The nature of the third-element effect in the oxidation of Fe-xCr-3 at.% Al alloys in 1 atm O2 at 1000 °C

The oxidation of an Fe-Al alloy containing 3 at.% Al and of four tertiary Fe-Cr-Al alloys with the same At content plus 2, 3, 5 or 10 at.% Cr has been studied in 1 atm O-2 at 1000 degrees C. Both Fe-3Al and Fe-2Cr-3Al formed external iron-rich scales associated with an internal oxidation of At or of Cr+Al. The addition of 3 at.% Cr to Fe-3Al was able to stop the internal oxidation of Al only oil 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 it 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 At, while internal oxidation was completely absent. Conversely, Fe-10Cr-3Al formed very thin, slowly-growing external Al2O3 scales, providing an example of third-element effect (TEE). However, the TEE due to the Cr addition to Fe-3Al was not directly associated with it prevention of the internal oxidation of A], but rather with the inhibition of the growth of external scales containing iron oxides. This behavior has been interpreted oil the basis of a qualitative oxidation map for tertiary Fe-Cr-Al alloys taking into account the existence of a complete solid solubility between Cr2O3 and Al2O3. (C) 2007 Elsevier Ltd. All rights reserved.