Leak failure at the TP316L welds of a water pipe caused by microbiologically influenced corrosion

Abstract

Water leaking through a pinhole from an ASTM A312-TP316L stainless steel pipe was investigated. The pipes had been used for transferring a large amount of potable water. Multiple leaks were found at the circumferential welds of the pipe after a short term of service. An analysis using an optical microscope, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) was conducted to explore the root cause of the failure.The results demonstrated that the perforation of the pipe was due to localized corrosion initiated from the inside at the many small pits penetrating to the outside of the pipe. Corrosion pits were located near the heat-affected zone (HAZ), which is the area most susceptible to a corrosion attack. The microbiologically influenced corrosion (MIC) was identified as the primary cause of perforation failure. Corrosion attacks occurred predominantly at the microstructure of ferrite stringers. Spheroidal corpuscles were observed with a high concentration of sulfur, which indicated that sulfate-reducing bacteria (SRB) were creating a biofilm, accelerating the MIC corrosion process. Slightly concave surfaces near the weld of the pipe exhibited evidence of bacterial attack or residue of microorganisms, where the leak occurred.