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The conditions for the transition between the coupled internal oxidation of two most-reactive components and the formation of external scales in the scaling of ternary alloys under oxidant pressures below the stability of the oxide of the most-noble component, denoted as a situation of intermediate oxidant pressures, are examined under a number of simplifying conditions which allow to develop an approximate analytical treatment. If the precipitation of the two oxides occurs at the same front of internal oxidation, the kinetics of internal oxidation as well as the critical B and C contents needed for the transition have a single solution under fixed conditions of all the parameters involved. Oil the contrary, in the presence of two different fronts, when the most-stable oxide forms at the innermost front, a whole range of possible solutions is predicted. In both cases, the critical-C content needed to avoid the simultaneous internal oxidation of B plus C is progressively reduced by the addition of B. This behavior provides the basis for a possible interpretation of the "third-element effect". However, the existence and the magnitude of this effect are complicated by the occurrence of other modes of oxidation for these systems. Thus, a general treatment of the third-element effect under intermediate oxidant pressures requires an exhaustive analysis of all the oxidation modes permitted for ternary alloys under these conditions.