Stress corrosion cracking of copper-base alloys

Abstract

This thesis describes the stress corrosion cracking of a series of aluminium bronze alloys and attempts to characterize the cracking in relation to the various mechanisms proposed. The theories of cracking applicable to the aluminium bronze/ammonia system include; the anodic dissolution, the film rupture and the embrittlement theories, which are described in the literature review. The copper aluminium alloy system was selected for investigation as it exhibits a martensite transformation, making the cracking of these alloys in ammonia solutions a usefulful comparison with the cracking of stainless steels in chloride solutions. The study used 'U-bend' specimens stressed in ammonia solution preconcentrated with copper, while the oxidation of the alloys under the same conditions was examined using unstressed specimens. Oxidation in this thesis is taken to mean an anodic dissolution process with or without the formation of a metallic oxide layer. The tests showed three binary alloys to be susceptible to intergranular and transgranular cracking, while two commercial alloys only cracked transgranularly and only in pH = 11 ammonia solution. Stress corrosion conditions gave rise to sharp, narrow cracks traversing all the phases in the alloy with matching features on opposing fracture surfaces, whereas oxidation gave rise to less distinct, more rounded features with a definite phase dependence. Further experiments showed the limited penetration and reactivity of a corrosive solution along narrow cracks. It is concluded that stress corrosion cracking and oxidation occur by different mechanisms and that the mechanism of stress corrosion cracking is unlikely to involve oxidation as a necessary step to cracking. The results of the present study support an embrittlement mechanism of stress corrosion cracking in aluminium bronze alloys.