Fundamentals of High Temperature Oxidation/Corrosion

Material degradation at high temperatures takes place due to loss in mechanical properties with increase in temperature as well as due to the chemical interaction of metal with the environment. This chemical interaction is further sub-divided into oxidation, sulfidation, and hot corrosion. While oxidation leads to the formation of oxide, which can be deleterious if the oxide is fast growing and spalls extensively, however, if the scale formed is adherent, thin and slow growing, it provides protection to the base metal or alloy. Sulfidation is a much severe degradation process and several times faster than the oxidation. In many industrial environments, it is a mixed gas environment, leading to oxidation and sulfidation simultaneously. Hot corrosion is another degradation mechanism which is even more severe than the oxidation and the sulfidation. Here, oxidation/sulfidation occurs in the presence of a molten salt on the surface of the substrate. Related issues, such as role of defect structure, active element effect and stress generation, during oxide growth process, have also been discussed. Finally, a guide to material selection for high temperature application is presented.