Surface Crack Growth Behavior of Pipeline Steel Under Disbonded Coating at Free Corrosion Potential in Near-Neutral pH S
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I.
INTRODUCTION
OIL and gas pipelines are crucial facilities in the distribution of oil and gas products. Although they have significant capital costs, they are nevertheless the most efficient and/or safest way of moving and distributing oil and gas products. It is estimated that the oil and gas pipeline network in Canada alone is long enough to stretch all the way from earth to the moon and back.[1] Because they often convey flammable products, they present a significant level of danger and may cause significant destruction and product loss in the event of an accidental pipeline rupture followed by an explosion. Since it was first reported, near-neutral pH stress corrosion cracking (NNPHSCC) has been responsible for a few accidental rupture/leakages[2] in Canada, some of which involved loss of property and significant release of either gas or liquids. NNPHSCC continues to be a major pipeline integrity issue in significant part because of imprecise underAFOLABI EGBEWANDE, formerly with the Department of Chemical and Materials Engineering, the University of Alberta, Edmonton, AB, T6G 2G6 Canada, is now with the Stantec Consulting Ltd., Calgary, AB, T2H 2M4 Canada. WEIXING CHEN, Professor, and REG EADIE, Professor Emeritus, are with the Department of Chemical and Materials Engineering, University of Alberta. Contact e-mail: [email protected] RICHARD KANIA, Technical Lead, and JENNY BEEN, Corrosion Specialist, are with the TransCanada Pipelines Limited, Calgary, AB, T2P 5H1 Canada. GREG VAN BOVEN, Team Leader – Systems Integrity, is with the Spectra Energy Transmission Ltd., Vancouver, BC, V6E 3P3 Canada. ROBERT WORTHINGHAM, formerly with the TransCanada Pipelines Limited, is now with the Worthingham Professional Services Inc., Calgary, AB, Canada. Manuscript submitted July 12, 2013. Article published online July 25, 2014 4946—VOLUME 45A, OCTOBER 2014
standing of the crack initiation and propagation mechanisms. Although it is generally accepted that the nearneutral pH environment plays a role in the development and propagation of cracks, proper understanding of the role played by the environment or the species in it is at best still evolving.[3–9] Eslami et al.[10] used a novel test set up to study the combined influence of pressure fluctuation, coating disbonding, and environmental parameters on the initiation of cracks in near-neutral pH environments at open-circuit potential. The study established that variations in the localized environments under the disbonded coating could result in varying stress corrosion cracking (SCC) initiation mechanisms. Such variations in the local environment under the disbondment could result in preferential crack initiation from the mouth of large pits, formation from persistent slip bands, initiation at the bottom of deep scratch lines, and from submicrometer pits. NNPHSCC cracks typically start as very fine surface cracks that grow both in the depth and at the pipe surface over time, sometimes until rupture occurs. Pipeline operators are generally aware that several segments of their pi
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