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The oxidation in 1 atm of pure oxygen of Ni-Cr-Al alloys with a constant aluminum content of 7 at.% and containing 5, 10 and 15 at.% Cr was studied at 900 and 1000 degrees C and compared to the behavior of the corresponding binary Ni Al alloy (Ni-7Al). A dense external scale of NiO overlying a zone of internal oxide precipitates formed on Ni-7Al and Ni-5Cr-7Al at both temperatures. Conversely, an external Al2O3 layer formed on Ni-10Cr-7Al at both temperatures and on Ni-15Cr-7Al at 900 degrees C, while the scales grown initially on Ni-15Cr-7Al at 1000 degrees C were more complex, but eventually developed an innermost protective alumina layer. Thus, the addition of sufficient chromium levels to Ni-7Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum. This effect is interpreted on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between internal and external oxidation of the most reactive component in binary alloys. (c) 2006 Elsevier Ltd. All rights reserved.