Corrosion characterization of ZrO₂ and TiO₂ ceramic coatings via air plasma spraying on 316 stainless steel in oxygenated sub- and supercritical water

Alloy 316 stainless steel (SS) is an important candidate material for supercritical water oxidation facilities, but severe corrosion negatively affects its life-span and potential application. In this work, ZrO₂ and TiO₂ were coated on the 316 SS substrate by air plasma spraying and then exposed in oxygenated sub- and supercritical water. The results show that the coatings could greatly improve the corrosion resistance of 316 SS in oxygenated sub- and supercritical water in the range from 300 °C to 480 °C at 24 MPa with 3 wt% of oxygen content for 100 h. Increasing temperature accelerated the corrosion of tested TiO₂- and ZrO₂-coated samples. ZrO₂ and TiO₂ coatings could inhibit the outward diffusion of metal elements in the 316 SS substrate and the inward diffusion of oxygen. The oxide film of the ZrO₂-coated sample was composed of Cr₂O₃, Fe₃O₄, Fe₂O₃, NiFe₂O₄ and NiCr₂O₄, while that of the TiO₂-coated sample consisted of Cr₂O₃, Fe₃O₄ and Cr₂MoO₆ at 480 °C. Compared with the ZrO₂ coating, the TiO₂ coating exhibited better protective effects on reducing 316 SS corrosion in oxygenated supercritical water. The TiO₂- coated sample led to lower absolute values of weight changes, more uniform and denser corrosion surface, and better inhibition on the outward diffusion of metal elements (especially Ni) and on the inward diffusion of O. This suggests that the TiO₂ coating on the 316 SS substrate via air plasma spraying is an effective approach to improve 316 SS corrosion resistance in tested oxygenated supercritical water.