The Impact of Pressure Fluctuations on the Early Onset of Stage II Growth of High pH Stress Corrosion Crack

Abstract

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.

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

“…A horizontal loading machine was used to apply different loading waveforms to the pinhole-loaded precracked CT specimens. The authors' previous research 39 showed that the precracked CT specimen is at Stage 1b of HpHSCC crack growth when K max is equal to or below 15 MPa p m. Therefore, a K max of 15 MPa p m was selected for all of the loading scenarios in this study. In total, nine constant-amplitude loading waveforms were designed to study the effects of either amplitude or frequency of load fluctuation on Stage 1b of HpHSCC crack growth.…”

“…Thus, this condition can be considered as crack propagation in Stage 1b. 39 Figure 5 depicts the area near the precrack tip on the side faces of the CT specimens tested under three different R-ratios after polishing away the passive film and etching in Nital (5%). The frequency load cycles with an R-ratio of 0.9 were 5 × 10 −2 Hz, whereas the frequencies for R-ratios of 0.7 and 0.5 were 10 −2 Hz.…”

“…[21][22] Metallurgical discontinuities assist the crack initiation by altering the stress state at the crack tip and locally providing the microscale threshold stress. 3,39,[45][46] Accordingly, the pre-existing cracks intensify the stress in the adjacent area and increase the chance for crack initiation. 39 Stage 1b in HpHSCC crack growth, where an intergranular crack exists on the free surface, represents the situation mentioned above.…”

“…3,39,[45][46] Accordingly, the pre-existing cracks intensify the stress in the adjacent area and increase the chance for crack initiation. 39 Stage 1b in HpHSCC crack growth, where an intergranular crack exists on the free surface, represents the situation mentioned above. In the same manner, the fatigue precrack CT specimens used in this study represent the same situation.…”

“…[19][20] It has been shown that the cracks at this stage have a semi-elliptical shape, where the crack length on the surface, 2c is larger than the crack depth, a. 3,[19][20]39 Given that these cracks are in the early stage of growth and given their short length on the surface, it can be estimated that these cracks are shallow. Hence, the stress intensification near these cracks' tips will be relatively small 20 and can be neglected in this study.…”