Critical Al content to form external-alumina scales on Cu–Al alloys

The oxidation of Cu–6.8Al (at.%) alloy has been studied at 800 and 900 °C in 1 × 105 Pa pure O2. The scales formed at 800 °C are composed of a thin outer CuO layer and an inner protective Al2O3 layer. On the contrary, at 900 °C different samples of the alloy present two kinds of different oxidation behavior: one is protective, very similar to that at 800 °C, while the other is intermediate between protective and non-protective, with formation of very thick scale on the partial surface, which is mainly composed of copper oxides. The different behavior presented on a single sample is probably caused by local inhomogeneities of the alloy. It is deduced that at 900 °C the critical Al content to form external-alumina scale on Cu–Al alloy is about 6.8 at.%.