The kinetics of the oxidation of aluminum—copper alloys in oxygen at high temperature

The oxidation behavior of high purity aluminum—copper alloys has been studied at temperatures from 475 to 575°C in dry oxygen at a pressure of 76 torr. The oxidation product was found to be duplex in nature consisting of both amorphous and crystalline γ-Al₂O₃. Roughly cylindrical crystals of γ-Al₂O₃ of constant thickness, at any given temperature and alloy content, grow into the metal from the amorphous oxide—metal interface by inward diffusion of oxygen through the overlying amorphous film. The amorphous oxide film, growing by outward diffusion of metal ions, forms between the crystals of γ-Al₂O₃ with accurately parabolic kinetics throughout the temperature and alloy composition ranges. Above the crystalline phase the amorphous oxide forms at a lower rate because of the additional resistance conferred to cation egress afforded by the crystalline oxide. In general, it was found that increasing copper content of the alloy in the range 0·1–4 per cent Cu caused a reduction in the crystal nucleation density and in the depth of intrusion of the crystals into the underlying alloy. Conversely, increasing copper content was also found to increase the rate of formation of amorphous γ-Al₂O₃ between the crystals of γ-Al₂O₃ and to increase the rate of radial growth of the crystals. These results are explained in terms of the defect nature of the oxidation products.