The oxidation of three Ni–6Si–xAl alloys in 1 atm O2 at 1000 °C

The oxidation of three ternary Ni–6Si–xAl alloys containing 6, 10 and 15 at.% Al and of the corresponding binary Ni–Al alloys has been studied at 1000 °C under 1 atm O2 to examine the effect of different Al additions on the behavior of ternary Ni–Al–Si alloys containing 6 at.% Si. Of the three binary Ni–Al alloys only Ni–15Al was able to form external alumina scales. Conversely, all the three ternary alloys formed an innermost layer of alumina directly in contact with the alloy following very similar and approximately parabolic kinetics after a short faster initial stage due to transient formation of NiO. Thus, the presence of silicon is very effective to reduce the critical Al content needed to form exclusive alumina scales with respect to binary Ni–Al alloys. The third-element effect due to silicon is interpreted on the basis of an extension of Wagner’s criterion for the transition from the internal to the external oxidation of the most reactive component in binary alloys.