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The effects of mechanical factors on crack growth behavior during the second stage of high pH stress corrosion cracking in pipeline steel were investigated by applying several loading scenarios on compact tension (CT) specimens. The main mechanism for stage 2 of intergranular crack propagation is anodic dissolution ahead of the crack tip which is highly dependent on crack-tip strain rate. The maximum and minimum crack growth rates were 3 × 10−7 mm/s and 1 × 10−7 mm/s, respectively. It was observed that several factors such as mean stress intensity factor, amplitude, and frequency of loading cycles determine the crack-tip strain rate. Low R-ratio cycles, particularly high-frequency ones, enhance secondary crack initiation, and crack coalescence on the free surface. This mechanism accelerates crack advance on the free surface which is accompanied with an increase in mechanical driving force for crack propagation in the thickness direction. These findings have implications for pipeline operators and could be used to increase the lifespan of the cracked pipelines at stage 2. For those pipelines, any loading condition that increases the strain rate ahead of the crack tip enhances anodic dissolution and is detrimental. Additionally, secondary crack initiation and coalescence could be minimized by avoiding internal pressure fluctuation, particularly rapid large pressure fluctuations.

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Steel pipelines undergo the following sequential stages prior to high pH stress corrosion cracking (HpHSCC) failure, viz., formation of environmental condition, initiation of the intergranular cracks followed by cracks coalescence to form critical crack size (Stage I), mechanically dictated crack growth with higher rate (Stage II) compared to Stage I, rapid crack propagation to failure (Stage III). From fracture mechanics perspective, the crack size reaches the critical value at the onset of stage II; consequently, stress intensity factor (K) ahead of the crack tip exceed the critical value (KISCC). Although many researches have been devoted to understanding HpHSCC behavior, the mechanical conditions that accelerate the onset of stage II remains unknown. This study investigates the mechanical loading conditions that yield to early onset of stage II with respect to the most severe loading condition in operating pipeline, underload-minor-cycle type of pressure fluctuation. In this study, several loading scenarios were applied to pre-cracked CT specimens exposed to 1 N NaHCO3-1N Na2CO3 at 40° C and −590 mVSCE. The first series of tests were conducted through applying variable amplitude loading waveforms to determine the K value below the KISCC. It was observed the crack growth rate decreases from 1.5 × 10−7 mm/s to 2.5 × 10−8 mm/s when Kmax decreases from 36 to 15 MPa·m0.5. Then, both constant amplitude and variable amplitude loading scenarios with the Kmax = 15 MPa·m0.5 were applied to pre-cracked CT specimens. It was observed that low R-ratio constant amplitude cycles yield to highest crack growth rate (3.6 × 10−7 mm/s), which was one order of magnitude higher than other waveforms. However, comparing the intergranular crack advancement per block resulted in similar crack growth rates for those waveforms containing low R-ratio cycles. These results imply that stage I of crack growth is assisted by fatigue due to low R-ratio cycles. It was observed that loading/unloading frequency of low R-ratio cycles has a direct relation with crack growth rate at stage I, i.e., high frequency cycles accelerate onset of stage II. The implication of these results for pipeline operator is that pressure fluctuation, particularly large and rapid pressure fluctuation at the sites susceptible to HpHSCC, threatens the pipeline integrity. Avoiding such pressure fluctuations, if possible, increase pipeline lifespan and prevents catastrophic damages by intergranular stress corrosion crack growth through delaying the onset of stage II of HpHSCC crack growth.