Stress Corrosion Cracking Observed in Ex-service Gas Pipelines: A Comprehensive Study

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ILST the consequences of catastrophic failure of oil and gas pipelines due to stress corrosion cracking (SCC) can be severe,[1] there still remains some uncertainty over the crack propagation mechanism in widely used pipe grades. SCC is an insidious form of environmental induced cracking that causes failure in pipelines, often, with little warning. SCC can occur in pipelines when three conditions are met[2,3]; 1. 2. 3.

The pipe is subject to an applied or residual tensile stress, Exposure to a corrosive environment, and The material is susceptible to SCC

SCC defects form in steel pipelines when coating disbondment occurs in cathodically protected pipes. When a sufficiently high cathodic potential is applied, water is dissociated to form hydroxyl ions, (OH), that create a localized, low pH region capable of combining CO2 dissolved in groundwater present in surrounding soil to form carbonate and bicarbonate ions.[4] Solutions

A. ROCCISANO and R. GHOMASHCHI are with the The University of Adelaide, Adelaide, SA 5005, Australia. Contact e-mail: [email protected] S. NAFISI is with the The University of Adelaide and also with the University of Alberta, Edmonton, Alberta T6G 1H9, Canada. Manuscript submitted March 15, 2019.

METALLURGICAL AND MATERIALS TRANSACTIONS A

with a pH greater than 9.3 cause the intergranular, high pH form of SCC, whereas a lower pH between 5.5 and 7.5 tends to encourage the transgranular, near neutral form of SCC.[4–6] The primary propagation mechanism for high pH SCC (SCC hereafter) is anodic dissolution of the steel pipe combined with passive film fracture at the crack tip.[7,8] Anodic dissolution occurs at the grain boundaries until cathodic protection forms a passive film at the crack tip that halts corrosion. When the film fractures due to the applied or residual tensile stress, anodic dissolution between grain boundaries can occur again. To mitigate failure in pipelines, there has been considerable research into the risk factors controlling the susceptibility of steels to SCC. Since SCC requires a tensile or residual stress, corrosive environment, and susceptible material to be present concurrently, removing any of these conditions prevents SCC. The magnitude and direction of the residual stress at the surface caused by manufacturing can dramatically alter the resistance to SCC as for instance tensile residual stresses at the surface can lead to increased SCC activity.[9,10] In addition, the replacement of failed pipeline coatings with modern coatings can dramatically reduce the SCC growth rate.[6] However, it is important to note, that recoating of pipelines may not halt crack propagation entirely as fatigue cracking can become the dominant mechanism in recoated pipes under load.[11] Steel surface roughness,[15,16] and composition,[12–14] [17–19] can also affect SCC susceptibility. microstructure

Recent investigation has focused on exploring the link between SCC susceptibility and crystallographic texture from both macro- and micro-texture viewpoints.[10,20,21] There are limited in