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The oxidation in 10(5)Pa of pure oxygen of three Ni-WxCr-7Al alloys (x = 0,5,10 at.%) was studied at 1173K. A dense external scale of NiO overlying a zone of internal oxidation formed on Ni-7Al and Ni-5Cr-7Al. On the contrary, an external Al2O3 layer formed on Ni-10Cr-7Al. Thus, the addition of sufficient Cr levels to Ni-7Al produced a classical third-element effect, inducing the transition between the internal and external oxidation of Al. This effect is interpreted on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between the internal and external oxidation of the most reactive component in binary alloys.