High-temperature corrosion continued

The extent to which low alloy steels react to high temperature corrosive environments is the subject of this chapter. In view of the commercial importance of these steels, the published literature on this topic is extensive and is being continually enlarged. The reader is encouraged to refer to the many excellent papers and current issues of the journals, referenced at the end of the chapter, for more detailed and contemporary information on the topic. 

If an alloy is to have acceptable resistance against high-temperature corrosion, it must react with the environment to form a continuous and adherent slow-growing scale which has sufficient mechanical properties to withstand the effects of both growth and thermal stresses. As discussed in Chapter 3 of this volume, ideal protective scale growth obeys diffusion-controlled, parabolic kinetics, i.e. ... 

Once a corrosion product layer is formed, the corrosion process may continue through the diffusion of at least one of the reactants through the corrosion product layer. Let us consider, for example, the case of nickel exposed to air at high temperature. Corrosion can theoretically continue through the nickel oxide layer by means of diffusion in either direction, alone or by counter-current diffusion, as illustrated in Fig. 15.9. 

The simplest form of a solid corrosion product on a metal surface is a continuous homogeneous surface scale consisting of one phase. Such a situation is encountered, e.g., in the oxidation of pure nickel where only nickel oxide is formed. Therefore, in many textbook examples, this type of reaction has been used to illustrate the principles of high-temperature corrosion. In the present chapter, the initial stages of the oxidation process are not addressed as this has been part of another chapter in this book. 

Nickel—Copper. In the soHd state, nickel and copper form a continuous soHd solution. The nickel-rich, nickel—copper alloys are characterized by a good compromise of strength and ductihty and are resistant to corrosion and stress corrosion ia many environments, ia particular water and seawater, nonoxidizing acids, neutral and alkaline salts, and alkaUes. These alloys are weldable and are characterized by elevated and high temperature mechanical properties for certain appHcations. The copper content ia these alloys also easure improved thermal coaductivity for heat exchange. MONEL alloy 400 is a typical nickel-rich, nickel—copper alloy ia which the nickel content is ca 66 wt %. MONEL alloy K-500 is essentially alloy 400 with small additions of aluminum and titanium. Aging of alloy K-500 results in very fine y -precipitates and increased strength (see also Copper alloys). 

It is claimed that the cured materials may be used continuously in air up to 300°C and in oxygen-free environments to 400°C. The materials are of interest as heat- and corrosion-resistant coatings, for example in geothermal wells, high-temperature sodium and lithium batteries and high-temperature polymer- and metal-processing equipment. 

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