Oxide scale formation, intermetallics

Thermodynamics determine when a potentially protective oxide scale is possible but kinetic issues ultimately determine whether or not a continuous layer will form. Without continuity, optimum protective oxide scale formation is not achieved. These kinetic issues include factors such as the rate of growth of other component oxides on the alloy, solubility and diffusivity of oxygen in the alloy, diffusivity of the alloy components, in particular Cr, Be, Al, or Si, and the form and distribution of Cr, Be, Al, or Si in the alloy. These issues will be discussed in detail for the various intermetallics in Sections 6.3-6.10. 

Vacancy injection, a phenomenon usually considered to be characteristic of cationic scale growth, was a rather popular topic after the mid-1990s. This phenomenon is again often invoked to explain the formation of pores and cavities within the metallic substrate beneath an oxide scale such as NiO or FeO and, more recently, AI2O3 for the oxidation of intermetallic aluminides. In that case, the metal-oxide interface is assumed to be unable to move freely to follow the recession of the metal lattice. Reaction [2.3] must then be replaced by the following reaction ... 

The areas concerning monolithic intermetallics which have been studied in recent years are (i) the formation of mctastable aluminas, and their transformation to stable a-alumina, (ii) the formation of interfacial voids and scale adherence and how these are influenced by reactive elements and sulfur, and (iii) accelerated oxidation at intermediate temperatures. Additionally the applications oriented areas of (iv) coatings, (v) oxidation of composites, and (vi) life predictions have received attention. 

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