Resistance to localized corrosion of lean duplex stainless steels after brief thermal treatments

Lean duplex stainless steels are biphasic steels with a low nickel and/or molybdenum content, which represent an interesting and cost effective alternative to the employment of the most common austenitic stainless steels (such as AISI 304 and 316) or traditional more expensive duplex steels (such as DSS 2205). However, during production and welding operations, a relatively brief permanence (a few minutes or tens of minutes) in the critical temperature range between 650 and 950 °C, induces the formation of undesirable secondary phases (mainly chromium and molybdenum carbides and nitrides) which influence the alloy corrosion resistance. This paper compares the pitting corrosion resistance of three lean duplex alloys, LDSS 2101, DSS 2304 and LDSS 2404, before and after heat treatments of 10 min at 650, 750 and 850 °C and analyses the correlated pitting propagation modes. With this aim, critical pitting temperature (CPT) measurements were performed in 0.1 M NaCl solution. The results show that concerning the solubilized alloys their localized corrosion resistance is in good agreement with PRENMn indications. Thermal treatments, and in particular that at 750 °C, reduce the pitting corrosion resistance of the alloys, mainly in the case of LDSS 2404 and 2101, due to the impoverishment in passivating elements (especially chromium and molybdenum) in regions close to precipitates. The heat treated DSS 2304 alloy shows a more limited reduction of its performances in comparison to the solubilized material and its CPT does not vary significantly with the treatment temperature. This phenomenon can be linked to the low nitrogen content of the alloy, which limits the volume of precipitates during heat treatments, allowing to reduce the extent of impoverishment in passivating elements.