Single Loop Electrochemical Potentiokinetic Reactivation Behaviour of Continuously Cooled SUS329J4L Duplex Stainless Steel

Abstract

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 H2SO4 + 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.