12 4. CONCLUSIONS Based on the results and discussion above, the following conclusions were made. 1. Blunt cracks were seen to have initiated from the corrosion pits on the pitted samples cyclically loaded in near-neutral pH environment and these blunt cracks were generally less than 0.5 to 0.6 mm deep, consistent with the observations from the field. These blunt cracks were transgranular and believed not to put pipelines at a risk from integrity management point of view. 2. The distribution of the nearby non-metallic inclusions significantly influenced the blunt crack growth by enhancing the stress-facilitated dissolution of the blunt cracks. 3. The orientation of the nearby non-metallic inclusions with regard to the blunt cracks was crucial for the blunt cracks to continue growing or keep dormant. Only when the orientation of the nearby inclusions was at a small acute angle to that of the pits or cracks, and the inclusions were in the same plane as crack initiation or extension, these inclusions would contribute to further crack growth. Thereafter, the inclusions would be eaten away by further growth of the blunt cracks. 4. When the inclusions were approximately perpendicular to the blunt crack growth, the cracks would become blunter and dormant. 5. The hydrogen trapped in the nearby inclusions and phase interface would cause clusters of tiny cracks to form, which appeared to be produced by hydrogen. These hydrogen generated cracks could be eaten away later by the stress-facilitated further dissolution of the blunt cracks. 6. When the blunt cracks reached a critical size, i.e., 0.5 to 0.6 mm deep, sharp cracks would be formed, engendered by hydrogen. 7. Different mechanisms could occur in pipeline steel in near-neutral pH environment. The transformation from one mechanism to another might result from complex relation between stress risers, dissolution driven by stress concentrations, inclusions, and trapped hydrogen ACKNOWLEDGMENTS The authors would like to acknowledge an NSERC Strategic Grant and Enbridge Pipelines Inc. for financial support, and an NSERC/AUAF facility access grant at CANMET Materials Technology Laboratory. The authors thank Scott Ironside of Enbridge for the experimental pipeline material. One of the authors also acknowledges helpful discussions with Dr. Roger Staehle. REFERENCES 1. NEB. Stress Corrosion Cracking on Canadian Oil and Gas Pipelines. Report of the Inquiry, National Energy Board. MH-2-95. December 1996. 2. CEPA, CEPA Stress Corrosion Cracking Database: Second Trending Report, August 17, 2000 3. Michael Baker Jr., Inc., Stress Corrosion Cracking Study, TTO Number 8, Integrity Management Program Delivery Order DTRS56-020D-70036, Final Report, January 2005. 4. B. Fang, R.L. Eadie, W. Chen, M. Elboujdaini, “Electrochemical Method Pits Generation and Its Application in Crack Initiation in Pipeline Steel in Near-Neutral pH Environment”, NACE International
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