The Intergranular Corrosion Susceptibility of Metastable Austenitic Cr–Mn–Ni–N–Cu High-Strength Stainless Steel under Various Heat Treatments

The intergranular corrosion (IGC) behavior of a new metastable austenitic Cr–Mn–Ni–N–Cu high-strength stainless steel under various heat treatments was studied. The samples were solution treated at 1050 °C for 30 min and then aged at 600 to 900 °C for 10 to 300 min, respectively. The IGC susceptibility of aged samples was investigated using a double-loop electrochemical potentiokinetic reactivation (DL-EPR) test in a solution of 0.1 M H2SO4 and 0.002 M KSCN and the 10% oxalic acid etch. The surface morphologies of samples were characterized using optical microscopy and the scanning electron microscopy after electrochemical tests. Two time-temperature-sensitization diagrams were plotted based on the DL-EPR test and oxalic acid etching. No IGC and precipitate were observed for samples aged at 600 °C and 900 °C. For samples aged at 650 °C to 750 °C, the IGC susceptibility and the amount of precipitate both increased with the extended aging time. For samples aged at 800 °C and 850 °C, the amount of precipitate increased as the aging time was prolonged. However, only the sample aged at 800 °C for 60 min showed slight intergranular corrosion in the DL-EPR test. The IGC of the Cr–Mn–Ni–N–Cu austenitic stainless steel originated from the precipitation of Cr23C6 and Cr2N at the grain boundaries. The chromium-depleted zones near grain boundaries stood as the corrosion nucleation sites, but the dissolution of the weak area followed a consistent crystallographic orientation along each grain boundary.