Corrosion behavior of nickel-based alloys in supercritical water oxidationsystems

There is a need to destroy both military and civilian hazardous waste and a n urgency, mandated by public concern over traditional waste handling metho dologies, 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 cri tical point (374 degreesC and 22.4 MPa for pure water), this technology pro vides rapid and complete oxidation with high destruction efficiencies at ty pical operating temperatures. Nevertheless, corrosion of the materials of f abrication is a serious concern. While Ni and Ni-based alloys are generally considered important for severe service applications, results from laborat ory and pilot-scale SCWO systems presently in operation indicate that they will not withstand some aggressive feeds. Significant weight loss and local ized 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 th e potential for corrosion control by judicious feed modification. Various a lloys were exposed in a SCWO system at 600 degreesC for 66.2 h. After expos ure, samples were coated with a thick outer salt layer and an inner oxide l ayer. It is considered likely that, at the high supercritical temperature e mployed during this test, the salt was molten and contained a substantial q uantity of gas. The inner oxide layer revealed the presence of numerous def ects and a thickness that is proportional to the corrosion rate determined by mass loss, suggesting the oxide layer is nonprotective. Of the alloys te sted, 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. The se experiments corroborate previous results from a failure analysis for C-2 76, suggesting a corrosion maximum in the subcritical region.