The Internal Oxidation of Ternary Alloys. V: The Transition from Internal to External Oxidation of the Most-Reactive Component under Low Oxidant Pressures

This paper examines the conditions for the transition from internal to external oxidation of the most-reactive component C of ternary A–B–C alloys by a single oxidant under gas-phase oxidant activities below the stability of the oxide of the two most-reactive components using Wagners criterion. For this, approximate relations between the solubility and diffusivity of oxygen and the composition of the binary A–B alloy matrix in the zone of internal oxidation, already developed previously, are used. The critical C content needed for the transition in ternary alloys is calculated as a function of the many parameters involved. At variance with the behavior of binary alloys, for ternary alloys this critical C content depends also on the ratio between the concentrations of A and B in the bulk alloy. The results calculated for ternary alloys are compared with those obtained for binary A–C and B–C alloys under the same values of all the relevant parameters. Finally, complete oxidation maps for ternary alloys under low oxidant pressures,including the condition for the stability of external scales of the C oxide, are also presented.