Some factors affecting initiation of stress corrosion cracking of an X80 pipe steel in near-neutral pH environment

Near-neutral pH stress corrosion cracking (NNpHSCC) continues to be a concern for existing high pressure pipelines used to transport oil and gas in Canada. Although several studies have focused on the role of pipe steel microstructure on the initiation and growth of NNpHSCC, most used specimens machined from sub-surface locations that did not preserve the original pipe surface, which is the material that ultimately exposed. In the present work, a series of test specimens were designed to preserve the external pipe surface and allowed shallow 0.05 mm root radius surface notches with depths from 0.1, 0.2 and 0.3 mm to be machined and tested. All specimens were machined in the hoop (transverse) direction from a 1067 mm diameter, 12.5 mm thick X80 pipe. The specimens were subjected to a constant load of 95% of the specified minimum yield strength (SMYS) (equivalent to 80% of the actual pipe hoop yield strength) using proof rings for extended durations, e.g., 110, 220, 440 or 660 days. The results show that there was no apparent SCC developed on the smooth specimens with the original surface even after being tested for up to 660 days. In contrast, SCC were found to have initiated at the machined notches, irrespective of their depth after testing for 220 days. To provide further understanding of specimen design, the same SCC testing conditions were applied to smooth round-bar test specimens machined in the hoop direction of this same pipe close to the external surface and the mid-wall locations. While minor SCC initiation was found in the near surface specimens, significant SCC was observed in the specimens taken from the mid-wall location. This finding suggests that the heterogeneous or variable microstructure through the pipe wall thickness plays a critical role in SCC initiation for the X80 pipe investigated. It also suggests that careful attention must be paid to the design of test specimens as well as the location that they are removed from a test pipe in order to realistically assess the SCC susceptibility of pipe steels.