EFFECTS OF ALPHA/BETA 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, there have been few studies on the corrosion resistance of duplex stainless steel in highly oxidizing nitric acid, such as hot nitric acid containing Cr6+ ions. In this study, the effect of Si content (Si = 0-4 %), austenitic phase (gamma) ratio in volume and N addition (N = 0.1 %) on the corrosion resistance of 23-25 Cr duplex stainless steel was investigated. Corrosion test was carried out in boiling 8N HNO3 + 0.2 g parallel-to Cr6+ using TIG welded joint specimens. The relationship between microstructures and configuration of the corrosion attack was analyzed. Results show that Si additon (Si > 3 %) to duplex stainless steels is effective in improving corrosion resistance. Si-bearing duplex stainless steel without addition of N, however, was attacked at HAZ when gamma ratio was less than 10% and more than 40 %, because of alpha phase sensitization and alpha phase formation, respectively. Nitrogen content affected gamma ratio, nitride and carbide precipitation. A gamma ratio more than 50 % was found to be most beneficial for the resistance to intergranular corrosion of the Cr-25-Ni-4Si-N duplex stainless steel.