![]() ![]() ![]() Higher strength steels will be more susceptible to environmentally assisted cracking mechanisms associated with hydrogen embrittlement eg SSCC, or hydrogen embrittlement due to CP overprotection. Finally, the pipeline material and its mechanical properties are contributing, as well. A corrosive environment or other impact on the surface of the pipeline – mechanical damage, for example – also support the initiation of cracking due to micro-embrittlement. Likewise, an axial load on the pipeline supports the occurrence of circumferential cracking.Īnother factor is the exposure of the pipeline surface. A pressurized pipeline experiences so-called hoop stress, which creates an environment for axial cracking. But pipeline cracks are always detected perpendicular to the main local stress direction of the pipe material. Therefore, cracks in pipelines can be categorized into environmentally assisted active cracking and mill-related or dormant planar anomalies.Ĭracking can be found at any location of the pipeline, be it in the pipe body or in the welded areas. For cracking, a time dependency is hard to observe. Therefore, similar to other typical pipeline anomalies, a fundamental question is the time dependency of crack-like anomalies. hook crack), and from an integrity perspective, cold welds, stitching and penetrators are all assessed as cracks.Ĭracking can be anywhere, but it needs some specific boundary conditions to initiate. Despite such features actually being related to fusion, the term cracking is still used (e.g. In EFW and ERW welds, “cracking” in most cases will be located on the mid-point of the bond line. While cold cracking generally occurs during or just after the welding process (it can be delayed), there are types of environmentally assisted cracking that can also readily occur in the weld metal. ![]() Hot cracks from solidification mechanisms can be in both the longitudinal and transverse directions as well as “crazed.” These cracks are usually formed in the pipe mill for seam welds or during pipeline construction for girth welds.Ĭold cracking in weld metal requires the presences of hydrogen, a susceptible weld metal microstructure and stress, so cracks typically form in the longitudinal direction. SAW seam and multiple process girth welds), cracks can be both hot and cold. This mechanism is referred as Sulfide Stress Corrosion Cracking (SSCC). In parallel events, once atomic hydrogen is absorbed within the microstructure, it can also diffuse into solid solution, leading to local crystal embrittlement and ultimately stress corrosion cracking in the presence of residual or applied tensile stresses. A variation of the HIC mechanism is Stress-Orientated HIC (SOHIC). Surface blisters may also contain cracks. Pipeline cracks may join up, at different levels through the pipe wall, to create through-wall cracks (Step-Wise Cracking – SWC). at inorganic inclusions (manganese sulfur) – and allowed to recollect with other trapped atomic hydrogen, this leads to the formation of molecular hydrogen and local buildup of pressure, which results in mid-wall blisters and cracking parallel to the wall. When atomic hydrogen is trapped at irregularities within the pipe steel – e.g. Following aqueous corrosion reactions in the presence of H2S on the pipe internal surface, atomic hydrogen is produced and absorbed in the pipe steel. The most common conditions for these are sour environments. Sour Cracking - HIC, Sulfide Stress Corrosion Cracking (SSCC)Įnvironmentally assisted cracking can also occur internally in pipelines. It is nevertheless accepted that some level of stress cycling is required for initiation and growth. Some investigations have also associated anaerobic microbial activity with the initiation of near-neutral pH SCC overall, however, the mechanism remains unclear and debated. It is thought to be most prevalent in high latitudes where there is a marked seasonal change in the carbon dioxide content of the soil. The cracks tend to be wider, with corroded crack walls and filled with corrosion products. In contrast to high pH SCC, the crack propagation through pipe wall thickness is of a transgranular nature. Near-neutral pH SCC cracking typically occurs in association with pits and general corrosion, as it occurs under freely corroding conditions, i.e. As for high pH SCC, the surface cracks generally form colonies in the axial direction of the pipe. Near-neutral pH SCC also occurs on external pipe surfaces under areas of coating disbondment but where CP is fully shielded. Near Neutral pH Stress Corrosion Cracking ![]()
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