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Stress Corrosion Cracking in Pipeline Steels

Clusters of fine stress corrosion cracks on the external surface of buried steel natural gas pipelines in contact with groundwater have been examined and studied. The growth rates of transgranular stress corrosion cracks have been modeled and determined by conducting laboratory tests under similar conditions to those recorded in practice. The steel samples were immersed in an anaerobic dilute, near neutral solution with an open circuit potential for various times under stress. Metallographic examination of the resulting stress corrosion cracks was then conducted. Transgranular fracture, similar to that observed in the field, was observed following tests carried out under low frequency cycling in combination with a high stress ratio (R= minimum load/maximum load). A quantitative relationship between the frequency and stress ratio was developed giving crack growth rates similar to those observed in practice. Also, a superposition model was developed and applied to the experimental data which gave very good agreement between the actual and predicted crack growth rates. Applying the superposition model to the operating natural gas pipeline data showed that realistic predictions of crack growth result when taking interaction of the cracks into account.