PSI - Issue 42

Mimoun Elboujdaini et al. / Procedia Structural Integrity 42 (2022) 1033–1039 Mimoun Elboujdaini / Structural Integrity Procedia 00 (2019) 000 – 000

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were unchanged. In these tests, some pre-existing cracks stopped propagating, and some disappeared as a result of lateral dissolution along the crack walls. This result explains the necessity of a certain magnitude of stress fluctuation in crack development, especially in maintaining crack sharpness, in this material-environment system, which does not show an active-passive transition. This result implies that the critical conditions for crack initiation from a smooth surface can be different from those for propagation. In the later stages of these experiments, the stress ratio was reverted to its initial value, 0.6, but three different environmental/electrochemical conditions were used -- NS4 solution, laboratory air, and NS4 solution with a catholically polarized potential, -850 mV (SCE). Substantial crack propagation was observed when the tests were conducted in solution, but not in laboratory air. This result indicates that fatigue alone, under the stress conditions applied, did not induce cracking, and that environmental reactions played a very important role in crack development. It was also found that cracks re-initiated at locations where previous cracks had developed, when a lower stress ratio was used, but disappeared when a higher stress ratio was applied. This result could indicate that pre-load history may influence subsequent stress- corrosion cracking behavior. The implication of this result to field operation is that cracking may not occur under certain operating conditions when the coating is still protective; however, once a critical environment is generated, cracks are likely to develop at locations where previous mechanical damage (caused, for example, by pressure fluctuations) accumulated. This also means that a relationship between corrosion cracking and fatigue property may exist. 5. Conclusions 1. Multiple types of crack initiation sites exist for stress corrosion cracking of pipeline steel in low-pH solutions. Stress corrosion crack initiation is a competitive process; cracks initiate at the most favorable sites first, then at other sites. 2. The number of cracks increased with increased test time. Crack interaction and coalescence significantly affect crack propagation when the crack density is high. 3. A large variation of crack growth rates exists for cracks with similar sizes, and crack growth rate distribution can be statistically described using the Weibull function. 4. Both maximum stress and amplitude of stress variation affect cracking, but the amplitude has a greater effect on crack initiation. 5. Reduction of the pressure fluctuations could be an effective way to improve the integrity of a pipeline without sacrifice of transmission capacity. References 1. Stress Corrosion Cracking Led to October 2018 Pipeline Rupture and Fire near Prince George, British Columbia,” Transportation Safety Board of Canada News Releases, March 4, 2020, https://www.tsb.gc.ca/eng/medias- 2. Elboujdaini, M, Research in Progress, NACE Corrosion/2009. 3. Kiessling, R., “Non - Metallic Inclusions in Steel, the Metals Society, London], (1978)” 4. Parkins, R.N., Materials Science and Technology, 1 (1985) p. 480. 5. Elboujdaini, M, Wang, Y.- Z., Revie, R.W., Shehata, M., de Sliveira, G., Parkins, R.N., “Initiation of Stress Corrosion Cracking in Pipeline Steel,” prepared for GRI, MTL report no. MTL 2001 -38(CF), August 2001 6. Elboujdaini, M., Li, J., Gertsman, V., Gu, G., Revie, W. Gao, M., and Katz, D.C., “Stress Corrosion Cracking: Microstructura l and Material Properties for Crack Initiation of 16” X -52 Line Pipe Steel - Corrosion 2004, NACE Conference Paper 04553, 2004. 7. Parkins R.N. and Delanty, B.S., “The Initiation and Early Stages of Growth of Stress Corrosion Cracks in Pipeline Steels Expo sed to a Dilute, Near Neutral pH Solution, Ninth Symposium on Pipeline Research, AGA Catalog No. L 51746, 1996, pp. 19-1 to 9-14. 8. Chen,W., King, F., and Vokes,E., Characteristics of Near-Neutral pH Stress Corrosion Cracks in an X-65 Pipeline, Corrosion, 58, 2002, pp. 267-275. 9. Parkins, R.N., Blanchard, W.K. and Delanty, B.S., “Transgranular Stress Corrosion Cracking of High-Pressure Pipelines in Contact with Solutions of Near-Neutral- pH”, Corrosion, 50, 394 -408 (1994). 10. Sutcliffe, J.M., Fessler, R.R., Boyd, W.K. and Parkins, R.N. “Stress Corrosion Cracking of Carbon Steel in Carbonate Solutions”, Corrosion, 28, 313 (1972). 11. Beavers, J.M., and Harle, B.A., “Mechanisms of High -pH and Near-Neutral- pH SCC of Underground Pipelines”, ASME International Pipeline Conference, Calgary, Alberta; June 1996. 12. Oriani, R.A. and Josephic, P.H., Metallurgical Transaction, 11A, 1980, p. 1809.