The Role of Pressure and Pressure Fluctuations in the Growth of Stress Corrosion Cracks in Line Pipe Steels

Stress corrosion cracking of pipeline steels involves the interaction of a susceptible material with a corrosive environment under appropriate loading conditions. The stress exerted on pipeline steel is a combination of the internal operating or hoop stress, cyclic stresses due to pressure fluctuations within the pipe, residual stresses developed during pipe manufacture and pipeline construction, as well as other sources of stress such as stress concentrators like corrosion defects and geotechnical stresses. As the internal pressure of an operating pipeline is never static the fluctuations in pressure result in cyclic loading of the pipeline material. Whilst there are many stresses acting on a pipeline the predominant stress is the hoop stress created as a result of the internal operating pressure. In this paper experiments are described which are aimed at determining the effect of pipeline hoop stress on the growth of stress corrosion cracks under neutral pH conditions. The propagation of stress corrosion cracks in pipeline steels at near neutral pH was studied under realistic environmental conditions. The impact of hoop stress on the growth of shallow surface cracks in an X60 pipeline steel was investigated at 40%, 70% and 100% of the specified minimum yield strength of the material. Crack growth rate is shown to be independent of the applied stress in the range studied for these shallow cracks. Cyclic stresses appear to be more important for crack propagation under neutral pH SCC conditions. In light of the results obtained in this test program an attempt was made to characterize the pressure changes occurring on an operating gas pipeline. A computer program was developed to enable characterization of the pressure fluctuations for operating gas pipelines. The results of characterization of pressure histories from a number of operating pipelines are presented.