Abstract Stress corrosion cracking (SCC) is a major pipeline degradation mode that often leads to leak or rupture during operation or hydrotest. Near neutral pH SCC was initially discovered to form beneath polyethylene-tape coatings in Alberta, Canada in the 1980s followed by reports from American operators. Unlike the well-studied high-pH SCC that is typically intergranular, previous metallurgical examination in laboratory studies suggest that near-neutral pH SCC is predominately wider and transgranular. The crack walls are often corroded heavily which makes studying the crack propagation mechanism challenging. In this work, advanced electron-microscopy techniques are applied to study the microstructural characteristics of near-neutral pH SCC in ex-service pipeline material. Firstly, 3D tomography of a SCC was carried out on a plasma FIB (focused ion beam), which revealed the development of cracks with a complex network structure from the external surface. Second, careful examination through scanning electron microscopy (SEM) revealed areas containing fine un-corroded cracks along the main path of the heavily corroded crack. To further reveal the characteristics of the fine cracks, through thickness trenches were made by FIB at the areas where un-corroded cracks were observed. The subsequent 2D imaging revealed that the cracks developed along grain boundaries, as opposed to what conventionally is believed i.e., that near-neutral pH SCC develops transgranularly. Lastly, TEM (transmission electron-microscopy) characterization was carried out on FIB lift-outs from the fine un-corroded cracks. Through electron diffraction, the intergranular nature of the fine cracks was determined. The local plasticity was also quantified by comparing the dislocation densities in the grains where fine cracks developed, to the overall dislocation densities in the pipeline. It is shown that no significant plasticity was involved with the development of fine cracks. Lastly, high resolution EDX (energy dispersive x-ray spectroscopy) revealed only oxidation occurred along the walls of the fine cracks, and no distinct foreign elements existed in the developing crack. The microstructural study revealed that the examined SCC did not require high stress intensity at the crack tip to propagate, which is in-line with the stress analysis based on operational history and bending strain measurement at the locations. Alternative crack propagation mechanisms will be hypothesized and confirmed by in-lab simulated studies.
