Practical High-Temperature Corrosion Problems

The oxidation rate laws described above are simple models derived from the behavior of pure metals. In contrast, practical hi -temperature corrosion problems are much more complex and involve the use of alloys. For practical problems, both the corrosive environment and the high- 

Oxidation is generally described as the most commonly encountered form of high-temperature corrosion. However, the oxidation process itself is not always detrimental. In fact, most corrosion and heat-resistant alloys rely on the formation of an oxide film to provide corrosion resistance. Chromium oxide (Cr203, chromia) is the most common of such films. In many industrial corrosion problems, oxidation does not occur in isolation rather a combination of high-temperature corrosion 

Apart from chromium, alloying additions used to enhance oxidation resistance include aluminum, silicon, nickel, and some of the rare earth metals. For oxidation resistance above 1200°C, alloys that rely on protective AI2O3 (alumina) scale formation are to be preferred over those forming chromia. Increasing the nickel content of the austenitic stainless steels up to about 30%, can have a strong beneficial synergistic effect with chromium. 

Fundamental metallurgical considerations impose limits on the amount of alloying additions that can be made in the design of engineering alloys. Apart from oxidation resistance, the mechanical prop- 

TABLE 3.6 Common Names and UNS Alloy Number of Alloys Used in High-Temperature Applications (Compositions Given in App. E) 

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High-temperature corrosion is a practical problem in most applications of metals and alloys at elevated temperatures in corrosive environments. In power plants, chemical and petrochemical process industries, for aircraft engines, heat treatment and other metallurgical processes, and new technologies such as waste-incineration plants, high-temperature fuel cells, and so on, the metallic materials must he carefully selected, to allow sufficient lifetime and avoid premature failure. Sometimes processes are not possible since the materials would not withstand the process conditions that must he adapted to the available materials. 

The physical properties of lithium metal were given in Table 4.4. Despite its obvious attractions as an electrode material, there are severe practical problems associated with its use in liquid form at high temperatures. These are mainly related to the corrosion of supporting materials and containers, pressure build-up and the consequent safety implications. Such difficulties were experienced in the early development of lithium high temperature cells and led to the replacement of pure lithium by lithium alloys, which despite their lower thermodynamic potential remained solid at the temperature of operation and were thus much easier to use. 

For similar reasons, other oxides with low melting points, in particular PbO and M0O3, reduce the corrosion resistance of alloys in high-temperature service. Generally, these oxides pose fewer practical problems than vanadium, however. 

Two different technological approaches are reflected by this classification. The main problem of low-temperature cells consists in finding efficient electrocatalysts so that the rates of the electrochemical reactions are still satisfactory at low polarization. Since the reaction rates increase with temperature, the role of the electrocatalyst is not so critical in the high-temperature cells. Other problems like corrosion and conductivity of the electrolyte become pertinent for the construction of a practical unit. 

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