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Pipeline Stress Corrosion Cracking (SCC)

Over 98% of pipelines are buried . No matter how well these pipelines are designed, constructed and protected, once in place they are subjected to environmental abuse, external damage, coating disbondments, inherent mill defects, soil movements/instability and third party damage. In pipelines this occurs due to a combination of appropriate environment, stresses (absolute hoop and/or tensile, fluctuating stress) and material (steel type, amount of inclusions, surface roughness.) (reference)

SCC propensity

There are two types of SCC normally found on pipelines, and known as high pH (9 to 13) and near-neutral pH SCC (5 to 7). The high pH SCC caused numerous failures in USA in the early 1960's and 1970's, whereas near-neutral pH SCC failures were recorded in Canada during the mid 1980's to early 1990's. The SCC failures have continued throughout the world including Australia, Russia, Saudi Arabia, South America and other parts of the world.

Intergranular cracking mode generally represents high pH SCC. A thin oxide layer is formed in the concentrated carbonate-bicarbonate environment, which around the crack surfaces provides protection. However, due to changes in loading or cyclic loading there is crack tip strain resulting in breakage of oxide film. This results in crack extension due to corrosion. Because of such a stringent environmental requirement for SCC initiation, this is not as prevalent as the near-neutral pH SCC. This type of SCC has been primarily noted in gas transmission lines (temperature.)

A cyclical load is critical for crack initiation and growth. There are field data that indicate that with a decreasing stress ratio there is an increased propensity for cracking. Hydrogen is considered a key player in this SCC mechanism, where it reduces the cohesive strength at the crack tip. Attempts have been made to relate soil and drainage type with SCC susceptibility, however limited correlation's have been noted.

There has been no correlation to a clear threshold for SCC initiation or growth. The morphology of the cracks are wide with evidence of substantial corrosion on the crack side wall.

See also: Basics of SCC, Controlling SCC, EL AL crash, Environments & SCC, Pipeline SCC, SCC Guide, SCC of aircraft component, SCC Mechanism, Swimming roof collapse, Testing strategy, Williams explosions