Ammoniacal Stress Corrosion Cracking of Alpha-Brass

Stress corrosion cracking of Cu-29.38 zinc in Mattsson's ammoniacal solution has been studied over the pH range 5.9 to 8.0. Electron diffraction studies conducted on the actual stress corrosion fracture surface established the presence of Cu₂O in all cases. Continuous loading tests on single crystals immersed in the environment established that neither a surface film of Cu₂O nor the environment have any effect on surface dislocation behavior. The results are consistent with Cu₂O films being very weak (almost powdery) in nature and probably forming by a precipitation process. Continuous loading tests conducted on polycrystalline specimens immersed in the environment yielded a relationship between grain size and stress to initiate fracture which was consistent with the observed effect of grain size upon flow stress behavior. It was not necessary to invoke surface energy considerations to account for the effect of grain size. The results are interpreted in terms of a modified form of the tarnish rupture model. Furthermore, this model is shown to be reasonably consistent with published kinetic data and applicable to other environments.