The Effect of Cr on the Oxidation of Ni–10 at% Al in 1 atm O2 at 900–1000°C

The oxidation of three Ni–xCr–10Al alloys with a constant Al content of 10 at% and containing 3, 5, and 10 at% Cr was investigated at 900–1000°C in 1 atm of pure oxygen and compared to the behavior of Ni–10Al. At both temperatures, an external NiO scale overlying a zone of internal-oxide precipitates formed on Ni–10Al and Ni–3Cr–10Al: in addition, a discontinuous Al₂O₃ layer formed at the front of the internal oxidation for Ni–3Cr–10Al. An exclusive external scale of Al₂O₃ formed at most places on Ni–5Cr–10Al at 900°C, while, at some sites, the same alloy formed an outer NiO layer overlying an internal oxidation zone. The scales formed on Ni–5Cr–10Al at 1000°C were complex, but eventually a protective Al₂O₃ layer developed either at the alloy surface or beneath a region containing a mixture of different oxides. Finally, an exclusive external Al₂O₃ layer formed on Ni–10Cr–10Al at both temperatures. Thus, the addition of sufficient chromium to Ni–10Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum under a constant Al content. A possible mechanism for the effect of chromium on the oxidation of Ni–10Al is discussed 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.