Steel pipelines undergo the following sequential stages prior to high pH stress corrosion cracking (HpHSCC) failure viz. formation of occluded environment, 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 (K~ISCC~).  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 NaHCO~3~-1N Na~2~CO~3~ at 40˚C and -590 mV~SCE~. The first series of tests were conducted through applying variable amplitude loading waveforms to determine the K value below the K~ISCC~. It was observed the crack growth rate decreases from 1.5×10^-7^ to 2.5×10^-8^ when K~max~ decreases from 36 to 15 MPam^0.5^. Then, both constant amplitude and variable amplitude loading scenarios with the K~max~=15 MPam^0.5^ 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 implying 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 avoiding the internal pressure fluctuation, particularly large and rapid pressure fluctuation at the sites susceptible for HpHSCC. This practice increases pipeline lifespan and prevent cataphoretic damages by intergranular stress corrosion crack growth through delaying the onset of stage II of HpHSCC crack growth. 


The Impact of Pressure Fluctuations on the Early Onset of Stage Ii Growth of High Ph Stress Corrosion Crack

Division / Sponsors: 
Pipeline Systems Division

Paper Proceedings: https://asmedigitalcollection.asme.org/IPC/IPC2020/volume/84447

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