Corrosion Behavior of Ni base alloys and 316 stainless steel inSupercritical Water under Alkaline conditions

摘要

Supercritical water (SCW) process is an attractive method for decompositon ofhazardous organic wastes and for upgrading of low-quality hydrocarbon resources.However, the corrosion of a reactor or a heat exchanger is known as the major difficultyto put the SCW processes into industrial applications. It is important to select suitablestructural materials, which are compatible with the SCW environment of each particularprocess, to ensure long term integrity of the reactor components.The objectives of this study are to evaluate the corrosion behavior of candidatecorrosion-resistant alloys in SCW environments under reducing atmosphere and tounderstand the corrosion mechanisms. Four kinds of Ni base alloys, Alloy 625, AlloyC-276, Ni-45Cr-1Mo (MC alloy) and Ni-19Cr-19Mo (MAT 21) and a Type 316stainless steel were used in this study. Corrosion experiments were carried out toinvestigate the influences of environmental variables such as temperature, anion, pHand the hydrogen partial pressure, on corrosion rate and corrosion morphology of thealloys. A static autoclave was used for the corrosion study. Exposure time was 50 hours.Applied pressure was 25 Mpa and testing temperature was 400℃. Susceptibility of thealloys to stress corrosion cracking (SCC) was also evaluated by slow strain ratetechnique (SSRT).Corrosion rate of type 316 stainless steel was always higher than that of any of the Nibase alloys in SCW containing NaOH. While weight change of type 316 stainless steelwas small in deionized water, weight gain was sighnificantly increased with increasingNaOH concentrations. In reducing atomsphere containing NaOH, the weight loss wasincreased with increasing hydrogen partial pressure. In SCW under reducingatmosphere containing NaOH, Ni base alloys containing both Cr and Mo, such as AlloyC-276 and MAT21, would be more resistant to corrosion than Ni-Cr binary alloys.SCC did not occur in the deionized water at 400℃/25MPa. With 0.001mol/L HCl or0.01 mol/l NaOH, cracking occurred in type 316 stainless steel at 400℃/25MPa. With0.001mol/L HCl or NaOH, cracking occurred in Alloy C-276 at 400℃/25MPa. With0.01mol/L HCl or NaOH, cracking occurred on Alloy 625 at 400℃/25MPa. Crackingsusceptibility of Ni base alloys decreased with increased Cr content of alloys in SCWeither containing HCl or NaOH.