Pipeline Circumferential Cracking in Near-Neutral pH Environment Under the Influence of Residual Stress
Creators: Shirazi, Hamed
Publisher: name: University of Alberta Library
Year: 2024

<p>In recent years, pipeline failures attributed to circumferential corrosion fatigue in near-neutral pH environments were reported. 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 occur when the pipeline&#39;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). Most cracks initiated from external pipeline corrosion pits cease growing when they reach a depth of about 1 mm and enter the dormant stage since the dissolution rate decreases as crack depth increases. Most crack-like features in NNpH-CF colonies remain dormant, and only a few can grow deeper. Using the digital image correlation (DIC) method, this research attempts to examine the effects of bending residual stress (as a source of axial stress) and cyclic loading (simulating pipeline pressure fluctuations) on these stages. The evaluation includes crack re-initiation from dormant cracks, crack growth, and failure mechanisms, considering various parameters such as applied cyclic loading, initial notch depth/position, and bending angle/direction. Fractography analysis, Crack Mouth Opening Displacement (CMOD), and Electron Backscatter Diffraction (EBSD) techniques were employed to gain deeper insights into the C-NNpH-CF failure mechanism.<br>
The study determined the threshold conditions for crack initiation, highlighting the intricate factors influencing this phase. The analysis of crack initiation in cross-section and fractography images revealed that different factors, such as applied loading, initial notch depth/ position, and bending parameters, contribute to crack initiation by influencing the stress distribution, stress cells, stress concentration factor (exclusively in the depth direction) and hydrogen diffusion into the notched region. In addition, there are time-dependent mechanisms governing crack initiation at the bottom of the pit, including dissolution rate and hydrogen-enhanced corrosion fatigue. <br>
Since integrity management measures should be implemented before Stage III (rapid crack propagation to rupture), crack growth rates at Stage II under various parameters were investigated and then 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 bend centerline in the inward direction, where maximum tensile stress is concentrated on the external surface. Stage III crack fracture 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.<br>
A deeper understanding of these mechanisms can improve knowledge of Circumferential Near-Neutral pH Corrosion Fatigue (C-NNpH-CF), which plays a significant role in pipeline integrity management. In addition to enhancing the accuracy of C-NNpH-CF predictions, gathering this knowledge can reduce the risks, costs, and environmental consequences of these failures.</p>