Manabu NOGUCHI
Hiroshi YAKUWA
Since a metal is obtained by applying external energy to a mineral, it is usually used in a thermodynamically unstable state. In the metal, a chemical reaction (corrosion) is spontaneously promoted because the metal tends to return to its mineral state (an oxide or sulfide). Accordingly, unless the metal is perfectly isolated from the atmosphere (an oxidizing agent), or unless its metal state is maintained by applying external energy, it is impossible to stop completely the chemical reaction (corrosion) which allows the metal to return to its mineral state. However, by selecting an appropriate combination of materials and environment, or by treating the metal appropriately, the corrosion rate can be reduced so that there is no problem in practical use. These are corrosion prevention technologies. Corrosion not only may shorten the life of equipment or devices but also may cause unexpected malfunction or destruction, which is accompanied by a shutdown and scattering of hazardous materials, resulting in serious economic loss and human damage. In addition, the loss due to corrosion and the cost of corrosion prevention measures in Japan are equivalent to about 1% of its GDP (about 500 trillion yen in 1997) (calculated by the Hoar method)1). Therefore, in addition to the soundness of equipment and devices, from the economic viewpoint, the development of corrosion prevention technologies is an important issue.
Corrosion is roughly classified into aqueous corrosion (or wet corrosion) and high temperature corrosion (or dry corrosion). Aqueous corrosion (wet corrosion) is associated with liquid water. Since this affects equipment using water, such as pumps that are our major products, various countermeasures have been taken. On the other hand, high temperature corrosion (dry corrosion) is related not to liquid water but to a substance such as high temperature gas or molten salt. This draws a lot of attention as a problem with high temperature equipment. If corrosion is associated with nuclear reactor water, boiler feedwater, etc., which is subject to high temperature but can be liquid water even at high pressure, it is not called high temperature corrosion, but is classified as aqueous corrosion. It seems that the frequency of high temperature corrosion is lower than that of aqueous corrosion. However, it should be considered that the corrosion is likely to occur in most of the equipment and devices which hold the key to resolving energy and resource issues and preventing global warming, including gas turbines, boilers, and plants related to metal refining.For this reason, if high temperature corrosion causes malfunctions in high temperature equipment, serious damage such as the shutdown of various plants and the shortage of energy may result. Furthermore, to solve global problems such as energy, environmental, and resource depletion problems, now more than ever it is urgent to reduce energy consumed by high temperature equipment and devices and to develop technologies for enhancing efficiency.To achieve these goals, materials always need to be improved to overcome the severe high-temperature corrosive environment. For continuous building of social infrastructure and economic development, it is important to understand high temperature corrosion and to take corrosion prevention measures.
We have developed technologies for preventing the high temperature corrosion that are used for devices such as waste disposal power plants and power recovery turbines for oil refining plants.Through six issues of this lecture, we show our research and development issues regarding high temperature corrosion of equipment related to environment and energy, describe the basic concepts for understanding high temperature corrosion and for taking corrosion prevention measures, and introduce typical hightemperature corrosion cases in our experience and our development of corrosion-resistant materials and corrosion prevention technologies. The first and second issues describe the basics of high temperature corrosion, and the third through sixth issues introduce high temperature corrosion in actual environments, and preventive measures.
1) Japan Society of Corrosion Engineering, Japan Association of Corrosion Control : in Study of the Cost of Corrosion in Japan, p2 (2001). Corrosion Cost Committee; “The Cost of Corrosion in Japan”, Zairyo-to-Kankyo, Vol. 50, p490 (2001).
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