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The nature of the third-element effect in the oxidation of Fe-xCr-3 at.% Al alloys in 1 atm O2 at 1000 °C
Authors:Y Niu  S Wang  ZG Zhang
Affiliation:a State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of sciences, Wencui Road 62, 110016 Shenyang, China
b Dept of Applied Chemistry, Shenyang Institute of Chemical Technology, 110142 Shenyang, China
c DICheP, Università di Genova, Fiera del Mare, Pad. D, 16129 Genova, Italy
Abstract:The oxidation of an Fe-Al alloy containing 3 at.% Al and of four ternary Fe-Cr-Al alloys with the same Al content plus 2, 3, 5 or 10 at.% Cr has been studied in 1 atm O2 at 1000 °C. Both Fe-3Al and Fe-2Cr-3Al formed external iron-rich scales associated with an internal oxidation of Al or of Cr+Al. The addition of 3 at.% Cr to Fe-3Al was able to stop the internal oxidation of Al only on a fraction of the alloy surface covered by scales containing mixtures of the oxides of the three alloy components, but not beneath the iron-rich oxide nodules which covered the remaining alloy surface. Fe-5Cr-3Al formed very irregular external scales where areas covered by a thin protective oxide layer alternated with others covered by thick scales containing mixtures of the oxides of the three alloy components, undergrown by a thin layer rich in Cr and Al, while internal oxidation was completely absent. Conversely, Fe-10Cr-3Al formed very thin, slowly-growing external Al2O3scales, providing an example of third-element effect (TEE). However, the TEE due to the Cr addition to Fe-3Al was not directly associated with a prevention of the internal oxidation of Al, but rather with the inhibition of the growth of external scales containing iron oxides. This behavior has been interpreted on the basis of a qualitative oxidation map for ternary Fe-Cr-Al alloys taking into account the existence of a complete solid solubility between Cr2O3 and Al2O3.
Keywords:A  Fe-Al alloys  C  Oxidation  C  Chromium effect
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