1.24 - High Temperature Corrosion of Alumina-forming Iron, Nickel and Cobalt-base Alloys

Abstract

This chapter discusses the oxidation behavior of alumina-forming alloys, particularly MCrAl and M(Al) where M is Fe, Ni, and to a lesser extent, Co. These alloys owe their high temperature oxidation resistance to the formation of a dense protective, external alumina scale. (Alumina-forming coatings and intermetallics are covered in a separate chapter.) Topics that are covered include the criteria for forming an alumina scale and the various stages of alumina formation in O 2 or air: the transient, steady-state, and breakaway oxidation (i.e., when the alloy can no longer form a protective scale). These sections include the formation of metastable cubic alumina phases, the growth mechanism, factors affecting scale adhesion including the addition of reactive (or rare-earth) elements and lifetime components. The last section covers the behavior of these materials in the presence of different oxidants, for example, H 2O, S, C, or N. The examples given include both commercial and model alumina-forming alloys and rely heavily on the oxidation database of the Corrosion Science and Technology Group in the Materials Science and Technology Division at Oak Ridge National Laboratory in Oak Ridge, TN.