Chain model of oxide melt formed in transverse channels in oxide layer during catastrophic oxidation of copper

摘要

Due to the outstanding thermal conductivity of copper, it is widely used as a heat sink. In industrial plants, where it conies to elevated temperatures, copper is often in contact with the refractory metals and alloys, on the surface of which the oxide films always exist. In this case copper may experience compatibility issues with some of these oxides as a result of accelerated oxidative degradation in air. It is known that at elevated temperatures in air many metals are exposed to accelerated oxidation by direct contact with low-melting oxides (V_2O_5, McO_3, WO_3, etc.), which is called catastrophic oxidation of metals (COM). In the case of COM, the oxide layer, forming on the surface of the oxidizable metal, has high oxygen ion conductivity due to the formation of transverse liquid channels at high temperatures. These channels, being formed as a consequence of melting of eutectic, provide the rapid transport of oxygen ions. Because the catastrophic oxidation of copper obeys parabolic rate law, the whole process is only limited by chemical diffusion of oxygen. Since COM is an important industrial problem, it is necessary to reach an understanding of its mechanism, for example, through the development of an adequate model of oxygen ion transport in the oxide melts on the basis of extensive experimental results. The catastrophic oxidation kinetics of copper in contact with WO3, oxygen ion transport number, microstructure and phase composition of the oxide layer formed on copper during COM have been investigated by TG, DSC, GC, SEM and XRD. It has been established that copper begin to rapidly oxidize at 870 °C in contact with WO3, and the parabolic rate constant is more than 10 times higher than that of pure copper and equal to 7.2-10~(-5) kg~2m~(-4)s~(-1) (specific mass of WO_3 0.35 kg m~(-2)) at 1000 °C. Composites WO_3 -5, 10 % CuWO_4, that have been synthesized for simulation of real oxide layer, conducts oxygen ions and electrons at 900-1000 °C. In the formed oxide layer, the existence of the transverse liquid channels providing a rapid mass transfer has been revealed. Also, the study of interaction of copper with WO3 under reduced oxygen partial pressure (to simulate the conditions at the interface "oxide layer/copper") shows the possibility of direct redox reactions of copper with components of the oxide melt. As a result, a model of gap - association of chains in the oxide melt have been proposed, allowing to quantitatively evaluate the parabolic rate constant and, thereby, to predict the rate of destruction of material in industrial conditions.