Influence of sigma-phase formation on the localized corrosion behavior of a duplex stainless steel

Because of their austenitic-ferritic microstructures, duplex stainless steels offer a good combination of mechanical and corrosion resistance properties. However, heat treatments can lower the mechanical strength of these stainless steels as well as render them susceptible to intergranular corrosion (IGC) and pitting corrosion. In this study, a low-carbon (0.02%) duplex stainless steel is subjected to various heat treatments at 450 to 950 degrees C for 30 min to 10 h, Tbe heat-treated samples then undergo ASTM IGC and pitting corrosion tests, and the results are correlated with the microstructures obtained after each heat treatment. In the absence of Cr23C6 precipitation, sigma-phase precipitates render this duplex stainless steel susceptible to IGC and pitting corrosion. Even submicroscopic sigma-phase precipitates are deleterious for IGC resistance, Longer-duration heat treatments (at 750 to 850 degrees C) induce chromium diffusion to replenish the chromium-depleted regions around the sigma-phase precipitates and improve IGC resistance; pitting resistance, however, is not fully restored. Various mechanisms of sigma-phase formation are discussed to show that regions adjacent to sigma-phase are depleted of chromium and molybdenum. The effect of chemical composition (pitting resistance equivalent) on the pitting resistance of various stainless steels is also noted.