Evolution of the sigma phase and its effect on the corrosion resistance of ASTM A890 3A duplex stainless steel

The sigma (σ) phase is a detrimental precipitation that affects the performance and application of duplex stainless steel (DSS). In this study, the evolution of the σ phase and its influence on the corrosion resistance of the ASTM A890 3 A DSS were analyzed quantitatively by aging at 750 °C for 3–96 h for the first time. With increasing aging time, the amount and rate of precipitation of the σ phase significantly changed. The fraction of the σ phase increased from 0.65% to 23.51%, accompanied by a decrease in ferrite content and an increase in austenite content. During the evolution of the σ phase, a combined process of nucleation and growth controlled by the eutectoid reactions in the first 3–24 h and a growth process controlled by diffusion in the subsequent 24–96 h were observed. The deteriorated corrosion resistance after aging was closely related to the fraction and distribution morphology of the σ phase. Under a precipitation fraction of the σ phase between 0% and 0.65%, the passive current density of the DSS increased, the pitting potential decreased. The width of the passivation region between 0.65% and 19.07% of the σ phase narrowed, and instability formed. The critical aging time for the transformation from passive to active DSS was between 48 h, corresponding to a content of the σ phase of 19.07%. The passivation region almost completely disappeared. In addition, the uniformity in distribution of the σ phase also influenced the corrosion resistance.