Failure analysis of service exposed austenitic stainless steel pipelines

Abstract

Several leaks appeared in Austenitic Stainless Steel (ASS) pipelines installed for transporting water in a test loop after a few years of operation at Indira Gandhi Centre for Atomic Research, Kalpakkam. The locations of leaks were mostly on pipe fittings like bends, but a few were noticed on the pipe away from the fittings too. This paper presents the results of failure analysis carried out on leaking of pipes and fittings. Investigation carried out include optical as well as scanning electron microscopy, energy dispersive spectroscopy, electron back scattered diffraction analysis and microhardness measurements. In addition, double loop electrochemical potentio-kinetic reactivation experiments were conducted on specimens extracted from the pipe side and fitting side of a weld that was found leaking. Further ASS pipe welds prepared with different surface finish conditions were exposed to the environment of the installed pipeline and surface degradation in these pipe welds were compared to reveal the effect of surface treatment on degradation of the welds. It is found that the sensitization along with residual stress generated during welding facilitated intergranular stress corrosion cracking in pipe fittings made of AISI 304 stainless steel resulting in the leaks observed in the pipe fittings. Cracks initiated from the corrosion pits present near the weld, which most likely would have formed due to improper cleaning given to the weld zone after completion of the weld. Leak observed in the pipe is attributed to the crevice corrosion that progressed from a defect present in the pipe making it grown across the thickness. The defect itself was result of an improper repair by arc welding, of a discontinuity that was found in the pipe. The pipe is produced from sheets by resistance welding and the origin of the discontinuity is the poor joint formation during resistance welding. The paper also gives recommendation on good fabrication practices to be followed so that similar kind of failures could be avoided in future. 2019 Elsevier Ltd