Criteria for the formation of protective Al 2O 3 scales on Fe–Al and Fe–Cr–Al alloys

Abstract

The conditions for the formation of external alumina scales on binary Fe–Al alloys and the nature of the third-element effect due to chromium additions have been investigated by studying the oxidation at 1000 °C in 1 atm O 2 of a binary Fe–10 at.% Al alloy (Fe–10Al) and of two ternary Fe–Cr–10 at.% Al alloys containing 5 and 10 at.% chromium (Fe–5Cr–10Al and Fe–10Cr–10Al, respectively). An Al-rich scale developed initially on Fe–10Al was subsequently replaced by a multi-layered scale containing mixtures of Fe and Al oxides plus a large number of Fe-rich oxide nodules: internal aluminum oxidation was essentially absent from this alloy. Addition of 5 at.% chromium to Fe–10Al did not suppress the formation of nodules, but they were eventually healed by the growth of an alumina layer at their base, resulting in a significant reduction of the oxidation rate. Finally, the alloy with 10 at.% Cr formed continuous external alumina scales without any Fe-rich nodule. Thus, the addition of sufficient amounts of chromium to Fe–10Al produces a third-element effect as expected. However, the process found in this alloy system does not involve a prevention of the internal oxidation of Al. Instead, it shows a transition from the growth of mixed Fe- and Al-rich external scales directly to an external Al 2O 3 scale formation. An interpretation of this kind of mechanism involving a third-element is presented along with a prediction of the critical Al contents required to produce the various possible scaling modes on binary Fe–Al alloys.