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 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.