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

Abstract

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.