Oxide Growth on Copper and Alpha‐Brasses in Aqueous Ammonia

Abstract

An ellipsometric study of the kinetics of oxide growth for pure copper and a series of alpha‐brasses immersed in tarnishing ammoniacal environments has established that the growth of the oxide, shown to be cuprous oxide, obeys a linear law in each case, and that the rate of growth increases significantly with (a) increasing zinc content of the solid, (b) increasing temperature, and (c) the application of anodic potentials. On the basis of these results it is suggested that the oxide is porous and that oxide growth involves the dissolution of metal ions at the base of the pore (anodic reaction) and their deposition at the oxide surface (cathodic reaction), electrons flowing through the oxide film. The influence of zinc content on the oxide‐growth rate is attributed to its effect on the dissolution kinetics.