An Improved Treatment of the Kinetics of Coupled Internal Oxidation by a Single Oxidant of the Two Most-Reactive Components of Ternary Alloys and of the Conditions for the Transition to Their External Oxidation

An improved treatment of the kinetics of coupled internal oxidation of the two most-reactive components of ternary alloys by a single oxidant, involving the presence of two different fronts of internal oxidation, as well as of the critical contents of these two components needed to avoid this oxidation mode, is developed both for the case of the absence and of the presence of external scales of the oxide of the most-noble component. These analyses are compared with the approximate solutions presented earlier based on the assumption of a simultaneous precipitation of the two oxides at the same front of internal oxidation. The most-important parameter to establish the magnitude of deviation of the double-front solution from that calculated under the assumption of a single front is the solubility product of the oxide of intermediate thermodynamic stability. Only when this parameter is relatively large the kinetics for the double-front solution differ significantly from the simpler case of single front. In the presence of external scales another important factor is the magnitude of the respective parabolic rate constant. An increase of this constant tends to reduce the differences between the single and double-front solutions under the same values of all the remaining relevant parameters. The same approach is adopted to calculate the critical contents of the two most reactive components required to avoid this form of internal oxidation. In this case, the assumption of the existence of a single front of internal oxidation represents an even better approximation than for the calculation of the kinetic parameters of internal oxidation.