High temperature scaling of two-phase Fe–Cu alloys under low oxygen pressures

The oxidation of binary two-phase Fe-Cu alloys containing 25, 50 and 75 wt% Cu and composed of mixtures of the two terminal solid solutions of copper in iron and iron in copper has been studied at 800-900 degrees C under oxygen pressures below the stability of copper oxides. The oxidation followed the parabolic law only in some cases, while in others it was rather irregular. The scaling rates of the alloys were lower than for pure iron and decreased with an increase in the copper content under constant temperature. The scales were always composed of an inner layer containing a mixture of copper metal and iron oxide and of an outer layer of iron oxide with very little dissolved copper. The interface between the two layers was generally regular and corresponded approximately to the location of the original alloys surface, while the distribution of the two phases in the mixed region followed closely that of the two metal phases in the original alloys. No significant iron depletion was observed in the alloys beneath the region of internal oxidation. The peculiar scale microstructures observed for these alloys are mainly a result of the limited mutual solubility of the two components and of their special microstructures involving the presence of two phases of widely different composition. (C) 1999 Elsevier Science Ltd. All rights reserved.