2 can only predict the occurrence of minor SCC rather than significant SCC. The mechanisms for NNpHSCC are not yet understood, and further research in this area is needed. Generally, the main part of pipeline life is consumed in the crack initiation process. Evidence from field failures suggests that corrosion pits might be a common site for crack initiation [3]. In laboratory investigation, it has been found that the earliest cracks appeared to initiate at corrosion pits [4-7] that formed around non-metallic inclusions and later cracks grew from corrosion pits that formed randomly on the surface [8-11], and some cracks induced by corrosion pits were related to stress cells caused by the difference of residual stress level over a much large area [12]. These pits may act as stress raisers to initiate cracks. Cracks can also be nucleated around other types of pits associated with metallurgical discontinuities. Wang et. al.[13] indicated that some corrosion pits can be formed preferentially along the heavily deformed metal in scratches on the surface. It was also reported that preferential corrosion occurs at the boundaries of pearlite colonies, and transgranular crack-like features can grow from such surface attack [14]. So NNpHSCC most commonly originates from corrosion pits. For pipelines in the field, it generally takes years for pits to grow and initiate cracks, and the pit growth may proceed under intermittent exposure conditions. Thus, it would be difficult to study NNpHSCC processes under the conditions close to those in the field. So an accelerating technique to generate pits was employed in this study, the details of which have been reported in another paper [15]. Basically it consists of an acid-immersion treatment to passivate the surface and then a second immersion in dilute hydrochloric acid, which leads to rapid pitting growth at sites where the passive layer is either removed with a needle or at innate weaknesses in the film where pits grow spontaneously. After the transition from pits to cracks has occurred in the field, tiny, elongated blunt cracks, frequently in crack colonies, are often seen in very large numbers [16]. The vast majority of these small cracks are found to become dormant and hence tend to be innocuous. However, if the small cracks can surpass a threshold depth, around 0.5 to 0.6 mm [17], these cracks can be activated and begin to propagate and may eventually lead to pipeline rupture if not detected and removed. So studies concerning the growth of these small cracks and how potential growth can be identified and avoided will contribute significantly to an understanding of NNpHSCC initiation and help in pipeline integrity management. 2. EXPERIMENTAL 2.1. SPECIMENTS X-52 line pipe steel from Enbridge Pipelines Inc. that had exhibited NNpHSCC crack colonies after thirty-four years of service was used in this project. The pipe was 34” (864 mm) in external diameter and 7.9 mm in measured wall thickness. The chemical composition can be seen in Table 1. The microstructure consisted of pearlite and ferrite. It was found that there were very few, if any, inclusions on the axial transverse (A-T) surface, while there were a lot of inclusions on the radial transverse (R-T) and the axial radial (A-R) surfaces [18]. In addition, the inclusions on the R-T surface were relatively smaller than those on the A-R surface. These non-metallic inclusions (MnS) were more abundant on the axial-radial section at mid-wall [18]. Hence, it was likely that the inclusions were elongated along the axial direction during the rolling processes.
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