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 resistanc e properties. However, heat treatments can lower the mechanical streng th of these stainless steels as well as render them susceptible to int ergranular corrosion (IGC) and pitting corrosion. In this study, a low -carbon (0.02%) duplex stainless steel is subjected to various heat tr eatments 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 res ults are correlated with the microstructures obtained after each heat treatment. In the absence of Cr23C6 precipitation, sigma-phase precipi tates render this duplex stainless steel susceptible to IGC and pittin g corrosion. Even submicroscopic sigma-phase precipitates are deleteri ous for IGC resistance, Longer-duration heat treatments (at 750 to 850 degrees C) induce chromium diffusion to replenish the chromium-deplet ed regions around the sigma-phase precipitates and improve IGC resista nce; pitting resistance, however, is not fully restored. Various mecha nisms of sigma-phase formation are discussed to show that regions adja cent to sigma-phase are depleted of chromium and molybdenum. The effec t of chemical composition (pitting resistance equivalent) on the pitti ng resistance of various stainless steels is also noted.