Effects of Isothermal Aging on the Microstructure Evolution and Pitting Corrosion Resistance of Lean Duplex Stainless Steel UNS S32003

UNS S32003 materials were aged at temperatures in the range of 873 K to 1173 K (600 °C to 900 °C) for 30 minutes and 2 hours. Nitrides were found to precipitate along the ferrite/austenite phase boundaries and within the ferrite phase. The precipitation kinetics was the fastest at 1073 K (800 °C), at which temperature the Cr-depleted zones were observed to be in the vicinity of the precipitates. Precipitation and resultant Cr-depleted zone led to the highest pitting susceptibility of the UNS S32003, aged at 1073 K (800 °C), when tested in 0.6 mol/L NaCl solution. To observe the precipitation evolution, the UNS S32003 specimens were aged at 973 K (700 °C) for different time periods for up to 120 hours. Within aging time up to 24 hours at 973 K (700 °C), only nitrides precipitates were observed and an intermetallic phase was observed after 120 hours of aging along with an increase in the percentage of precipitates. Pitting potential dropped with increasing aging time and metastable pitting rate increased with longer aging time period, both indicating a negative effect of aging on pitting corrosion resistance of the lean duplex stainless steel UNS S32003.