Investigation of failure of steel steam generating evaporator tube involving delamination defects and corrosion

Abstract

A low carbon steel tube burst at a coal powered power station (Orot-Rabin station, Hadera-Israel). The tube was composed of material specified to ASTM A210M-02 Gr. A1. The bursting of the tube caused a steam/water mix to escape through a hole during initial unit activation after maintenance repairs. The tube is a diagonal ribbed steam-generating tube, mainly used as a physical support for a superheater coil assembly. During internal visual inspection using a borescope, it was seen that a whole layer of metal had partially detached from the area around the hole, causing the tube wall to thin considerably prior to failure. Moreover, it was seen that the same area had undergone corrosion at an advanced stage. Optical metallography revealed that the metal layer had detached from the tube's internal surface due to a wide and thick two-dimensional delamination defect present beneath the internal surface, and that a large amount of copper was present near the detached layer's edge. Scanning Electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) verified the existence of an abnormal concentration of copper and zinc at the same location. This led to the final conclusion that the hole in the tube had formed due to a combination of a wide delamination defect left over from production, and severe galvanic corrosion enabling the defect's exposure and delamination, eventually causing wall thinning and failure.