ESSC & DUPLEX 2019

Duplex stainless steels (DSS) are composed of ferrite and austenite in approximately equal amounts combining properties of both phases. Pitting Resistance Equivalent Number “PREN” is a theoretical way to measure pitting corrosion resistance of steels based on the composition of the material. Nevertheless, in a duplex structure the alloy elements may be partitioned between the phases resulting in different PREN values for α and γ. This work aims to investigate the relation between PREN of α and γ and the pitting potential of UNS S31803 DSS, solution-treated between 1040 and 1150 °C, generating samples with different α and γ fractions and, consequently, different alloy element partition between ferrite and austenite. Thermo-Calc ® thermodynamic simulations were performed to predict the chemical composition of each phase allowing the PREN calculations. Potentiodynamic polarization tests were conducted in 0.6M NaCl solution at 70°C for pitting potential (Epit) determination. It was found that increasing solution-treatment temperature leads to the increase of ferrite volume fraction, as predicted by Thermo-Calc ® . However, as ferrite volume fraction increases, its PREN decreases due to the alloy elements partition. Opposite behavior is observed for austenite, but the increase of PREN is less significant for this phase. The increase of ferrite volume fraction leads to the decrease of Epit, and this may be related to the decrease of Cr and Mo content in α phase that reflects directly in PREN.