Analysis of Pore Formation at Oxide–Alloy Interfaces—II: Theoretical Treatment of Vacancy Condensation for Immobile Interfaces

摘要

Void generation at scale–alloy interfaces during the oxidation of binary alloys as a result of matter transport through the scale and of diffusion of the metal components in the alloy is examined under a number of simplifying assumptions. In particular, it is assumed that the scale–alloy interface does not displace with time and that all the metal vacancies at this interface condense there to form voids, which amounts to calculating the maximum rate of interface void production. The analysis is developed both for the case of the parabolic rate law and for a general kinetics behavior, using the experimental information concerning the kinetics of weight gain. Application of this treatment to the oxidation of FeAl at 1000°C, taking into account the gradual transformation of the initial form of alumina into the stable alpha form, predicts the presence of a maximum in the overall void volume, in qualitative agreement with experimental observations. However, the overall volume of voids measured is much smaller than the values calculated under various conditions, suggesting that an effective mechanism of vacancy annihilation must be present at the scale–alloy interface.