A study of the initial oxidation of copper in 0.1 MPa oxygen and the effect of purity by metallographic methods

To clarify the initial oxidation mechanism of copper, the oxidation was carried out at 400 °C in 0.1 MPa oxygen using 99.9999% (6 N) and 99.5% (2 N) pure specimens. Oxidation of 6 N copper after 60 s showed that the number density of the oxide nuclei varied with the face of copper crystals, while the nucleation occurred preferentially at the grain boundaries. A metallographic examination indicated that the products of initial oxidation consist of both CuO and Cu₂O. CuO is firstly formed as a thin uniform film on the copper surface, and then Cu₂O nucleates and grows beneath the CuO film. This result is different from the conclusion reached in the literature that CuO does not appear until the laterally growing Cu₂O nuclei have covered the whole surface using other methods. In contrast to 6 N copper, nucleation of Cu₂O was much delayed for 2 N copper, though a thin CuO film was similarly formed on 2 N copper surface. Impurities in 2 N copper should be responsible for slow nucleation of Cu₂O and slow growth of nuclei.