A unified mechanism of stress corrosion and corrosion fatigue cracking

Abstract

A mechanism of stress corrosion cracking (SCC) is outlined in which anodic dissolution at film rupture sites relieves strain hardening and reduces the fracture stress at the crack tip. Experimental evidence is cited to suggest that relief of strain hardening occurs by interaction of subsurface dislocations with divacancies generated by the anodic dissolution. A transgranular crack propagates by accumulation of divacancies on prismatic planes which then separate by cleavage under plane strain conditions at the crack tip. At appropriate metallurgical and chemical conditions, anodic dissolution and/or divacancy migration may be enhanced at grain boundaries, leading to an intergranular failure mode. Evidence is also available to indicate that cyclic loading relieves strain hardening. Relief of strain hardening by combined cyclic loading and corrosion accounts for the higher incidence of corrosion fatigue cracking (CFC) without the requirement of any critical dissolved species. Data on fatigue of stainless steel at elevated temperature in both vacuum and air provide additional support for the proposed mechanism.