Pre-cyclic-loading-enhanced Stage-1b stress corrosion crack growth of pipeline steels

Pipeline steels are highly susceptible to high pH stress corrosion cracking (SCC). Such cracks are often more pronounced in corrosive environments and can potentially lead to damage or catastrophic failure of the component if not properly managed. High pH SCC mitigation requires a thorough understanding of factors affecting crack growth rate and its propagation mechanism, which is often characterized by anodic dissolution at grain boundaries and continuous rupture of passive films.

The cracking process has been broadly divided into five main stages. The incubation stage is characterized by the coating damage and formation of concentrated bicarbonate/carbonate solution. Stage 1a involves crack formation via selective grain boundary attack, while stage 1b involves the extension of the crack length mainly via the self-induced microcracks within the plastic zone ahead of the crack tip. Lastly, stages 2 and 3 are characterized by direct crack growth from the main crack tip and fast growth to pipe rupture, respectively.

Pipeline steels are generally subjected to load-controlled pressure fluctuations that can produce pure mechanical damage or residual stress alteration to the existing defects and cracks. Stage-1b crack growth is affected by numerous factors, including the magnitude of the stress cycles, cyclic loading before exposure to a corrosive environment, heat treatment following pre-cyclic loading, and stress intensity factor. While stage 2 crack growth is well articulated in the literature, stage-1b is still underexplored. Knowledge and in-depth understanding of stage-1b are of great significance as it bridges the early stages of crack initiation and stage 2 crack growth.

Now, in a rigorous new study by Dr. Shidong Wang and Professor Weixing Chen from the University of Alberta, together with Dr. Lyndon Lamborn from the Enbridge Pipelines Inc., they described the influence of pre-cyclic loading on Stage-1b SCC growth of X52 pipeline steels. The steels were subjected to a corrosive environment containing concentrated carbonate and bicarbonate solutions. Moreover, they investigated the effects of the magnitude of the stress cycles and heat treatment following the pre-cyclic loading. The studies were focused on Stage-1b crack growth that is self-induced by existing cracks (the so-called mother-daughter relationship, which can be found in Corrosion Science 178 (2021) 109056,  Journal of Materials Science 57 (2022): 15967-15989, Corrosion 79 (2023): doi.org/10.5006/4168). Their research work is currently published in the peer-reviewed journal, Corrosion Science. The authors showed that pre-cyclically loaded specimens subjected to subsequent cyclic loading corrosion exposure exhibited Stage-1b SCC growth when the mechanical driving force is below the threshold for Stage 2 growth (at maximum stress intensity of 16 MPa m^0.5, frequency of 0.01 Hz and stress R ratio of 0.5). Specimens with pre-cyclic loading recorded higher crack growth rates than those without; although pre-cyclic loading introduced compressive residual stress at the crack tip, which was believed to be detrimental to the crack growth in traditional SCC theory. Results proved that the compressive residual stress at the main crack tip induced by pre-cyclic loading facilitates microcrack coalescence to the main crack tip, which assists the Stage-1b growth. Additionally, an increase in the magnitude of the pre-cycling loading accelerated the SCC crack growth and its subsequent transition from Stage-1b to Stage 2. Compared with specimens with lower prior cyclic stress, those with higher stress experienced significant localized exfoliation and rupture/cracking of the passive films in their plastic zone, suggesting the enhanced Stage-1b crack growth.

A combination of the pre-cyclic loading and heat treatment contributed to the occurrence of strain-aging-assisted SCC crack growth. However, the contribution of the static corrosion-induced intergranular attack to Stage-1b crack growth was less significant. Based on the results, specimens subjected to pre-cyclic loading at maximum stress intensity of 28 MPa m^0.5 and aging were 4.9 ×10^-8 mm/s and 6.1 ×10^-8 mm/s, which was about 1.1 and 1.2 times higher than those free from heat treatment, respectively.

In summary, the research work has addressed the most difficult challenge encountered in understanding why the cyclic loading prior to corrosion exposure could enhance the Stage-1b SCC crack growth which is the bridging between SCC crack initiation and Stage 2 growth. The compressive residual stress at the main crack tip caused by pre-cyclic loading (typically overloading, hydrostatic testing, etc.) facilitates the crack growth in the category of Stage 1b by assisting the coalescence of nucleated microcracks in the plastic zone to the main crack tip.

This novel finding and proposed new mechanism herein overthrow the old ideas in traditional SCC theory that compressive residual stress would always suppress the crack initiation and growth, which is of great significance to the future engineering, scientific and industrial research. It would undoubtedly inspire researchers in many fields to have a better understanding of the enhanced early-stage SCC crack growth in the presence of compressive residual stresses. According to the authors, the pipeline steels’ susceptibility to cracking could be reduced by removing the plastically deformed regions or the existing cracks before recoating or rewelding. In a joint statement to Advances in Engineering, the authors explained their study provided useful insights that could contribute to the effective mitigation of SCC in pipeline steels.