Corrosion Behavior of Nickel-Based Alloys in Supercritical Water Oxidation Systems

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 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 °C 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.

Corrosion Properties of Candidate Materials in Supercritical Water Oxidation Process

“…Iron based alloys showed a weaker corrosion resistance than nickel based alloys in previous SCWO corrosion studies, especially in the chlorinated solution (38). The 316L alloys showed the highest corrosion rate about 50 mmyr -1 among all alloys, indicating austenitic stainless steel was not suitable for chlorinated feed in SCWO (29). When the test of decomposing organic matter was conducted, it was always found that the carboncontaminated covered the surface of alloys due to the incomplete degradation of organic matter (26,28).…”

“…It is known that the high temperature subcritical water has higher corrosive and solubility than supercritical water. Alloys always get a higher corrosion rate at subcritical condition than supercritical condition and the higher destruction efficiency of organic matter at subcritical condition because of the more dissolved catalytic alloying elements by severe corrosion (16,29,(39)(40)(41)(42)(43)(44)(45)(46)(47)(48). The high pressure 60 MPa could cause intergranular corrosion and higher corrosion rate than 25 MPa by more than two orders of magnitude, indicating the importance of density of water on corrosion behavior of alloys (49).…”

“…However, the acidic solution would lead to NiO instability in high temperature (58,63). The most mass reduction and the most severe morphological change were both observed under subcritical condition (29,30,45,64,65). In contrast to 304L SS, 316L SS and Inconel 690, the oxide on Inconel 625 is too thin to measure in deaerated supercritical water over the temperature range 400 °C to 550 °C (36).…”

“…However, the Inconel 625 also presented unacceptably high corrosion rate 18 mmyr -1 when exposed to oxygenated ammoniacal sulphate solutions in the high-density supercritical region (50). The G30 exhibited the highest corrosion resistance and lowest corrosion rate 5 mmyr -1 among various alloys exposed to an aggressive chlorinated SCWO feed (29). The Cr-base alloy Ducrolloy displayed a reduction of weight loss by a factor of 3 for preoxidized in air before SCWO exposure, however the alloy G30 showed no significant effect of the preoxidation on corrosion rate (66).…”

“…In oxidizing supercritical water, due to its high chromium content, Inconel 625 showed better corrosion resistance than Hastelloy C-276. However, Hastelloy C-276 showed a lower corrosion rate than Inconel 625 in supercritical water containing chloride because of the high molybdenum content (28)(29)(30). In addition, the mechanical strength of Hastelloy C-276 is weaker than that of Inconel 625 on exposure to high temperature condition (T＞550 °C).…”