1 CRACK INITIATION: STRESS-CORROSION CRACK IN X-52 PIPELINE STEEL IN NEAR-NEUTRAL pH ENVIRONMENT Mimoun Elboujdaini1,*; Bingyan Fang2; and Rrg Eadie3 1- CANMET Materials, Natural Resources Canada, 3303 - 33rd Street NW, Calgary, AB, Canada T2L 2A7 2- G.E. Oil & Gas PII Pipiline Solution, 4908 – 52nd Street SE, Calgary, AB, Canada, T2B-3R2 3- Dept. of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada, T6G 2G6 * {Corresponding author: melboujd@nrcan.gc.ca} ABSTRACT Specimens from a failure X-52 pipeline that had been in-service for 34 years were pitted using passivation/immersion method developed by the authors to simulate pitted pipelines observed in service. The resulting pitted samples were then cyclically loaded in near-neutral pH environment sparged with 5% CO2 / balance N2 gas mixture at high stress ratios (minimum stress/maximum stress), low strain rates and low frequencies which are close to those experienced in active flowing service. It was found that the majority of cracks initiated from the corrosion pits and were less than 0.5 to 0.6 mm deep, which were generally quite blunt. These cracks were transgranular in nature and designated as Stage I cracks and were typical of cracks found in most crack colonies. However, the further growth of these short blunt cracks was significantly influenced by the distribution of the nearby non-metallic inclusions. Inclusions enhanced the stress-facilitated dissolution crack growth, which is the crack growth method proposed by the authors in a related paper. When the orientation of the inclusions was at an acute angle to the orientation of the pits or cracks, and the inclusions were in the same plane as crack initiation or advance, these inclusions would enhance crack growth, or even trap hydrogen which further resulted in the formation of clusters of tiny cracks, which appeared to be caused by hydrogen. The hydrogen produced cracks could be eaten away later by the stress-facilitated further dissolution of the blunt cracks. If these cracks can grow sufficiently however they pose an integrity risk as they can initiate long cracks (near-neutral pH SCC). These hydrogen caused cracks in Stage I were rare. It was nevertheless suggested that cracks deeper than 0.5 to 0.6 mm in the field should be removed to reduce or avoid the threat of rupture. If active corrosion can be prevented such as by cathodic protection then smaller cracks are innocuous. 1. INTRODUCTION Near-neutral pH stress corrosion cracking (NNpHSCC) has been a significant integrity issue for gas and oil pipelines for over two decades since it was first documented in 1985 [1], and it has been investigated extensively. More than 18,000 colonies of NNpH SCC had been reported on Canadian pipelines up to August 2000 [2], and more than 95% of these colonies became dormant. A small percentage, however, which might grow until failure if not detected and removed, would cause a considerable risk to pipeline integrity management. Pipeline failures have been reported to decrease recently, possibly because more rigorous pipeline integrity management programs have been in place. However, there still have been one or two ruptures a year in Canada since 1995 and numerous cracking discovered through ILI (in-line inspection) or hydrotesting. Although soil models and simulations have been developed to help assess risk [3], these are rather imprecise, and
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