Corrosion behaviour of nickel base alloys and 316 stainless steel in supercritical water under alkaline conditions

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.