Progress Toward Understanding the Stress Corrosion Problem

This paper describes current understanding of the propagation of transgranular stress corrosion cracking (TGSCC) in engineering alloys. In contrast to intergranular stress corrosion cracking (IGSCC), which proceeds by preferential anodic dissolution at the crack tip, TGSCC is considered to propagate by discontinuous brittle fracture (cleavage). First, the propagation process is described, outlining current knowledge of the crystallography of the cleavage surfaces and reviewing the evidence for the discontinuous nature of cracking. An evaluation follows of the models that are currently proposed to explain how interaction with the chemical environment induces brittle fracture in normally ductile alloys, focusing, in particular, on a new approach, the film-induced cleavage model. Finally, attention is directed to step formation between the parallel, but displaced, primary cleavage facets. It is suggested that step formation, which in face-centered cubic (fcc)alloys occurs by highly localized plastic shearing on {111} planes, represents a new approach to combatting this important SCC form.