Effect of α/γ Phase Ratio and N Addition on the Corrosion Resistance of Si-bearing Duplex Stainless Steel in Nitric Acid

Duplex stainless steel is widely used in various service conditions due to its high mechanical properties and high corrosion resistance. However, the corrosion resistance of the duplex stainless steel in highly oxidizing nitric acid has not been made clear yet. In this study, the effect of Si content ( Si = 04%), γ phase ratio ( γ=2100%) and N addition (N=0.1%) on the corrosion resistance of 23 25Cr duplex stainless steel in 8N HNO3 + 0.2 g/1Cr6+ boiling solution were investigated with TIG welded joint specimen. An intergranular corrosion depth and corrosion rate were measured after the corrosion test of 48 h×5 periods. And then a relationship between their microstructures and configuration of the corrosion attack was considered.Si addition ( Si>3%) to the duplex stainless steels was effective to improve the corrosion resistance. The steel without N, however, was attacked at HAZ in case of γ<10% and γ>40%, because of α phase sensitization and σ phase formation, respectively.N content affected α/γ ratio, nitride and carbide precipitation. As for the adequate ratio of α/γ for the resistance of intergranular corrosion of the duplex steel, obtained value experimentally was more than 50% in γ phase, in the case of 25Cr-Ni-4Si-N steel.