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The oxidation of a ternary Cu-Ni-Co alloy containing about 20 at.% Ni and 75 at.% Co, balance Cu (Cu-20Ni-75Co), was studied at 600-800°C in 0.1MPa pure O2. The kinetic curves for the present alloy are irregular and deviate from the parabolic rate law and are composed of three quasi-parabolic stages. The oxidation rates decrease continuously with time more rapidly than required by the parabolic rate law. These indicate that the scales become more protective with the increment of time. The oxidation rates of the present alloy increase with the increment of temperature. Cu-20Ni-75Co alloy formed a dark external layer composed of Co3O4 followed by a slightly lighter internal layer of CoO. Obviously, adding 20 at% Ni to Cu-75Co alloy is able to decrease the critical Co content required to form the most stable oxide and finish a transformation from the mix internal oxidation to the formation of the exclusive external scales of Co oxides.

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