Stress includes pipeline operation working stress, residual stress and secondary stress, etc. It causes stress concentration at the crack tip and destruction of the material passivation film; the sensitivity of the pipeline is related to the manufacturing process and the type, grade, impurity content and surface conditions of the steel; The specific corrosive environment refers to the corrosive environment formed by the peeling off of the inner and outer coatings of the pipeline, the hydrogen sulfide in the gas transmission medium, the presence of carbonic acid, nitric acid, chlorine, and sulfate ions in the soil and groundwater. A prominent feature of SCC failure is that there are many axial surface crack groups on the gas pipeline. The groups are connected to form long and shallow cracks. The fracture surface is covered with black magnetite or iron carbonate film.
The stress corrosion cracking of gas transmission pipeline is the result of the combined action of corrosion and stress. The cracking caused by the combined action of corrosion and stress is far greater than the result of a single factor acting separately and then superimposed. If any one of the two factors is eliminated, the damage caused by the other becomes insignificant. The medium that produces SCC is generally not very corrosive. If there is no stress, the gas pipeline is sufficiently resistant to corrosion in this medium; vice versa, the stress that generates SCC is not necessarily very large. If there is no corrosive medium Exist, the gas pipeline can run for a long time without cracking and failure.
Stress corrosion often causes major economic losses, catastrophic accidents and environmental pollution to oil and gas fields. If appropriate protective measures are taken, 30 @% of the economic losses caused by corrosion can be recovered. Strengthening corrosion and protection research can bring huge economic benefits to the oil and gas industry. SCC is a form of environmental cracking. The local environment, the potential of the material, the applied stress, the condition of the protective coating, the operating temperature, and the heat treatment of the material play an important role in this failure. The stress corrosion cracking on the pipeline develops from the small cracks lurking on the surface of the pipeline. At first, these cracks are not visible to the naked eye. The stress corrosion cracking generally develops very slowly. It may exist on the pipeline for many years without problems, but once If the crack becomes large enough, the pipeline will break and leak or break. China is currently constructing the West-East Gas Pipeline Project and the natural gas pipeline from Xinjiang to Shanghai with an investment of more than 110 billion yuan and a total length of 4,200 km. Strengthening the research work on the SCC and life prediction of gas pipelines is very important to ensure the safe operation of such long-distance gas pipelines.
The stress in the gas pipeline mainly comes from the pressure of the pipeline operation, secondary stress, residual stress and stress concentration. The cracking direction of SCC is related to the direction of the local maximum stress of the pipe wall, and it extends in the direction perpendicular to the maximum stress. In most cases, SCC is mainly subjected to circumferential stress, and cracks initiate and propagate along the axial direction of the pipeline, that is, axial cracks; while in the case of circumferential SCC, pipelines are mainly subjected to axial stress, and the cracks extend along the circumferential direction, which is the circumferential To crack. Internal pressure can produce circumferential stress, which is an important source of stress. The direction of most SCC is along the pipe axis.
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