Chemical States in Oxide Films on Stainless Steel Treated in Supercritical Water

Abstract

A preliminary study of the corrosion films of stainless steel SUS304 was carried out by X-ray photoemission spectroscopy for supercritical water having no oxidant or reactant. The corrosion films produced by the supercritical water at 450°C and 50 MPa were compared with those by anodic polarization in H 2 SO 4 aqueous solution. The chemical states in the films were determined by factor analysis of Fe, Cr, and Ni 2p X-ray photoelectron spectra. Factor analysis of the passivated SUS304 surface was successfully carried out using four reference spectra, Fe metal, Fe 3 O 4 , Fe 2 O 3 , and FeOOH, with careful treatment excluding wüstite Fe 1−x O phase from the standpoint of thermochemistry. In the surface treated by supercritical water, the ratio of iron and nickel increased even when compared to the bulk composition of SUS304 and the major component of nickel was attributed to NiFe 2 O 4 . In contrast, the content of chromium decreased remarkably. This was explained by effective removal of CrO 4 2− and its protonated form, HCrO 4 − , which are predicted in the Pourbaix diagram prepared for chromium in supercritical water ͑450°C and 50 MPa͒.Supercritical water ͑Ͼ647 K and Ͼ22.1 MPa͒ for decomposing nonflammable organic materials present in low-level radioactive waste has been extensively studied. 1-4 Although materials highly resistive to supercritical water are desirable, corrosion effect of the supercritical water is not well understood. In view of the practical use, stainless steels are preferable. However, it has been questioned whether stainless steels are tough to high temperatures of the supercritical water and corrosive species in it. An example of the corrosive condition is hydrogen peroxide in 2% H 2 SO 4 + 2% H 2 O 2 at 400°C and 28.5 MPa, which is used to oxidize cation-exchange resin.This preliminary study of the corrosion films of stainless steel SUS304 was done by X-ray photoelectron spectroscopy ͑XPS͒ for supercritical water having no oxidant or reductant. The corrosion films by the supercritical water at 450°C and 50 MPa were compared with those by anodic polarization in H 2 SO 4 aqueous solution. The chemical states in the films were determined by factor analysis of Fe, Cr, and Ni 2p XPS spectra. The results were discussed with Pourbaix diagrams. ExperimentalPreparation of SUS304 specimens for treatment with supercritical water.-Specimens with dimensions of 10 ϫ 5 ϫ 1 mm were used. In order to investigate the passivation layer formed in each treatment, after the rolling plane of the samples was finished with no. 8000 emery paper, they were washed twice with ethanol in an ultrasonic cleaner. Each sample was then weighed and loaded into a supercritical batch-wise reactor made of hastelloy C-22, whose nominal inside volume was 10.8 cm 3 , 1,2 with 3 mL of water. In order to control the temperature of the reactor, five cartridge heaters were attached to the reactor vessel and a thermocouple was used to measure the temperature inside the reactor. The reaction was carried out at a temperature and pressure ...

Effect of molybdenum and tungsten polyoxometalates on the composition of surface layers and electrochemical behavior of stainless steel in sulfuric acid

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