Effects of Loading Spectra on High pH Crack Growth Behavior of X65 Pipeline Steel

Abstract

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.

Crack Growth Sensitivity to the Magnitude and Frequency of Load Fluctuation in Stage 1b of High-pH Stress Corrosion Cracking

“…The authors' previous work has demonstrated intergranular cracking under the test conditions using potentiodynamic polarization tests at two scan rates. 30 The corrosion fatigue tests started 24 h after setting up the test cell to allow the cell to reach equilibrium in temperature and potential. It is worth mentioning that a black film covered the side faces the CT specimens after 24 h of exposure to the test environment.…”

“…Second, the differences between the fast and slow scan rate must be greater than one order of magnitude. 28,30,35,[41][42] According to Figure 4, the HpHSCC potential range is between −625 mV SCE and −575 mV SCE .…”

“…3,[26][27] Thereafter, Stage 2 starts when the mechanical driving force at the crack tip causes sustainable crack growth through the film rupture mechanism. 12,[28][29][30][31][32] The crack growth rate increases significantly in comparison with the previous stage. Therefore, the intergranular crack advances in the pipe's thickness within a relatively short time frame compared to Stage 1b.…”

“…A recently published review paper that studies the chronological crack evaluation during HpHSCC failures 20 showed that much research is devoted to the incubation stage, [33][34][35] Stage 1a, 18,22,24,36 and Stage 2. [28][29][30]32 In contrast, less attention had been paid to Stage 1b of HpHSCC. 20 The development of the bathtub model rests on the assumption that crack initiation and crack coalescence are random.…”

Study of SCC of X70 Steel Immersed in Simulated Soil Solution at Different pH by EIS

“…High pH-SCC usually occurs in a concentrated carbonate/bicarbonate electrolyte resulting in an intergranular cracking mode. The mechanism of high pH SCC is attributed to anodic dissolution resulting from selective dissolution at the grain boundary and repeated rupture of passivating film that form over the crack tip [ 10 ]. On the other hand, the occurrence of near-neutral pH SCC is associated with a special environment characterized with anaerobic, dilute bicarbonate solutions and the stress corrosion crack has a transgranular, quasi-cleavage crack morphology [ 11 , 12 , 13 ].…”