Effects of thiosulfate in artificial seawater on corrosion behaviour of 25Cr-3Ni-7Mn-0.66N new duplex stainless steel

Abstract

In this research, the 25Cr-3Ni-7Mn-0.66N new duplex stainless steel (DSS) was fabricated by the vacuum arc re-melting (VAR) process, then deformed by hot-forging process, and subsequently heat-treated at 1250°C for 1 hr + water quenched + 1050°C for 1.5 hr + water quenched., based on the calculation of the phase diagram by Thermo-Calc software. The material characterisations and chemical compositions were examined by various methods. The corrosion behaviours of the new duplex in artificial seawater (ASW) after ASTM D1141 and in artificial seawater mixing with thiosulfate was studied at 25°C and compared with that of the 2205 standard DSS by electrochemical methods. The final microstructures are ferrite and austenite. The phase ratio of ferrite to austenite is 59:41. The secondary phases were not found in the new duplex grade. The electrochemical test results show that the corrosion behaviours of the new duplex grade were better than that of 2205 DSS in the ASW, but it was reversed in the acidified ASW mixing with thiosulfate. According to the Polarisation and Electrochemical Impedance Spectroscopy test results, it is found that the corrosion behaviours on both duplex grades were more deteriorated when increasing the thiosulfate concentration in the acidified ASW mixing with thiosulfate. The pitting resistance of ferrite is lower than that of austenite since the pit exhibits in ferrite. It is confirmed by the PREN16 calculation results of ferrite and austenite. The Mott-Schottky analysis showed the bi-layer structure of the passive film in both the ASW and the acidified ASW mixing with thiosulfate. The passive film structure consists of the p-type semiconductor inner layer and the n-type semiconductor outer layer. In the ASW, the densities of cation and anion vacancies in the inner and outer layer of new duplex steel are lower than that of 2205 DSS, respectively. However, in the acidified ASW mixing with thiosulfate, the densities of cation and anion vacancies in the inner and outer layer of new duplex steel are higher than that of 2205 DSS, respectively. The corrosion behaviours of two duplex grades in both the ASW and the acidified ASW mixing with thiosulfate were discussed based on the Pourbaix diagram and the Point Defect Model (PDM).