This research has focused on the effect of surface roughness and surface scratch orientation on the development of neutral pH SCC on pipeline steels. The susceptibility to neutral pH SCC was assessed in this study using slow strain rate testing on X-65 line pipe steel. The surfaces of the test samples were ground using either #240 or #600 sandpaper to introduce different scratch roughness. Scratches were produced with an orientation parallel, perpendicular and inclined at 45° to the loading direction, respectively. The test samples were exposed to a synthetic neutral pH ground water at both open circuit potential (OCP) and −800 mV (SCE).
It has been found that the reduction in ductility due to a near neutral pH environment was more than 20% larger for the specimens with perpendicular scratches than those with parallel scratches, either at OCP or −800 mV (SCE). Roughness appeared to have little effect on the ductility of the specimen with parallel scratches. However, it has some effect on the sample with perpendicular scratches. For these samples, a finer scratch caused more reduction in ductility than the rougher scratches, particularly under cathodically protected conditions. The reduction in ductility for the specimen with scratches 45° inclined to stress axis was in between those with parallel and perpendicular scratches.
The difference in ductility arising from scratch roughness and orientation was consistent with the observation of the surface conditions after test. For samples with perpendicular and angle scratches, cracks were seen to coincide with scratch lines. For those with parallel scratches, only short cracks developed in a direction approximately 45° to the stress axis. Mechanisms concerning this scratch-facilitated crack formation are discussed.
