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which can react with oxygen to form stable, protective, external oxide scales on the component surface, thereby inhibiting further high-temperature degradation. In many industrial processes, however, alloys are rarely exposed to pure oxygen under isothermal conditions. Environments are more variable and complex, and thermal and stress cycles or interrupted on-off loads may occur. Differential thermal contraction between the alloy and protective scale can induce high stresses in the system, sometimes resulting in failure of the scale. Also, the environments may contain oxidizing species in addition to oxygen. For example, many sources of energy and raw materials for chemical, metallurgical, and power-generation industries are hydrocarbon fuels—oil, coal, or gas—which contain impurities such as sulfur, chlorine, or nitrogen, depending on the particular supply. Several modes of metal degradation by the gaseous environment are thus possible, including oxidation, carburization, sulfidation, chloridation, and nitridation. In addition, molten salts can deposit on metallic components, giving significant hot salt corrosion problems. On combustion, coals tend to produce large volumes of ash that can contain sodium, potassium, sulfur, and vanadium. Low-melting-point compounds formed from these elements can rapidly destroy a protective oxide scale. The presence of water and acidic vapors, such as SO2 and SO3, introduces the risk of dewpoint corrosion if the temperature drops below some critical value. Chemical potential and temperature gradients can also considerably affect corrosion processes. Problems of oxidation and corrosion can be combated by developing, selecting, and designing materials resistant to the specific environment. Such materials have the necessary physical, mechanical, and corrosionresistance properties for some applications. For others, however, the requirements of high-temperature strength and formability

may not be compatible with corrosion resistance, and it becomes necessary to select a material based on mechanical property requirements and apply a corrosion-resistant coating for protection. Although considerable strides have been made in developing materials and coating systems for many applications, a major challenge for the future is the need to increase operating temperatures for improved efficiency. For instance, increasing the inlet temperature of a gas turbine from 900 to 1250cC can result in a 30% increase in the energy output of that turbine for a given fuel consumption. Turbine inlet temperatures have risen by 6 to 9°C per year over the past 30 years, mainly because of improvements in the mechanical properties of alloys and in their resistance to corrosion, particularly through the use of protective coatings. This trend is likely to continue, with speculation regarding inlet temperatures approaching 1800 to 2000°C. Hence, there is considerable interest in materials that can maintain strength to higher temperatures than can conventional alloys— e.g., intermetallics, ceramics, cermets, and comp