Complex ions and stress-corrosion cracking in α-brass

A study has been made of the stress—corrosion cracking, rate of weight loss and surface condition of an alloy containing 70% copper and 30% zinc in ∼ 15 N aqueous ammonia. The behaviour is shown to be strongly dependent on the concentration of the cupric complex ion Cu(NH3)5 2+ present in the environment. Stress—corrosion life decreased significantly with increasing complex-ion concentration, the relationship exhibiting a well-defined inflection at a critical concentration. Specimens immersed in solutions of concentrations exceeding the critical value were tarnished, while those exposed to less concentrated solutions appeared tarnish-free. These and other observations indicate that two mechanisms of stress-corrosion cracking are operative, one in the presence of tarnish and the other in the absence of this layer. Possible mechanisms are discussed. A model has been developed to explain the dependence of the tarnishing reaction on the chemical composition of the environment.