Corrosion behavior of nickel-based alloys in supercritical water oxidation systems

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 degreesC and 22.4 MPa 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 Ni and Ni-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 and dealloying, are seen in some environments. Although exotic liners such as platinum are currently promoted as a solution to aggressive conditions, some evidence suggests the potential for corrosion control by judicious feed modification. Various alloys were exposed in a SCWO system at 600 degreesC for 66.2 h. After exposure, samples were coated with a thick outer salt layer and an inner oxide layer. It is considered likely that, at the high supercritical temperature employed during this test, the salt was molten and contained a substantial quantity of gas. The inner oxide layer revealed the presence of numerous defects and a thickness that is proportional to the corrosion rate determined by mass loss, suggesting the oxide layer is nonprotective. Of the alloys tested, G-30 exhibited the highest corrosion resistance. Experiments in which a C-276 tube was instrumented with thermocouples and exposed to a HCl feed indicate for this simple non-salt-forming influent that there is a strong correlation between temperature and the extent and form of corrosion, with the most pronounced degradation being at high subcritical temperatures. These experiments corroborate previous results from a failure analysis for C-276, suggesting a corrosion maximum in the subcritical region.