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The corrosion properties of three austenitic steels with different Si contents were studied under oxygen-saturated liquid Pb–Bi condition for 3000 h. The three austenitic steels did not exhibit appreciable dissolution of Ni and Cr at 450 °C. At 550 °C, the thick ferrite layer produced by dissolution of Ni and Cr was found in JPCA and 316SS with low Si contents while the protective oxide film composed of Si and O was formed on 18Cr–20Ni–5Si steel and prevented dissolution of Ni and Cr. [Copyright &y& Elsevier]

Corrosion tests of austenitic stainless tube were done under flowing Pb–Bi conditions for 3000 h at 450 °C. Specimens were 316SS produced as a tubing form with 13.8 mm outer diameter, 2 mm thickness and 40 cm length. During operation, maximum temperature, temperature difference and flow velocity of Pb–Bi at the specimen were kept at 450, 50 °C, and 1 m/s, respectively. After the test, specimen and components of the loop were cut and examined by optical microscope, SEM, EDX, WDX and X-ray diffraction. Pb–Bi adhered on the surface of the specimen even after Pb–Bi was drained out to the storage tank from the circulating loop. Results differed from a stagnant corrosion test in that the specimen surface became rough and the corrosion rate was maximally 0.1 mm/3000 h. Mass transfer from the high temperature to the lower temperature area was observed: crystals of Fe–Cr were found on the tube surface in the low-temperature region. The sizes of crystals varied from 0.1 to 0.2 mm. The depositing crystals were ferrite grains and the chemical composition ratio (mass%) of Fe to Cr was 9:1. [Copyright &y& Elsevier]

The supercritical water process is an attractive method for the decomposition of hazardous organic wastes and for the upgrading of low quality hydrocarbon resources. However, corrosion of reactors or heat exchangers is a major problem in industrial applications. It is important to select suitable structural materials, which are compatible with the supercritical water environment of each particular process, to ensure the long term integrity of the reactor components. The objectives of the present study were to evaluate the corrosion behaviour of candidate corrosion-resistant alloys in supercritical water environments under reducing atmospheres and to improve understanding of the corrosion mechanism. Four kinds of Ni-base alloys, Alloy 625, Alloy C-276, Ni–45Cr–1Mo (MC alloy) and Ni–19Cr–19Mo (MAT 21) and a Type 316 stainless steel were used in the present study. A parametric study was carried out to investigate the influences of environmental variables such as temperature, anion content, pH and the hydrogen partial pressure, on the corrosion rate and corrosion morphology of the alloys. A static autoclave was used in the present study. The exposure time was 50 h, the applied pressure was 25 MPa and the test temperature was 400°C. The susceptibility of the alloys to stress corrosion cracking was also evaluated by the slow constant strain rate technique. The corrosion rate of Type 316 stainless steel was always higher than that of any of the Ni-base alloys in supercritical water containing NaOH. While the weight change of Type 316 stainless steel was small in deionised water, the weight gain significantly increased with increasing NaOH concentrations. In a reducing atmosphere containing NaOH, the weight loss increased with increasing hydrogen partial pressure. In supercritical water under a reducing atmosphere containing NaOH, Ni-base alloys containing both Cr and Mo, such as Alloy C-276 and MAT 21, would be more resistant to corrosion than Ni–Cr binary alloys. Stress corrosion cracking did not occur in deionised water at 400°C/25 MPa. With 0·001 mol L-1 HCl or 0·01 mol L-1 NaOH, cracking occurred in Type 316 stainless steel. With 0·001 mol L-1 HCl or NaOH, cracking occurred in Alloy C-276. With 0·01 mol L-1 HCl or NaOH, cracking occurred in Alloy 625. The cracking susceptibility of Ni-base alloys in supercritical water containing either HCl or NaOH decreased as their Cr content increased. [ABSTRACT FROM AUTHOR]