Steels continued oxidation resistance

Metals vary greatly in their corrosion-resistance - chromium and titanium have good resistance, while steel readily corrodes. The oxide film formed on chromium and titanium closely adheres to the surface and protects the metal from further oxidation. In the case of steel, the oxide film in the form of rust is loose, allows moisture to be retained, and promotes further corrosion. If corrosion is allowed to continue, the steel will eventually be completely consumed, i.e. the metal will have returned to the condition of the ore from which it was extracted. 

Steel is aluminized (i.e., coated with aluminum) by hot dipping or spraying and, to a lesser extent, by cementation. Molten baths of aluminum for hot dipping usually contain dissolved silicon in order to retard formation of a brittle alloy layer. Hot-dipped coatings are used for oxidation resistance at moderately elevated temperatures, such as for oven construction and for automobile mufflers. They are unaffected by temperatures up to 480°C (900°F). At still higher temperatures, the coatings become refractory, but continue to be protective up to about 680 °C (1250 °F) [34]. 

The use of equipment close to the temperature at wliich the material was diffusion treated will result in continuing diffusion of chromium, aluminum etc., into the substrate, thus depleting chromium with consequent loss in oxidation and corrosion resistance. For aluminum, this effect is noticeable above 700°C in steels, and above 900°C in nickel alloys. For chromium, the effect is pronounced above 850°C for steels and above 950°C for nickel alloys. 

A dense and electronically insulating layer of LiA102 is not suitable for providing corrosion resistance to the cell current collectors because these components must remain electrically conductive. The typical materials used for this application are 316 stainless steel and chromium plated stainless steels. However, materials with better corrosion resistance are required for longterm operation of MCFCs. Research is continuing to understand the corrosion processes of chromium in molten carbonate salts under both fuel gas and oxidizing gas environments (23,25) and to identify improved alloys (29) for MCFCs. Stainless steels such as Type 310 and 446 have demonstrated better corrosion resistance than Type 316 in corrosion tests (29). 

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