Failure analysis of a welded stainless-steel piping system with premature pitting

A failure analysis of welded pipes (ASTM A312 Grade TP 316L) in a water treatment plant was done and it is presented in this paper. The premature failure was detected by several water leakages caused by pitting corrosion three months after hydrostatic tests and before transferring the plant to the operator. The leakages were located near the bottom of the circumferential butt welds of the entire piping system. The samples were analyzed using visual inspection, radiographic examination, optical emission spectrometry, ferrite number (FN) measurements, metallographic examination and Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectrometry (EDS). The results showed that the corrosion rate was extremely high compared to other studies available in the literature for austenitic stainless steel and the pits were in the weld metal and the Heat Affected Zone (HAZ) of the circumferential welds. The lack or deficiency of a purging gas during the field welding generated heat-tinted zones around the welds. Visual inspection showed that the pits were preferentially located at heat-tinted zones and they have large sub-surface cavities. The SEM inspection confirmed the presence of structures compatible with sulfate-reducing bacteria and iron-oxidizing bacteria and revealed that the failure was caused by microbiologically-influenced corrosion (MIC). The review of standards and codes used to perform hydrostatic testing of austenitic stainless steel piping systems showed that the water used for hydrostatic tests should have been treated before testing or the surfaces should have been deeply drained and dried. The failure was caused by the water used during the hydrostatic testing since it was not fully drained from the piping system and remained stagnant leading to MIC.