Leakage failure analysis of a 60 m3 LNG storage tank

This work aimed to analyze a leak that occurred in a 60 m3 LNG storage tank made of 304 stainless steel. Optical microscopy (OM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), transmission electron microscope (TEM), and mechanical property and hardness tests were conducted to characterize the failed storage tank. The results show that the crack originates near the circumferential weld of the lower head and propagates perpendicular to the weld. There are dense corrosion pits near the crack paths, and high contents of Cl and S in the crack and pits. Besides this, deformation-induced martensite transformation occurs during the cold spinning process of the head, and the microstructure undergoes embrittlement. In service, the lower head subjected to greater internal pressure, plus the structural stress caused by martensite transformation and the welding residual stress, combined with the action of corrosive medium, stress corrosion cracking (SCC) occurs near the circumferential weld of the lower head. To prevent SCC on storage tank, it is suggested to utilize 304 stainless steel with a low content of delta-ferrite, and apply solution treatment after spin molding of the head of tank.