Examining Sigma Phase in Super Duplex Stainless Steels (UNS S32750) after Isothermal Aging, Focusing on Its Influence on Etching and Pitting Corrosion with Optical Microscopy

Super duplex stainless steel (SDSS), known for its ferritic–austenitic microstructure and exceptional corrosion resistance, is widely used in petrochemical and marine industries. The impact of sigma (σ) phase formation on the pitting corrosion behavior of isothermally aged SDSS across varying aging times (15, 30, 45, 120, 420 min) at 1000 °C has been investigated in this study. Phase sizes and morphologies, which have evolved over aging time, have been measured after electrochemical etching. Optical microscopy and XRD analysis revealed the evolution of ferrite (α), austenite (γ), and secondary phases (σ and γ₂). As aging time increased, the fractions of γ₂ and σ phases rose at the expense of the α phase. Pitting corrosion tests, conducted according to ASTM-G48 standards, indicated a decline in corrosion resistance with increased secondary phase fraction due to compositional variances, leading to electrochemical potential changes and micro-galvanic cell formation. Prolonged aging resulted in more pronounced Cr- and Mo-depleted areas, increasing the corroded surface fraction and pits volume-to-depth ratio. Correspondingly, the overall corrosion performance was assessed based on pit volume, calculated using optical microscopy, and escalated with aging time, indicating progressive corrosion damage. The study concludes that the σ phase negatively impacts pitting corrosion resistance, requiring careful management of high-temperature applications, welding, and heat treatments to control σ phase formation in SDSS. These insights are crucial for optimizing SDSS in petrochemical and marine industries, enhancing service life and reliability in corrosive environments.