THE INTERNAL OXIDATION OF 2-PHASE BINARY-ALLOYS BENEATH AN EXTERNAL SCALE OF THE LESS-STABLE OXIDE

The internal oxidation of two-phase binary A-B alloys by a single oxid ant at high temperatures, under partial pressures sufficient to also f orm external scales of the less-stable oxide, is examined by means of quantitative models and compared with the corresponding behavior of si ngle-phase alloys. It is shown that, depending on various factors, par ticularly on the solubility and diffusivity of the most-reactive compo nent B in the most-noble component A, this process may or may not invo lve a diffusion process of the alloy components, leading to different scale morphologies. It is also concluded that even when the solubility and diffusivity of B in A are sufficiently high, so that the internal oxidation of the common type occurs, the restriction to the diffusion of B in the alloy due to its limited solubility affects the kinetics of internal oxidation, producing an inn ease of the rate of internal o xidation and of the critical concentration of B in the alloy required for the transition to the external oxidation of B with respect to sing le-phase alloys under the same values of all the relevant parameters. The lower the solubility of B in A, the larger these effects.