The kinetics of the oxidation of copper. Part I. -The initial oxidation of copper at low pressures

The oxidation of copper has been the subject of several investigations. It was first treated quantitatively by Pilling and Bedworth in their study of the oxidation of metals. They showed that metals, in so far as the kinetics of their oxidation is concerned, may be divided into two classes, according as the oxide occupies a greater or less volume than that of the metal from which it is formed. In the first case it is clear that the oxide will protect the remaining unoxidised metal from attack, while in the second, owing to the necessarily porous nature of the oxide no such protective action can arise. Copper belongs to the first group. Pilling and Bedworth pointed out that when the oxide protects the metal, as soon as a complete oxide layer is formed, the factor controlling the rate of oxidation is the rate of diffusion of oxygen through the oxide. If now it is assumed that the rate of diffusion is inversely proportional to the thickness of the oxide layer, then we may write dx / dt = k / x , (1) where x is the weight of oxide formed at time t and k a constant. Integrating we obtain x 2 = kt . This equation was found to describe the rate of oxidation of metals belonging to the first group accurately. Copper, however, provided an exception; for, although the increase of volume accompanying oxidation is in the ratio of 1 to 1•67, it was only at high temperatures that the oxidation process followed equation (2). At lower temperatures the rate of oxidation varied in a capricious manner with time. This was attributed to the cracking of the oxide film. More recently, Dunn has reinvestigated the low temperature oxidation of copper. He finds that between 200° and 300°C. the oxidation of commercial copper follows (2) over short ranges of time. The behaviour of copper activated by oxidation and reduction was, however, anomalous. The results could not be expressed by (2) even over the shortest ranges of time.