Criteria for the formation of protective Al2O3 scales on Fe–Al and Fe–Cr–Al alloys

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 O2 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 Al2O3 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.