Precyclic-loading-induced stress corrosion cracking of pipeline steels in a near-neutral-pH soil environment

Two pipeline steels of different grades were used to study the effect of cyclic loading prior to corrosion exposure upon the initiation of stress corrosion cracks in a near-neutral-pH soil environment. A cyclic loading scheme was constructed based on pressure fluctuation data collected on operating pipeline systems. The specimen surface was either mirror-polished or polished to a 600-grit finish. It was found that precyclic loading caused the formation of microcracks on the specimen surface during subsequent corrosion exposure under constant Load and under mild cathodic polarization. No such microcracks occurred at the open-circuit potential because of uniform corrosion. For specimens polished to a 600-grit finish, the microcracks were either inclined at 45degrees to the loading direction or aligned along the polishing scratches: for mirror-polished specimens. only microcracks inclined at 45degrees were observed. The formation of microcracks is attributed to the presence of Localized deformation, such as persistent slip bands. These microcracks established the site for crack initiation and facilitated the development of stress corrosion cracking during subsequent slow strain rate testing.