PSI - Issue 42

Mimoun Elboujdaini et al. / Procedia Structural Integrity 42 (2022) 1033–1039 Mimoun Elboujdaini / Structural Integrity Procedia 00 (2019) 000 – 000

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3.5. Complexities in SCC Phenomena SCC in steels for oil and gas pipelines is a very complex and challenging phenomenon. This complexity is reflected in the changes, with time, of the diverse parameters influencing the cracking phenomena, whereas the biggest challenge is in obtaining field-relevant reproducible laboratory data . SCC encompasses major effects from metallurgical, mechanical, and environmental parameters, all of which can be dominant under specific conditions. Adding to the complexity are the loading conditions in operating pipelines that define the mode of failure as SCC or, considering dynamic loading, possibly as corrosion fatigue (CF).

Fig. 4. Development of cracks showing the interaction and coalescence of cracks. (Observations were made on replicas).

Fig. 5. Surface morphology of specimen containing cracks that developed during pipeline operational service. The cracks numbered linked during testing in NS4 solution saturated with 5%CO 2 /N 2 , leading to failure.

4. Summary Many investigators have found that only cyclic loading or dynamic straining , but not constant load , promotes transgranular cracking in pipeline steels exposed to the simulated groundwater that is associated with low-pH SCC in the field. The current research further confirmed the importance of dynamic loading on the cracking process. Although cracks were readily observed to initiate within a few days from corrosion pits in a specimen subjected to a maximum stress of only 60 %YS, and R = 0.6, no cracking was detected in a specimen after exposure for 20 times this period when a higher maximum stress (90% YS) but lower stress fluctuation (R = 0.9) was applied. The cracks not related to surface geometrical discontinuities were detected only after a substantial exposure time (number of load cycles) was applied, which correlated with the maximum stress level at constant R ratio. It is generally true that any cracking process in metallic materials requires a certain amount of plastic deformation, on either a macro or a micro scale, even if the cracking is controlled by a chemical process, such as anodic dissolution.