Comparative corrosion performance of super duplex and cast duplex stainless steels in synthetic seawater and 3.5 wt% NaCl

The corrosion behavior of two wrought super duplex stainless steels (SDSS), UNS S32750 and UNS S32760, and a cast duplex stainless steel (DSS), DIN/EN 1.4517, was evaluated in 3.5 wt% NaCl and ASTM D1141 synthetic seawater at 25, 50, and 70 °C. All materials were assessed in their as-received (as-cast/as-rolled) metallurgical condition, without additional homogenization or solution annealing, to determine their baseline stability limits. Both SDSS grades preserved passivity in synthetic seawater up to 70 °C. In NaCl, S32760 maintained resistance to localized attack at 70 °C, whereas S32750 initiated pitting. The cast alloy showed stable pitting from 50 °C in both electrolytes. Impedance values above 5 × 10⁵ Ω·cm² at 25 °C decreased with temperature, most significantly for 1.4517, while corrosion-current densities increased three- to five-fold per 25 °C increment but remained below 2 µA·cm² for S32760 in seawater. Pitting potentials for 1.4517 decreased to approximately −50 mV vs. SCE at 70°C in NaCl solution. Performance differences were strongly linked to microstructure and composition: the wrought SDSS displayed finer inter-austenitic spacing (∼11–12 µm) and higher PREN (>42), whereas the cast DSS exhibited coarser morphology (∼29 µm) and lower PREN (∼38). XPS analysis revealed passive films mainly composed of chromium and iron oxides/hydroxides, with chloride exposure promoting Cr2O3 depletion and hydroxide enrichment, correlating with the observed reduction in impedance and passive range. These findings clarify the corrosion-resistance hierarchy of the studied alloys and support material-selection decisions for subsea equipment, desalination systems, and other components operating in aggressive marine environments.