Corrosion high-temperature alloys

Ferritic stainless steels depend on chromium for high temperature corrosion resistance. A Cr202 scale may form on an alloy above 600°C when the chromium content is ca 13 wt % (36,37). This scale has excellent protective properties and occurs iu the form of a very thin layer containing up to 2 wt % iron. At chromium contents above 19 wt % the metal loss owiag to oxidation at 950°C is quite small. Such alloys also are quite resistant to attack by water vapor at 600°C (38). Isothermal oxidation resistance for some ferritic stainless steels has been reported after 10,000 h at 815°C (39). Grades 410 and 430, with 11.5—13.5 wt % Cr and 14—18 wt % Cr, respectively, behaved significandy better than type 409 which has a chromium content of 11 wt %. 

Tips of platinum, platinum—nickel alloy, or iridium can be resistance-welded to spark-plug electrodes for improved reHabiHty and increased lifetime. These electrodes are exposed to extremely hostile environments involving spark erosion, high temperature corrosion, thermal shock, and thermal fatigue. 

The most common form of corrosion is uniform corrosion, in which the entire metal surface degrades at a near uniform rate (1 3). Often the surface is covered by the corrosion products. The msting of iron (qv) in a humid atmosphere or the tarnishing of copper (qv) or silver alloys in sulfur-containing environments are examples (see also SiLVERAND SILVER ALLOYS). High temperature, or dry, oxidation, is also usually uniform in character. Uniform corrosion, the most visible form of corrosion, is the least insidious because the weight lost by metal dissolution can be monitored and predicted. 

Corrosion Resistance Possibly of greater importance than physical and mechanical properties is the ability of an alloy s chemical composition to resist the corrosive action of various hot environments. The forms of high-temperature corrosion which have received the greatest attention are oxidation and scaling. 

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. 

Mrowec et examined the resistance to high-temperature corrosion of Fe alloys with Cr contents between 0.35 and 74 at% Cr in 101 kPa S vapour. They found that the corrosion was parabolic, irrespective of the temperature or alloy composition, and noted that sulphidation takes place at a rate five orders of magnitude greater than oxidation at equivalent temperatures. At less than 2% Cr, the alloys formed Fe, j.,S growing by outward diffusion of Fe ions, with traces of FeCr2S4 near the metal core. 

An excellent reference book for the high-temperature corrosion resistance of materials of construction is George Y. Lai, High-Temperature Corrosion of Engineering Alloys, ASM International, Metals Park, Ohio, 1990. 

T. A. Ramanarayanan and C. M. Chun, Chapter 6 Metal Dusting Corrosion of Metals and Alloys, New Development in High Temperature Corrosion and Protection of Materials, Ed. W. Gao and Z. Li, Woodhead Publishing Ltd., Cambridge, UK p.80-116 (2008). 

H.J. Grabke. Surface and interface reactions and diffusion during the high-temperature corrosion of matals and alloys // Defect Diffusion Forum.- 2001.- V.194-199.-P.1649-1660. 

Chromium-containing cobalt alloys have been developed for use requiring wear resistance and high temperature corrosion resistance. The nominal composition of some wear-resistant cobalt alloys is given in Table 4.49. 

Internal or subsurface attack (oxidation). High-temperature corrosion can be identified by simple visual observation of the surface. However, subsurface phenomena within the matrix of the alloy, as well as obscured relations at the interface of the alloy with the surface films formed in many high temperature exposures can be seen in Figure 6.24. Electrochemical corrosion at high temperature at the interface also involves the diffusion of the aggressive gas phase to the vulnerable phase in the subsurface, leading to corrosion most of the time. 

High-temperature corrosion and wear is encountered in various industries such as waste incineration, fossil energy, pulp and paper, petroleum refining, chemical and petrochemical, mining and smelting operations. One of the methods to combat corrosion and wear and its control is to select suitable material, i.e., an alloy, for the plant design and maintenance. The selection of proper material for plant design and fabrication is followed... 

The development of CVD films for high-temperature corrosion protection of ferrous alloy has focused on methods with and without addition of oxygen-active elements. CVD processes, which employ vapor-transport and heat treatment of a stabilized alloy substrate to... 

The objective of surface modification by CVD is to develop additional corrosion protection for high-temperature alloys (e.g., Fe-25Cr-0.3Y) beyond that achieved by reactive-element additions which will be effective for a long period. Because CVD is a high temperature process, the following topics must be discussed before we can develop successful high-temperature corrosion-proteetion coatings. ... 

A durable protective coating for high-temperature alloys can be achieved by CVD. Normally, we must consider alloy stabilization in addition to chemical reaction in a controlled environment. The results define the nature of coatings for high-temperature corrosion protection, namely, a thin (1-2 pm) diffused silicon layer that covers the surface and penetrates even the smallest defects, cracks, etc., on the alloy to be protected. This surface modification treatment by CVD can be adapted to other alloys and is technologically simple and relatively inexpensive. 

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