In general, practical use has divided these environments according to the most common causes of failure of a component
in service. The most common failures are associated to the attack by a specific element,such as oxygen (which causes oxidation), carbon (carburization and metal dusting), sulfur (sulfidation), halogen (e.g., chlorination), and nitrogen
(nitridation). Other modes of failure, such as molten metal attack and hot corrosion, are less specific. Detailed descriptions of such modes of attack are given in Ref 1 and 2. The most common high-temperature degradation mode is oxidation, and the protection against oxidation, in general, is given by the formation of a chromium oxide scale. The presence of a small amount of aluminum or silicon in the alloy may improve the resistance against oxidation of a chrome forming alloy (Table1). Attack by other elements, such as chlorine and sulfur,depends strongly on the partial pressure of oxygen in the
environment (Ref 2).