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Vol.9 No.1 AASP16 (122)-AA123-124 GA21 NT83-84

Academic Articles

Regular Paper

Vol.9 No.1 (2017) p.26 - p.32

Characterization of the Corroded Surface Morphology of a Carbon Steel Piping Elbow Affected by Flow-Accelerated Corrosion

Hiroshi ABE^1,*, Yutaka WATANABE¹, Makoto NAKASHIMA², and Tadashi TATSUKI²

¹ Graduate School of Engineering, Tohoku University, 6-6-01-2, Aoba, Aramaki, Aoba, Sendai Miyagi, 980-8579, Japan
² Research and Development Center, Tohoku Electric Power, 7-2-1, Nakayama, Aoba, Sendai, Miyagi, 981-0952, Japan

Abstract

It has been widely recognized that the inner surface of carbon steel piping affected by relatively severe flow-accelerated corrosion (FAC) shows typical morphology in terms of roughness in many cases. Although hydrodynamic parameters would determine the characteristics of the rough surface developed in such cases, the detailed mechanism of the roughness formation is not fully known. In this study, the local FAC rate profile and morphology in terms of surface roughness for an entire 90-degree carbon steel piping elbow used in a fossil power plant have been investigated under two-phase flow in order to discuss the relationships between the FAC rate and characteristics of the inner surface morphology. For the axial direction, the FAC rate gradually increased with distance from the inlet, and it reaches the peak value in the downstream side, beyond which, the value kept decreasing slightly. Distributed horse-shoe pits and scalloped patterns were observed on the extrados side. The finding suggests that the extrados side of the pipe wall was covered with a continuous liquid film (single-phase). The result shows the possibility of the occurrence of FAC by a single-phase flow in the area where there is a sudden change in flow regime, even when the nominal phase state of the fluid is a two-phase flow regime. Both the local FAC rate and scallop coverage increase steeply near the middle parts of the elbow. There was a good correlation between the two, but it was not a one-to-one relationship because a significant decrease in FAC rate was observed in the downstream side; in contrast, the entire surface was covered with a fine scalloped pattern. The change in the morphology in terms of surface roughness clearly indicated that the overlapping of horse-shoe shaped pits gave a scalloped appearance. The results revealed that the inner surface shape affected by FAC is considered to essentially depend on the number density of nucleation sites for pit formation.

Keywords

Pipe wall thinning, Flow-accelerated corrosion, Turbulence, Surface roughness, Scallop pattern, Horse-shoe pit, Carbon steel, Piping elbow, Fossil power plant

Full Paper: PDF EJAM Vol.9 No.1 pp.21-27 "Characterization of the Corroded Surface Morphology of a Carbon Steel Piping Elbow Affected by Flow-Accelerated Corrosion"

Article Information
Article history:
Received 12 October 2016
Accepted 29 May 2017


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