Influence of oxide grain morphology on formation of the CuO scale during oxidation of copper at 600-1000°C

To explain the very slow growth rate of the CuO scale during copper oxidation at high temperatures above 850°C as reported in the literature, the influence of the morphology of the CuO grains on scale growth was investigated by oxidizing copper with different purities in 0.1 MPa oxygen atmosphere at 600-1000°C. Oxidation of 99.99% copper shows that the growth of the CuO scale depends on the morphology of the CuO grains. The very thin CuO scale observed at 1000°C is related to flat CuO grains resulted from fast lateral growth of the CuO grains, whereas the thicker CuO scale at lower temperatures results from the fine CuO grains. This dependence of scale growth on oxide grain morphology was confirmed by oxidizing 99.9999% and floating zone refined (>99.9999%) copper at 800°C. It reveals that grain boundary diffusion can favour the growth of the CuO scale, and its contribution is related to the morphology of the CuO grains depending on the purity of copper specimens.