11 cracks tips were large enough to trap more hydrogen, leading to hydrogen related growth. At this time, the crack shape became sharp [18], and was significantly different from Stage I cracks. Thus, the cracks in this period were named Stage II cracks. Stage II cracks would grow intermittently, and in the end, they could surpass the critical threshold for long cracks, after which the cracks would grow in a typical corrosion fatigue model. This implied that for pipeline steel in near-neutral pH environment, these different mechanisms of stress corrosion cracking could have occurred within a continuous spectrum, with a gradual transition from one to another as the dominance of corrosive processes was replaced by stress or strain, led readily to the notion that steel microstructure, electrochemistry and stress or strain might interact in a variety of ways, and that the transformation from one mechanism to another might result from complex relation between stress risers, dissolution driven by stress concentrations, inclusions, and trapped hydrogen. This is the first time that different crack types and mechanisms have been observed and reproduced in the laboratory. These different mechanisms are necessary for NNpHSCC crack initiation and early growth. These are quite consistent with the observations in the field. The sharp cracks in Stage II can create a significant risk to pipeline integrity. If they can’t be detected and removed, they will likely keep growing and eventually once the long crack corrosion fatigue threshold is exceeded, cracks will grow much faster, even leading to pipeline rupture or leakage. Compared to the long crack growth stage, the blunt crack growth in Stage I and the sharp crack growth in Stage II consume a majority of pipeline life. So Stages I and II cracks are very important for pipeline integrity management and should be addressed intensively. Unfortunately, so far no research has touched Stage I blunt cracks and little has paid attention to Stage II sharp cracks, while most has focused on long crack growth. For in-service pipelines, removing Stage II cracks that are deeper than 0.5 to 0.6 mm would benefit the pipeline integrity and enhance the remaining life. If Stage I crack growth can be impaired, that would significantly extend the remaining life. If active corrosion can be prevented such as by cathodic protection then smaller cracks are innocuous. Figure 9. Tiny hydrogen produced cracks along the phase boundary between pearlite and ferrite ahead of the main crack. (b) (a)
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