L52090 Near-Neutral pH SCC - Dormancy and Re-Initiation of Stress Corrosion Cracks

Abstract

The objective of this research project was to identify the environmental, metallurgical, and mechanical conditions that lead to dormancy and re-initiation of previously dormant stress corrosion cracks. These parameters would assist pipeline operators in mitigating near-neutral pH SCC on their systems, and allocating resources for pipeline maintenance. Unload-reload transients were found to increase the crack growth rate in the majority of the experiments and to re-initiate dormant stress corrosion cracks in several cases. On the other hand, there was no consistent effect of the presence or magnitude of overloads on crack growth behavior. The simulated hydrostatic tests had relatively little effect on the crack velocities for near dormant conditions, but consistently inhibited subsequent crack growth for actively growing cracks. The results of analyses of the data and modeling suggest that the R ratio (ratio of minimum to maximum pressure) and frequency of pressure fluctuations on an operating pipeline can be used as a tool to rank segments of pipelines base on the mechanical driving force for propagation of near neutral pH SCC. The concept of a critical crack tip strain rate, which can be related to the R ratio and frequency, also can be used to reasonably predict whether a given set of loading conditions will lead to dormancy.