PSI - Issue 42

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

1038

6

Surface geometrical discontinuities cause stress concentration, and thus, micro-plastic deformation could occur around these discontinuities if an appropriate combination of the geometry of the discontinuity and stressing condition, external and/or internal, is achieved. It is also possible that hydrogen facilitates the local deformation process. The most effective method of introducing local plasticity without overall yielding is by applying cyclic loading to the material. The electrochemical nature of the steel-environment system confirms the necessity and the importance of a dynamic loading component in causing SCC in low-pH solution. Our investigations have confirmed that, from the potentiodynamic polarisation curves, there is no active-passive transition, and the fast (10 mV/s) and slow (0.1 mV/s) sweep rate curves are virtually coincident from -1.0 to 0.0 V (SCE). The lack of any tendency towards an active passive transition in the system can have a significant impact on crack development since there is no transition to prevent the lateral dissolution of the crack walls. Therefore, the sharpness of a crack depends mainly on factors other than the environmental and metallurgical factors , i.e., mechanical loading, so that more restrictive loading conditions are required compared with systems showing an active-passive transition. For the current system, dissolution and hydrogen evolution occur simultaneously once the material is exposed to the environment; in the meantime, the specimen is subjected to dynamic loading. Those factors all interact, and their relative importance varies in the different stages of crack development. Though a dynamic component is necessary in mechanical loading to promote cracking, the amplitude of this component, reflected by the stress ratio, R, could be smaller (higher R) if a rougher surface, e.g., the original intact pipe surface, is involved, and/or when a crack is well developed . The observed morphology of cracks (Fig. 6) is not different from that produced in pipeline, and similarly, cracks tended to develop at the depressed regions.

Fig. 6. Morphology of cracks in a X-65 specimen exposed to CO 2 saturated distilled water after 1x10

6 cycles at 

max =95% YS, R = 0.6 and

F = 1Hz.

The results also indicate that a large range of crack growth rates can exist for cracks with similar sizes and under the same test conditions. This simply reflects the variation of the local conditions of all the metallurgical (microstructural), environmental and mechanical factors involved and the changes in the local conditions with time. Metallurgical and environmental factors may have more profound roles to play when cracks are small or during the initiation stage. However, when a crack becomes large, the growth rate is controlled by the mechanical loading conditions, and the range of crack growth rates becomes small. Both the maximum stress and the stress amplitude affect cracking behavior, but the stress amplitude has a more significant effect on crack initiation , especially when smooth test surfaces are used. From the viewpoint of pipeline operation, a cost-effective approach to SCC control is to develop methods to stabilize the pressure, thereby reducing the amplitude of pressure fluctuation. 4.1. The implications of the results: In experiments, cracks developed from specimens with the original linepipe surface at the cyclic stress ratio, R= 0.6 in NS4 solution saturated either with pure CO 2 or 5%CO 2 /N 2 when a sufficient number of load cycles was applied. In subsequent test intervals, higher stress ratios (R): 0.7, 0.75, 0.8 and 0.85, were employed and all other test conditions