Failure analysis of electric-heater tube for heat-storage tank

This paper presents an investigation of a failure of the 316L stainless steel electric-heater tube in a heat-storage tank. Visual examination was performed to discover the characteristics of the fracture zone. The fracture appearance and the ingredient analysis of the filler material were examined using scanning electron microscopy and X-ray diffraction. Also, the region that is susceptible to corrosion was identified using an electrochemical method. From those investigations, the surface defects that were caused by improper die finish during drawing process acted as the initiation points of the corrosion process. The influent water changes the filler material from magnesia to magnesium hydroxide, which causes the volume expansion. The failure of tube occurred due to the combination of the pitting corrosion on the external surface and internal pressure from the volume expansion of the filler.