velocity, and the nature of corrosion products also affect the atmospheric corrosion of ceramics. 

RA McCauley. Corrosion of Ceramics. New York Marcel Dekker, 1995. 

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