Alloy 625 is frequently used as reactor material for Supercritical Water Oxidation (SCWO) applications. TMs is due to the favorable combination of mechanical properties, corrosion resistance, price and availability. Nevertheless, the corrosion of alloy 625 like the corrosion of other Ni-base alloys during oxidation of hazardous organic waste containing chloride proceeds too fast and is a major problem in SCWO applications.
In these experiments high pressure, high-temperature resistant tube reactors made of alloy 625 were used as specimens. They were exposed to SCWO conditions, without organics, at temperatures up to 500ºC and pressures up to 37 MPa for up to 150 h. Simultaneously, coupons also made for alloy 625 are exposed inside the test tubes.
The most important corrosion problem for alloy 625 is pitting and intercrystalliie corrosion at temperatures near the critical temperature, i.e. in the preheater and cooling sections of the test tubes. Under certain conditions, stress corrosion cracking appears and leads to premature failure of the test reactors. The corrosion products were insoluble in supercritical water and formed thick layers in the supercritical part of the reactor, Under these layers only minor corrosion occurred.
Supercritical Water Oxidation (SCWO) is a very effective process to destroy hazardous aqueous wastes containing organic contaminants. In the supercritical fluid phase all organics react rapidly and quantitatively with oxygen to form CO2 N2 and H2O. The main target applications in the USA are the destruction of DOD and DOE wastes such as rocket fuels and explosives, warfare agents and organics present in low level radioactive liquid wastes1.The first commercial installation started operation in Texas in 1994 with the oxidation of amines, glycols and long-chain alcohols 2. In Germany and Japan experimental work has been focused on the destruction of industrial aqueous toxic wastes 3,4. Recently organo Corporation, Tokyo, set up a commercial supercritical water oxidation unit in Japnn.
If halogens are absent in the feed, corrosion is under control, But frequently, hazardous wastes containing heteroatoms like CI, S or P are present, which are convened during SCWO to the acids HCI, H2SO4, and H3PO4, respectively. The complex load at high temperatures, pressures and fluid densities from 100 to 1000 kg/m3, on the one hand, and a highly corrosive aqueous environment on the other hand, lead to fast corrosion of the construction materials of SCWO reactors. Even ceramic bulk materials frequently show high corrosion rates under these conditions5. Besides the precipitation of inorganic salts in the supercritical part of the reactor, corrosion is a major problem for the safe operation of SCWO systems.
Chloride is the most important corrosive species in SCWO treatment of wastes. After the reaction, frequently the pH of SCWO solutions is rather low. To keep the chemistry as simple as possible we used hydrochloric acid or sodium chloride to simulate chlorine containing solutions in SCWO environments. First screening tests on the corrosion of pressure tubes in SCWO environments showing high corrosion rates in the temperature transition zones of the tubes have been reported by Boukis et al. 6,7.
