Mechanisms of Exclusive Scale Formation in the High Temperature Oxidation of Alloys

The high temperature oxidation of alloys is most often considered within the continuum framework developed by C. Wagner. We argue that in order to make progress in understanding exclusive scale formation, one needs to examine the atomic-scale kinetic processes that today are amenable to a variety of experimental, computational and theoretical approaches. In our discussion, we suggest that there is an analogy between the kinetic processes required for exclusive scale formation and thin film layer-by-layer growth. In both cases, the magnitude of the Ehrlich-Schwoebel barrier sets an effective length scale for growth, that if too large, results in three-dimensional rather than lateral island or oxide growth. In this event, exclusive scale formation can only occur if there is a sufficiently large density of oxide nucleation events at the surface of the alloy. We suggest that a suitably identified, dilute alloy component, could serve as a "surfactant"that segregates to the surface and is capable of enhancing the nucleation density.