Investigation on the Corrosion Behavior of Lean Duplex Stainless Steel 2404 after Aging within the 650-850 °C Temperature Range

This paper reports the effects of thermal aging between 650 and 850 degrees C on the localized corrosion behavior of lean duplex stainless steel (LDSS 2404). Critical pitting temperature (CPT) and double loop electrochemical potentiokinetic reactivation (DL-EPR) tests were performed. The localization of pitting attack and intergranular corrosion (IGC) attack after DL-EPR was investigated by optical (OM) and scanning electron microscopy (SEM) and by focused ion beam (FIB) coupled to SEM. Thermal aging caused the precipitation of mainly chromium nitrides at grain boundaries. Aging at 650 degrees C or short aging times (5 min) at 750 degrees C caused nitride precipitation mainly at alpha/alpha grain boundaries as a result of fast diffusion of chromium in this phase. Aging at 850 degrees C or aging times from 10 to 60 min at 750 degrees C also allowed the precipitation at the alpha/gamma interface. Nitrides at gamma/gamma grain boundaries were observed rarely and only after long aging times (60 min) at 850 degrees C. Electrochemical tests showed that in as-received samples, pitting attack only affected the alpha phase. Conversely, in aged samples, pitting and IGC attack were detected close to nitrides in correspondence of alpha/alpha and alpha/gamma grain boundaries depending on aging temperatures and times.