Early Corrosion Characterization of 316 Stainless Steel (Passivated in Supercritical Water with Na₃PO₄) in Subcritical Water Containing Oxygen and NaCl

Corrosion has severely hindered the development and application of high-temperature hydrothermal oxidation technologies for disposing of chloride-containing organic wastewater. This study explored the early corrosion behaviors of 316 stainless steel (316 SS) (UNS S31600), passivated by sodium phosphate (Na₃PO₄) in supercritical water in advance, in subcritical water (at 100, 160, 230, and 300 °C and 25 MPa) having high contents of oxygen (O₂) and sodium chloride (NaCl) for 50 h. The results show that the corrosion rate of the phosphate-passivated 316 SS was approximately half of that of the passivated 316 SS (pretreated in supercritical water without Na₃PO₄) under the same conditions. The surface oxides of the phosphate-passivated 316 SS consisted of Cr₂O₃, Fe₂O₃, CrPO₄, FePO₄, and Ni₃(PO₄)₂. The passivation pretreatment of 316 SS in supercritical water with Na₃PO₄ could form a stable and protective oxide film containing phosphates. These phosphates significantly improved the corrosion resistance of 316 SS in subcritical water (containing oxygen and chloride) by preventing corrosive substances from contacting the alloy substrate.