One of the outstanding problems in understanding near-neutral pH stress corrosion cracking (NNpHSCC) in pipelines is the determination of the factors that cause a very small number of the cracks in some colonies to continue growing while the majority of cracks and colonies become dormant at a depth that is less than 1 mm. To answer this question, we have intensively studied two colonies of surface cracks from an oil pipeline exhibiting what is commonly termed NNpHSCC. The cracks were characterized using optical microscopy and scanning electron microscopy. More than 400 cracks were progressively exposed by serial grinding along the crack surface length (pipe longitudinal direction). The cracks in one colony (pipe 1) were very similar to the cracks in the characterization published earlier on near-neutral cracks in gas pipelines by Chen, et al. These cracks were wide, particularly at the crack mouth, often blunt at the tip, and contained significant amounts of corrosion product. Neither the linced cracks nor the independent cracks showed evidence of growth in the depth direction beyond about 0.5 mm. The effect of crack coalescence on crack propagation, which has been suggested as an important mechanism in preventing dormancy is considered in relation to the linked cracks in this pipe. There was no evidence in this study that coalescence was able to forestall the onset of dormancy. The colony in pipe 2 was found to have longer and deeper cracks, and the cracks were sharp and retained their sharpness even as they grew in depth. The cracks contained no loose corrosion products. This colony is somewhat sparser than the colony in pipe 1 and there was little crack coalescence. While this sparseness may have some effects on the cracking, we do not believe it can explain the different morphology of the cracking. The cracks in this colony appeared to continue to grow. Their shape suggested that growth was being enhanced in depth relative to length as the cracks got deeper. Conditions in these cracks appeared to have rendered the crackflanks passive so that dissolution was only active at the crack tips. The cracks were transgranular unlike high-pH cracks. This appears to be a variant of anaerobic cracking, combining the transgranular nature of near-neutral pH cracking with the passive crack flank surface of high-pH cracking. This combination would provide a mechanism, whereby small cracks are able to continue cracking beyond the dormancy stage that has been commonly observed around depths of 0.5 mm. Surface characterization has not revealed any differences in the surface chemistry so far, and further investigation will be required to develop an understanding of these cracking variations. It is suggested, however, that the surface passivity was responsible for the cracks continuing to grow. If the controlling conditions can be found and controlled, then this holds promise as a means of reducing the risk from NNpHSCC.
Research and cracking implications from an assessment of two variants of near-neutral pH crack colonies in liquid pipelines
Untersuchung der Rissbildung und logische Schlußfolgerung aus einer Abschätzung von zwei pH-neutralen Varianten von Rissanhäufungen in Flüssigkeit-Pipelines
Corrosion, Houston ; 63 , 7 ; 648-660
2007
13 Seiten, 15 Bilder, 2 Tabellen, 42 Quellen
Article (Journal)
English