Characteristics of scalloped surface and its relation to FAC rate of carbon steel piping elbow

Scallop pattern is widely recognized as a typical morphology on the corroded surface by high flow-accelerated corrosion (FAC) rate in single-phase flow. Although turbulence flow condition could be closely related to scalloped appearance, the detailed process of scallop development is not fully known. In this study, detailed characterization of scalloped appearance (scallop size, depth, and density) and oxide layer (nano-void structure, thickness, and compositional profile) on the inner surface of the carbon steel piping elbow used in a fossil power plant have been carried out. The extrados of the piping elbow showed maximum FAC rate (approximately 0.25 mm/year). On the other hand, the intrados of that showed no FAC. The FAC rate lineally decreased from the center of extrados to intrados side, while the scallop patterns were observed in a limited region on the extrados. Approximately half of the maximum FAC rate was shown even on the smooth surface. It has been notable that there was no significant increase of the FAC rate by scallop formation although the local turbulence was increased by hydrodynamic effect due to scallop surface. The relationship among scallop size, depth, and density obtained in this study indicated that the occurrence of overlapping horse-shoe shaped pits that give a scalloped appearance. The surface morphologies under FAC condition is considered to essentially depend on the nucleation density of pits. Nanometer-scaled thin porous oxide layer was formed on both extrados and intrados, and there was no significant difference in nano-void structure, thickness, and crystalline structure obtained by transmission electron microscope characterizations despite a significant difference in the FAC rate. Since no Cr enrichment was observed in the oxide layers formed on the both sides, FAC suppression by Cr content of carbon steels could be negligible in this condition.