Corrosion characteristic comparisons of ZrO₂-, TiO₂-coated and uncoated 316 stainless steel samples in supercritical water oxidation of municipal sludge

Supercritical water oxidation (SCWO) can efficiently and harmlessly dispose municipal sludge (MS), but typically faces severe reactor corrosion problems. In this work, 316 stainless steel (316SS) sample surface was processed by atmospheric plasma spraying to form a protective ZrO₂ or TiO₂ coating. The 316SS, ZrO₂/316SS and TiO₂/316SS samples were exposed to a typical MS SCWO environment (at 450 °C, 24 MPa and 1.2 of oxidation coefficient) for 20 h–150 h. The results show that prolonging exposure time increased corrosion and surface coating could inhibit 316SS corrosion. ZrO₂/316SS showed the best corrosion resistance among 316SS, ZrO₂/316SS and TiO₂/316SS, and its maximum corrosion rate was 0.0097 mg·cm⁻²·h^-1. The oxide film of ZrO₂/316SS after 150 h of corrosion exposure consisted of ZrO₂, FeO, Fe₂O₃, NiO and Cr₂O₃, and its inner layer was rich in Cr₂O₃. The ZrO₂ coating was much more stable than the TiO₂ coating, and can be considered as the coating of the 316SS structure material for a MS SCWO reactor.