Influence of Dissolved Oxygen on DL-EPR Behavior of SUS329J4L Duplex Stainless Steel after Convection Cooling

抄録

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/cm² solution and simulated small structures exposed to 20 mL/cm² solution, and with almost the same high sensitivity as the immersion corrosion test.