“Canadian Experience in SCC of Pipelines and Its Remedies” Recent Progress in SCC of Pipelines in Near-Neutral pH Environment

Abstract

Evidence from a failure pipeline that had been in-service for 34 years showed that there are often quantities of tiny blunt cracks, frequently in crack colonies, in the pipeline. The vast majorities of these small cracks are seen to be dormant and hence tend to be innocuous. However, if the small cracks can surpass a threshold depth, around 0.5–0.6 mm, these cracks can be activated and begin to grow and may eventually lead to pipeline rupture if not detected and removed. The mechanisms are far from being understood. This paper is to review pipeline stress corrosion cracking (SCC) to help understand the mechanisms on pit-to-crack transition and early growth to contribute to pipeline integrity management so that rupture can be avoid or reduced. Specimens were pitted using two different techniques and then cyclically loaded in near-neutral pH environment sparred 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 during pipeline operations. The crack morphologies produced in laboratory with these techniques were found to be very similar to those from the failed pipeline in the field, which has never been reported before. It was proposed that two different mechanisms were responsible for the early-stage crack growth. For cracks less than 0.5–0.6 mm deep, they were blunt, engendered by anodic dissolution (localized corrosion), which was facilitated by stresses. So it was called stress-facilitated dissolution crack growth. Once crack depth was larger than the critical value, around 0.5–0.6 mm, the cracks would be reactivated and more hydrogen would be trapped in the plastic zones. Thus, hydrogen would play an important role in crack propagation. So in this stage, cracks tended to become sharp and the mechanism was referred to hydrogen assisted cracking. The observations from the field can be interpreted very well by using the proposed models. It was suggested that cracks deeper than 0.5–0.6 mm in the field should be removed to reduce or avoid the threat of rupture. If active corrosion and hydrogen related Stage II growth can be prevented then smaller cracks are innocuous.