Criteria for the formation of protective Al2O3 scales on Fe-Al and Fe-Cr-Al alloys |
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Authors: | ZG Zhang PY Hou |
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Affiliation: | a State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Wencui Rd 62, 110016 Shenyang, China b DICheP, Università di Genova, Fiera del Mare, Pad. D, 16129 Genova, Italy c Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA |
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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 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. |
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Keywords: | FeCrAl Ternary alloys Oxidation Cr effect |
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