Influence of sigma-phase formation on the localized corrosion behavior of a duplex stainless steel |
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Authors: | K M Adhe V Kain K Madangopal H S Gadiyar |
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Affiliation: | (1) Metallurgy Division, Bhabha Atomic Research Centre, 400 085 Trombay, Bombay, India |
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Abstract: | 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 °C for 30 min to 10 h. The 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, σ-phase precipitates render this duplex stainless steel susceptible to IGC and pitting corrosion. Even submicroscopic
σ-phase precipitates are deleterious for IGC resistance. Longer-duration heat treatments (at 750 to 850 °C) induce chromium
diffusion to replenish the chromium-depleted regions around the σ-phase precipitates and improve IGC resistance; pitting resistance,
however, is not fully restored. Various mechanisms of σ-phase formation are discussed to show that regions adjacent to σ-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. |
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Keywords: | anodic polarization duplex stainless steel intergranular corrosion pitting corrosion sigma phase |
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