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 th e most-reactive component, (2) the formation of composite external sca les containing a mixture of the two compounds, or finally (3) the excl usive growth of the most-stable compound as an external scale. The con ditions for the stability of each scale structure depend on a number o f thermodynamic and kinetics parameters, whose effects are examined qu antitatively in this paper. The conditions for the stability of the va rious structures and the criteria for the transitions among them are a lso examined. The maximum number of possible scale structures is four, but it can reduce to three and, in some cases, only to two. In partic ular, the internal oxidation of the most-reactive component may not oc cur if the stabilities of the two oxides are not sufficiently differen t from each other.