Alumina Scale Formation: A New Perspective

Oxidation of Al 2 O 3 scale‐forming alloys is of immense technological significance and has been a subject of much scientific inquiry for decades. The oxidation reaction is remarkably complex, involving issues of alloy composition, kinetics, thermodynamics, microstructure, mechanics and mechanical properties, crystallography, etc. A brief overview of the formation of passivating, thermally grown oxide Al 2 O 3 scales will be given in the light of recent findings on the defect structure and associated transport behavior of α‐Al 2 O 3 . It is inferred that the electronic structure of Al 2 O 3 is of direct relevance to understand the Al 2 O 3 scale growth. We also discuss the effect of the so‐called “reactive” elements (REs)—Y, Zr, and Hf—on reducing the rate of Al 2 O 3 scale thickening by reducing the outward flux of aluminum. An important aspect of the “new perspective” is the suggestion that the REs change the electronic structure of Al 2 O 3 —the relevant near‐band‐edge defect (grain boundary) states that are crucial to vacancy creation both at the scale/gas and scale/metal interfaces.