Weixing Chen

The Effect of Microstructure on Pit-to-Crack Transition and Crack Growth in an X-52 Pipeline Steel in Near-Neutral pH Environment

X-52 pipeline steel specimens that had been pitted using a passivation/immersion technique were cyclically loaded in a near-neutral pH solution sparged with 5% CO2 /balance N2 gas mixture at a peak normal stress of 109% of the yield strength (YS), a stress ratio, R, of 0.8, and a frequency of 0.0001 Hz. Blunt cracks were seen to have initiated from the corrosion pits. There were many more cracks on the radial transverse (R-T) surface than on the axial transverse (A-T) surface. On the R-T surface, there were a lot of non-metallic inclusions particularly at mid-wall in this steel and these resulted in the nucleation of large pits that were particularly prone to pit-to-blunt-crack transition. At higher peak normal stress, 109% of YS, compared to previous studies at a little lower stresses, there was more rapid crack formation on the R-T surface. In the end, the cracks along the large elongated inclusions penetrated into the steel samples and led to failure. The crack path was transgranular in nature and the fracture surface displayed quasi-cleavage features. Analysis revealed that the pit depth to width ratio for individual pits was a little higher than that for linked pits, however, the ratio of crack depth to crack mouth width was observed to be much larger than the ratio for the linked pits. Strong preferential dissolution was believed to be responsible for the pit nucleation from these non-metallic inclusions, with the plastically deformed regions at the pits acting as the anodic phases.Copyright