When stainless steels are exposed to hot gases, chemical reactions take place between the steels, or rather between their alloying elements, and elements or compounds in the gases. As a rule, these reactions will lead to the formation of gas metal interface layers of reaction products. These surface layers are more or less protective against further attack from the gas. In practice, all stainless steels rely on the formation of an oxide layer. All other reaction products either form a porous, less adherent, and hence non-protective scale or are liquid or even volatile and will flux off any existing scale or evaporate. Thus, one condition that must be fulfilled for good high temperature corrosion resistance is that the gas must be oxidizing, i.e. it must contain enough oxygen for the formation of a protective layer, consisting of oxides of one or several of the alloying elements.
There exist several types of high temperature corrosion (oxidation, sulphidation, carburisation and nitridation, furnace gases, etc) depending on the temperature’s level, the time the steel is exposed, the gases’ composition etc. When choosing material for high temperature applications, one must have an extensive knowledge of existing or expected service conditions, such as gas temperature and composition, and material temperature. After consultation with a reputable supplier, one may come to choose the most suitable type of stainless steel for each corresponding high temperature environment.
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