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When pipelines fail by near-neutral pH SCC (NNpHSCC) there are usually
quantities of tiny blunt cracks, frequently in crack colonies, near these failures.
These small cracks are usually dormant and innocuous. The critical step in
NNpHSCC failures is the process by which these cracks are able to grow in depth
beyond 0.5 mm. This mechanism is far from being understood. This paper
reviews our experiments to help understand the mechanisms of pit-to-crack
transition and early growth from the blunt cracks.
Specimens were pitted using techniques developed in our laboratories and then
cyclically loaded in near-neutral pH environment sparged with 5% CO2 / balance
N2 gas mixture at stress ratios, strain rates and frequencies which were close to
those experienced during oil pipeline operations. The conditions under which the
pits become blunt cracks and under which those blunt cracks can sharpen and
grow beyond 0.5 mm are reported. The changes are related to the plastic
deformation near the pits for blunt cracks and the ability of the crack to
concentrate hydrogen near the crack tip for crack sharpening.