The Role of Stress-Assisted Localized Corrosion in the Development of Short Fatigue Cracks

The initial development and growth of defects from 'engineered' surfaces, i.e. fine abraded, polished, shot peened etc., often dominates the resulting component lifetime, particularly for materials of high strength and limited ductility. When subject to the conjoint action of stress and environment this lifetime is impaired and reductions in fatigue strength are often observed resulting from a reduction in defect development time, often termed 'initiation', and enhancement in defect growth rate. A number of factors exist which influence the rate at which defects, such as pits/cracks, develop. Included in these are; physical and chemical material surface condition, the nature of the loading mode, test frequency and electrochemical micro-climate at the metal/solution interface. Based upon corrosion experiments conducted under cyclic and static stress, using low and high strength steels and stainless steels in chloride environments, the following events; surface film breakdown, pit development and growth, pit/crack transition and environment-assisted stage I and stage II crack growth have been observed. Included in these experiments is that of the Scanning Reference Electrode a technique adapted to evaluate stress-assisted localised corrosion, a process considered to be of primary importance during the early stages of stress corrosion and corrosion fatigue cracking; particularly for actively corroding systems.