Role of Mn on σ-phase evolution and corrosion degradation of 22-23% Cr ultra-low Ni low-Mo duplex stainless steel during 800 °C aging

To optimize low-nickel duplex stainless steel (DSS) composition by reducing high-temperature precipitates and improving post-thermomechanical corrosion resistance, the effect of Mn content (8.39–16.79 wt%) on σ-phase precipitation and corrosion behavior was systematically examined in a 22.5% Cr ultra-low Ni–low Mo DSS during 800 °C aging. Results show that the combination of a relatively low Mn content (8.39%) and low Mo content slowed σ-phase precipitation, achieving kinetics close to 2205 DSS. Extending aging from 1.5 to 30 hours reduced the pitting potential (E b ) difference between the 8.39% Mn alloy and 2205 DSS from ∼600 mV to 200 mV, indicating increased σ-phase dominance. Passive film thickness and polarization resistance declined linearly with aging across all Mn levels. Early degradation was due to Mn-induced film loosening and Cr-depleted zone expansion, gradually lowering E b in 8.39% and 10.95% Mn DSS. Prolonged aging enhanced σ-phase formation, raising n-type iron oxide content while reducing protective Cr 3+ and Mo 6+ concentrations and increasing Fe 2+ /Mn 2+ levels and donor/acceptor densities (ND/NA), collectively impairing film stability. Although 8.39% Mn DSS maintained intergranular corrosion resistance (IGC) comparable to 2205 DSS, its lower E b resulted from decreased Crox/Crhy and Mo 6+ /Mo 4+ ratios. In contrast, higher Mn contents accelerated σ-phase precipitation, expanded Cr-depleted zones, and significantly degraded IGC resistance.