Oxidation of copper-manganese alloys under pure oxygen

Abstract

Oxidation, in oxygen gas at atmospheric pressure, of copper-manganese alloys (Mn content less than 40 at.%) has been investigated between 600 and 850° C. The reaction kinetics, determined by thermogravimetry, follow a parabolic law for alloys having a low manganese content (less than 10 at.% Mn) but are more complex for higher concentrations, particularly in the first stages of the oxidation process. Whereas in the early stages of oxidation the kinetics are controlled by surface reactions which accompany the formation of the different oxide layers, they are later controlled by the diffusion of a mobile species when the parabolic law is followed. In this condition an apparent activation energy may be determined from the rate constants. These energies are of the order of 120–140 kJ mol⁻¹, comparable with that for oxidation of pure copper (134 kj mol⁻¹), indicating a similar oxidation mechanism.

The oxide layers formed were identified by cross-checking results of X-ray diffraction, electron microprobe analysis, and from glow discharge spectrometry. External layers of CuO and Cu₂O formed on alloys of lower manganese concentration, evolving towards one or several mixed copper-manganese oxide layers with increasing manganese content. Under the external layers, which were weakly adherent to the sample, an internal-oxidation layer formed, which was adherent and consisted of precipitates of Mn₃O₄/MnO dispersed in the copper lattice. For alloys richer in manganese (36 at. % Mn) and at temperatures above 850°C (20 at.% Mn), the internal-oxidation layer evolved into two zones: MnO particles beneath a zone of Mn₃U₄ particles.