Electrochemical Potentiokinetic Measurement of Phase Decomposition in Duplex Stainless Steel by Aging

Duplex stainless steel SUS 329J2L is degraded in mechanical properties and corrosion resistance when aged in the temperature range between 400 and 500°C. This degradation is caused by the decomposition of ferritic-phase into Cr-rich and Fe-rich phases. The decomposition during aging is characterized by Mossbauer spectroscopy and electrochemical potentiokinetic reactivation (EPR) measurement. Mossbauer spectroscopy measurement shows that the spinodal decomposition occurs in the ferritic-phase at the early stage of aging at 450°C. Clusters of Cr-rich and Fe-rich regions are formed when aged for 1-3 hours and begin to aggregate after 300 hours. The activation peak potential shifts to more noble direction and the activation peak current density increases with aging time after aging for 30 hours. Reactivation peak appear after aging for 300 hours. This leads us to the conclusion that the activation peak potential and peak current density are more sensitive to the microscopic fluctuation of Cr content even in the early stage of phase decomposition compared with the passivation and reactivation behavior. Because this may be that the dissolution behavior more precisely reflects the degree of Cr-depletion than the passivation and reactivation behavior, which is affected by the size of the Cr-depleted region as well as the degree of the Cr-depletion, as suggested by one of the present authors.