Microstructural Evolution, Sigma Phase Morphology, and Localized Corrosion Behavior in UNS S32750 Superduplex Stainless Steel

The effects of sigma (σ) phase morphology and distribution on the corrosion behavior of superduplex stainless steel (SDSS) were investigated. The aim of this study was to establish the relationship among distinct σ-phase morphologies, chromium redistribution, and the resulting corrosion mechanisms using electrochemical and microstructural characterization techniques. Heat treatments at 700 °C and 800 °C produced distinct σ-phase morphologies and volume fractions. Microstructural characterization was performed using optical microscopy (OM) and scanning electron microscopy (SEM), and corrosion behavior was evaluated using open-circuit potential (OCP), cyclic polarization, and double-loop electrochemical potentiodynamic reactivation (DL-EPR). Statistical analysis of chromium (Cr) distribution was used to link microstructural evolution to corrosion susceptibility. Lamellar σ promotes selective attack between lamellae of σ and secondary austenite due to localized Cr depletion, whereas σ at grain boundaries causes sensitization and intergranular corrosion. Notably, the sample aged at 800 °C for 30 h did not exhibit pitting during cyclic polarization. However, it showed clear signs of sensitization and selective attack in the DL-EPR test, indicating that temperature-driven Cr partitioning increases Cr gradients around σ. These gradients, together with the morphology and fraction of σ, ultimately govern the transition between lamellar-selective corrosion and grain-boundary sensitization. Overall, the findings underscore the importance of combining microstructural and electrochemical assessments to accurately predict localized corrosion and sensitization in SDSS.