Environmental Assisted Cracking Behavior of Sigmatized Super Duplex Stainless Steel in Oilfield Production Brine

The slow strain rate test (SSRT) technique was used to investigate the stress corrosion cracking (SCC) susceptibility of sigmatized super duplex stainless steel (SDSS) grade UNS S32760, exposed to a simulated oilfield brine containing carbon dioxide/hydrogen sulfide (CO₂/H₂S) at 80°C. The sigma phase was precipitated by heat treatment (HT) at 850°C for various durations (0 to 12 min). Results show that low levels (less than 2%) of sigma have no profound impact on the mechanical and corrosion properties of SDSS. However, at higher levels (greater than 2%) of sigma phase, there is severe loss in ductility and increased corrosion rate. Fractographic observations using scanning electron microscopy (SEM) show that the fracture mode shifts from ductile to transgranular cracking with increasing levels of sigma phase. Electron-backscattered diffraction (EBSD) results show that crack nucleation occurs in the sigma grains. The cracking mechanism involves brittle fracture of sigma phase and slip-assisted anodic dissolution in the case of North Sea brine environments. The EBSD strain map reveals that high strains are associated with sigma and ferrite phases, and the crack propagation is associated with plastic strain at the crack tip.