General reviews of electrochemical and corrosion phenomena under conditions associated with supercritical water oxidation

Abstract

Supercritical Water Oxidation (SCWO) is a promising technology for destroying highly toxic organic compounds present in aqueous waste streams. Organic wastes that have been identified as possible targets for destruction by SCWO include EPA-regulated organic wastes, organic components of DOE mixed low-level radioactive wastes, and DOD chemical weapons stockpiles. SCWO capitalizes on the properties of water in the supercritical phase to affect spontaneous and rapid oxidation of hydrocarbons to form CO2, H2O, and, depending on the species of heteroatom present in the organic waste, one or more acids. HCl, H2SO4, and H3PO4 are the acids most frequently encountered in SCWO process streams. The formation of acids in SCWO feeds at high temperatures and pressures under highly oxidizing conditions leads to severe corrosion of the process unit for even the most corrosion resistant constructional alloys. Currently, the existence of a constructional material that can withstand the extremely aggressive conditions present in all sections of the SCWO process stream for all candidate organic wastes is extremely unlikely. Previous attempts to identify such materials have proved unsuccessful. This has led to more fundamental research addressing physical chemistry, electrochemistry, and corrosion phenomena in aqueous systems under hydrothermal conditions. This review addresses this research as it pertains to SCWO technology, and based on these findings, discusses potential methodologies for reducing corrosion damage in SCWQ systems. Currently, it appears that proper selection and/or development of construction materials in conjunction with precise control of feed stream chemistry may be a promising option for corrosion control in SCWO process environments.