Effect of Mn addition on the precipitation and corrosion behaviour of 22% Cr economical duplex stainless steel after isothermal aging at 800 °C

The microstructural and corrosion behaviour of Fe-22Cr-1.9Ni-2.3Mo-0.2N-xMn low nickel type duplex stainless steel (DSS) with Mn contents ranging from 4.3 to 9.7 wt% were studied after aging treatment at 800 °C lasting from 30 to 930 min. The corrosion behaviour was investigated in 3.5 wt% NaCl solution by measurements of potentiodynamic polarisation and electrochemical impedance spectroscopy. The microstructural analysis shows that the σ precipitates mainly nucleated along γ/δ phase boundaries and grew into the adjacent δ-ferrite by prolonging the aging time, having the same eutectoid structures as commercial high-Ni DSS. At high Mn contents, the growth rate of σ precipitates was initially high with aging times from 30 to 150 min, but decreased gradually up to 930 min with more Mn participation. A Mn content of 4.3 wt% brought only about a 150 mV pitting potential (Eb) drop for the aged specimens, while further addition of Mn significantly lowered Eb at the early stage of aging with more aging-induced σ precipitates. The metastable pit nucleation resistance decreased with Mn content from 4.3 to 6.9 wt% and was suppressed by a higher Mn addition of 9.7 wt% for aging treatments from 150 to 930 min. The pits mainly initiated along δ/γ boundaries and within δ ferrite, then propagated around σ precipitates with increasing aging time, and more Mn gives rise to both the number and size of pits in the early stage of aging treatment. The charge transfer resistance (Rt) depends strongly on the existence of defects caused by the σ precipitates, while increasing Mn content slightly decreases the passive film resistance (Rfilm) in specimens aged from 60 to 930 min.