Abstract
There is a need to destroy both military and civilian hazardous waste and an urgency, mandated by public concern over traditional waste handling methodologies, to identify safe and efficient alternative technologies. One very effective process for the destruction of such waste is supercritical water oxidation (SCWO). By capitalizing on the properties of water above its critical point, (374°C and 221 atm for pure water), this technology provides rapid and complete oxidation with high destruction efficiencies at typical operating temperatures. Nevertheless, corrosion of the materials of fabrication is a serious concern. While nickel and nickel-based alloys are generally considered important for severe service applications, results from laboratory and pilot-scale SCWO systems presently in operation indicate that they will not withstand some aggressive feeds. Significant weight loss and localized effects, including stress corrosion cracking (SCC) and dealloying, are seen in chlorinated environments. Ultimately, the practicality of SCWO may be limited by the ability to mitigate or predict corrosion damage. This paper assesses the recurring failure of Alloy C-276 preheater tubes that displayed cracking during SCWO experiments employing dilute methylate chloride feed solutions. An attempt has been made to correlate the failure mode to the results generated during a controlled experiment employing a nominally identical Alloy C-276 tube instrumented with thermocouples. The results reveal the importance of both stress and temperature and indicate a restricted temperature region within which corrosion is most severe.
