Introduction

In addition to attack by reactive gases, alloys used in practical environments, particularly those involving the combustion products of fossil fuels, undergo an aggressive mode of attack associated with the formation of a salt deposit, usually a sulphate, on the metal or oxide surface. This deposit-induced accelerated oxidation is called hot corrosion. The severity of this type of attack, which can be catastrophic, has been shown to be sensitive to a number of variables including deposit composition, and amount, gas composition, temperature and temperature cycling, erosion, alloy composition, and alloy microstructure. A number of comprehensive reviews on hot-corrosion have been prepared. The purpose of this chapter is to introduce the reader to the mechanisms by which hot corrosion occurs. The examples used will be those associated with Na2SO4 deposits which are often encountered in practice. However, the effects of some other deposits will be briefly described at the end of this chapter.

Once a deposit has formed on an alloy surface the extent to which it affects the corrosion resistance of the alloy will depend on whether or not the deposit melts, how adherent it is and the extent to which it wets the surface, and the status of equilibrium conditions at the interfaces. A liquid deposit is generally necessary for severe hot corrosion to occur although some examples exist where dense, thick, solid deposits have, apparently, resulted in considerable corrosion.