Pipeline Circumferential Cracking in Near-Neutral pH Environment Under the Influence of Residual Stress

Abstract Despite the implementation of protective coatings and cathodic protection in buried pipeline steels, the occurrence of Near-Neutral pH Corrosion Fatigue (NNpH-CF) remains a critical pipeline integrity management challenge. (Note that this mechanism has previously been termed near-neutral pH stress corrosion cracking.) This form of corrosion fatigue can only occur when the pipeline’s protective coating is damaged, exposing the pipe to corrosion conditions. Circumferential near-neutral pH corrosion fatigue (C-NNpH-CF), where residual and axial stresses play a pivotal role in service failures, is less understood than the more common axial form. C-NNpH-CF failure involves several stages, including crack initiation and early growth (Stage I), sustainable crack growth (Stage II), and rapid crack propagation leading to failure (Stage III). Using the digital image correlation (DIC) method, this research examined the effects of bending residual stress (as a source of axial stress) and cyclic loading (simulating pipeline pressure fluctuations) on these stages. The evaluation included crack re-initiation from simulated dormant cracks, stage II crack growth, and failure mechanisms, considering various parameters such as applied loading, initial notch depth/position, and bending angle/direction. The study determined the threshold conditions for crack reinitiation, highlighting the intricate factors influencing this phase. Additionally, crack growth rates during Stage II were compared with those from earlier works on longitudinally oriented NNpH-CF, providing a comprehensive understanding of the circumferential near-neutral pH corrosion fatigue process. Under the influence of maximum bending angle and axial cyclic loading, the highest crack growth rate was obtained at the bent centerline in the inward direction, where maximum tensile stress is concentrated on the external surface. Stage III was primarily governed by a high stress intensity factor, contrasting with Stage II, which was influenced by a medium stress intensity factor and hydrogen-enhanced fatigue. The stress gradient in the depth direction of a bent pipeline emerged as a significant factor affecting crack growth rates at different stages. This research has enhanced circumferential cracking modelling accuracy, aiding operators in mitigating risks, reducing costs, and minimizing environmental consequences associated with these failures.