This paper explores the initiation of stress-corrosion cracking (SCC) on the exterior of gas transmission piping. Initiation is taken here to include processes leading to the formation of a crack-like feature as well as early growth. Initiation is characterized in terms of the microstructural factors and the mechanics conditions that influence initiation and subsequent growth.
Field and laboratory data are introduced to identify features typical of, but somewhat unique, to SCC initiation on gas-transmission pipelines. It is shown that the cracking behavior can be grouped as a function of the crack spacing perpendicular to the maximum principal stress. Patches of cracks are defined as sparse or dense in terms of this spacing, with a circumferential crack spacing on the order of 0.2x (wall thickness) separating these two cracking patterns. These results show that cracks with dense spacings tend to dormancy whereas the sparse spacings continue to grow.
Fracture mechanics analysis is used to rationalize the field cracking patterns. It is shown that compliance changes due to the presence of adjacent crack tips account for the field cracking behavior. The results show that initiation patterns, which lead to closely spaced crack arrays, will tend to a dormant state as the cracks grow, because this growth leads to shielding of the tips from the service stresses. In contrast, growth can continue in arrays where the conditions at initiation give rise to sparse crack spacings. In addition, the fracture mechanics results are used to evaluate the suitability of the tapered-tension test -- a commonly used procedure to characterize SCC initiation. Laboratory and modeling results are shown to be consistent with the field behavior.