The Structure and Composition of Thick Tarnish Films on Alpha-Phase Copper Alloys

Abstract

Electron microprobe analysis, X-ray diffraction, and metallographic studies have been carried out on thick tarnish films formed on a series of alpha-phase Cu-Zn, Cu-AI, and Cu-Ni alloys by exposure to an ammoniacal solution. The results established that the tarnish films consist of an inner layer of Cu₂O and an outer layer of CuO, the latter being formed after long exposures. The microprobe data indicated that both oxides are essentially depleted with respect to the alloying elements, and appear to contain less copper than the stoichiometric values. These observations are consistent with the view that the Cu₂O is formed by a dissolution-reprecipitation mechanism; however, the mechanism of CuO formation is not yet understood.