External and Internal Oxidation Properties of Ternary Fe-Si-Al, Ni-Cr-Al and Ni-Si-Al Alloys

Abstract

<p>The oxidation properties of binary and ternary Fe-Si-Al alloys containing 0-6 at% Si and 0-6 at% Al were investigated at 1073 K in 0.04-0.07 torr and 760 torr of oxygen. In combination with Si, the Al concentration required to form a protective Al₂O₃ scale on the ternary alloys is significantly lower than that for binary Fe-Al alloys. Theoretical and experimental results are consistent with the hypothesis that there is a synergistic effect due to the combined presence of Si and Al, resulting in improved oxidation resistance of the ternary Fe-Si-Al alloys. Si appears to limit the oxidation of Fe during the transient stage of oxidation and increase the activity and outward diffusion flux of Al to re oxide/alloy interface through ternary interactions during the subsequent stage of parabolic oxidation kinetics.</p> <p>A criterion has been established for the onset of internal oxidation beneath the external scale when oxidizing conditions favor formation of the oxide of the least noble metal in a dilute ternary alloy. This criterion was applied to the oxidation of ternary Fe-Si-Al and Ni-Cr-Al alloys. Theoretical predictions of minimum Al concentrations required to form a protective Al₂O₃ scale on these two alloys are consistent with the experimental observations.</p> <p>Attempts were made to give expressions describing the kinetics of internal oxidation of dilute ternary alloys during simultaneous internal oxidation of the two solutes. Good agreement was obtained between theoretically predicted and experimentally observed values of the depths of internal oxidation zones in Ni-1at%Si-4at%Al alloys at 1073 K in Ni/NiO packs or in 760 torr of oxygen.</p>