The internal oxidation of ternary alloys. VIII: The transition from the internal to the external oxidation of the two most-reactive components under high-oxidant pressures

Ternary A-B-C alloys, where A is the most-noble and C the most-reactive component, exposed to oxygen pressures above the stability of the least stable oxide AO (high-oxidant pressures) may present two different types of internal oxidation, i.e. either a coupled internal oxidation of both B and C beneath an external AO scale or a single internal oxidation of C beneath an external scale of the oxide of B. This paper examines the conditions required to avoid the coupled internal oxidation of B plus C beneath external AO scales, considering both the case of formation of a single and a double front of internal oxidation. The analysis, based on an extension to ternary alloys of the criterion defined by Wagner for the transition between the internal and external oxidation of the most-reactive component of binary alloys, shows that the addition of B to binary A-C alloys is very effective in reducing the C content needed for this transition in comparison with binary A-C alloys. This results provides a basis for a possible explanation of the third-element effect. However, the actual possibility of its occurrence depends also on the ability to avoid other oxidation modes, not examined here.