The criteria for the transitions between the various oxidation modes of binary solid-solution alloys forming immiscible oxides at high oxidant pressures

The possible high-temperature corrosion modes of binary solid-solution alloys forming two immiscible compounds by a single oxidant include (1) the exclusive growth of external scales of the most-noble component, which may or may not be associated with the internal oxidation of the most-reactive component, (2) the formation of composite external scales containing a mixture of the two compounds, or finally (3) the exclusive growth of the most-stable compound as an external scale. The conditions for the stability of each scale structure depend on a number of thermodynamic and kinetics parameters, whose effects are examined quantitatively in this paper. The conditions for the stability of the various structures and the criteria for the transitions among them are also examined. The maximum number of possible scale structures is four, but it can reduce to three and, in some cases, only to two. In particular, the internal oxidation of the most-reactive component may not occur if the stabilities of the two oxides are not sufficiently different from each other.