The influence of surface tarnish on the stress-corrosion of ?-brass

Abstract
The structure and mechanism of formation of the tarnished layer formed on αj-brass when it is immersed in a copper sulphate–ammonium sulphate–ammonium hydroxide solution of pH 7·2 has been studied. The mode of formation of cracks in this layer when the brass is deformed has been investigated and the various shapes of crack observed are explained in terms of the orientation of the slip vector relative to the surface. It is shown that these cracks localize further chemical reactions when the specimens are subsequently re-immersed in the solution and that, by a process of repeatedly and separately deforming the specimen in tension and re-immersing it in the solution, small, transgranular, crack-like penetrations can be propagated into the brass. The relation of these observations to the phenomenon of stress corrosion of α-brass is discussed, and the general importance of passive, embrittled, surface layers to the stress-corrosion of other systems is indicated.