Sigma phase generally precipitates in duplex stainless steel during slow cooling, which usually causes loss in both corrosion resistance and fracture toughness. To develop a non-destructive method to detect the sigma phase in duplex stainless steel for in situ use in the quality management of large structures, the double-loop electrochemical potentiokinetic reactivation behaviour of continuously cooled duplex stainless steel SUS329J4L at different cooling rates was investigated under different conditions. The polarisation condition with the solution of 0.5 M H 2 SO 4 + 0.01 M KSCN + 0.5 M NaCl and the potential scanning rate of 0.83 mV s −1 were determined as suitable and sensitive.

Sigma phase precipitates in duplex stainless steel during slow cooling from high temperature, which generally induces great loss in both corrosion resistance and impact toughness. To develop a nondestructive method for detecting the presence of sigma phase in duplex stainless steel for on-site use in the quality management of ready-made large machines, the single loop electrochemical potentiokinetic reactivation (SL-EPR) behaviour of SUS329J4L duplex stainless steel continuously cooled from the solution annealing temperature with various rates by adjusting fluxes of cool nitrogen gas were investigated under several polarization conditions. The reactivation current density of SL-EPR test was used to detect the precipitated σ phase and compared with the ferric chloride test, the modified DL-EPR test and the impact test, as well as observation of the dissolution morphology. As the result, the high reactivation current density in the SL-EPR polarization was obtained on the cooled SUS329J4L steel at WQ ~ 0.02 K/s in solution of 0.5 M H 2 SO 4 + 0.01 M KSCN + 0.5 M NaCl at potential sweep rate of 0.83mV/s. In the polarization, the potential holding at 0.3 V vs. SCE for 300 s before sweeping the potential to negative side is necessary to stabilize the passive film and thus improved the detection sensitivity of sigma phase in duplex stainless steel. As comparison, the modified SL-EPR test which checks the presence of sigma phase has a quasi-equivalent detection sensitivity to impact test on the WQ ~ 0.02 K/s cooled SUS329J4L steel, with higher sensitivity in SL-EPR method than the modified DL-EPR method.

Sigma phase precipitates in duplex stainless steel during slow cooling, degrading the corrosion resistance and the notch toughness. The double loop electrochemical potentiokinetic reactivation (DL-EPR) method in a solution of 0.5 M H2SO4 + 0.01 M KSCN + 0.5 M NaCl is known to be good for detecting sigma phase. However, the influence of dissolved oxygen (DO) in solution had not been elucidated, and on-site application of DL-EPR had not been carried out. In this study, SUS329J4L stainless steel was continuously cooled at different time constants by convection using nitrogen, and its DL-EPR behavior was measured in solution with different DO concentrations. A cathodic current partially appeared at potentials nobler than the corrosion potential due to the reduction of oxygen. Elimination of DO suppressed the cathodic reaction, resulting in adequate sensitivity for detection of precipitated sigma phase. The detection sensitivity was almost the same as that for laboratory samples in 700 mL/cm 2 solution and simulated small structures exposed to 20 mL/cm 2 solution, and with almost the same high sensitivity as the immersion corrosion test.