Corrosion studies in supercritical water oxidation systems

Supercritical water oxidation (SCWO) is a technology which can effectively destroy various civilian and military wastes, including nerve gas, by oxidation in water at high temperature and pressure. Although, as a technology SCWO can destroy such wastes, the process must be carried out in a reactor which will withstand not only the high temperature and pressure conditions but also a very corrosive environment. As process conditions are severe, corrosion could, ultimately, be the limiting factor in the useful application of this technique; nevertheless, little information relevant to degradation within such systems is available in the current literature. Results from laboratory and pilot-scale SCWO systems presently in operation indicate that wastage (accelerated general corrosion), pitting and stress corrosion cracking (SCC) are corrosion phenomena likely to be encountered in the various sections of SCWO systems. In order to increase our understanding of corrosion phenomena within such systems, systematic testing employing both exposure and electrochemical experiments will be carried out. Exposure samples are monitored for weight loss and subsequently examined to provide comprehensive information on both the rate and form of degradation. Additionally, in order to provide a fundamental understanding of degradation phenomena, an elevated temperature electrochemical facility has been constructed. This incorporates an Inconel-625 vessel designed to permit standard d.c. techniques as well as electrochemical impedance spectroscopy (EIS). Finally, results from the analysis of failures which have occurred within other SCWO systems have been employed to provide a more comprehensive understanding of potential failure modes.