Corrosion of Copper-Gold Alloys by Oxygen-Containing Solutions of Ammonia and Ammonium Salts

Abstract

Copper‐gold alloys, ranging in gold content from zero to 15 at. %, were exposed to stirred aqueous solutions of ammonia and ammonium salts under oxygen partial pressures of up to 6.8 at. Only copper dissolved under these conditions, a gold‐rich film being left behind on the surface of the alloy. As the gold content of the alloys was increased, the rate of solution of copper generally fell off and the rate law shifted from linear, usually to parabolic. Alloys containing 15 at. % gold corroded slowly in solutions containing ammonia only, but were passive in the presence of ammonium sulfate. The effect on the kinetics of solution composition, oxygen pressure, and temperature are described, and some of the factors which limit the rate of corrosion are discussed. It is concluded that, under certain conditions, copper oxides are formed which contribute to the film structure and to limiting the corrosion rate.