Corrosion Behavior of Nickel-Based Alloys and Type 316 Stainless Steel in Slightly Oxidizing or Reducing Supercritical Water

The objectives of this study were to evaluate the corrosion behavior of candidate corrosion-resistant alloys in slightly oxidizing or reducing supercritical water (SCW) environments containing hydrogen chloride (HCl) and to understand the corrosion mechanism that controls the corrosion resistance of these alloys in the upgrading of the low-quality hydrocarbon resources process. Four kinds of Ni-based alloys, UNS N06625 (Alloy 625), UNS N10276 (Alloy C-276), Ni-45Cr-1Mo (MC alloy), and Ni-19Cr-19Mo (MAT 21), and a Type 316 (UNS S31600) stainless steel were used in this study. The test temperature varied from 350°C to 550°C. The test pressure was fixed at 25 MPa. A parametric study was carried out to investigate the influences of environmental variables such as temperature, anion, pH, and hydrogen partial pressure on the corrosion rate and corrosion morphology of the alloys. A static autoclave was used in this study. In reducing SCW, the corrosion rate of Type 316 stainless steel was always higher than that of other Ni-based alloys. While weight changes of Type 316 stainless steel and MC alloy were small in deionized water, weight losses were significantly increased with the increasing hydrogen partial pressure. In slightly oxidizing SCW containing HCl, the weight loss of Type 316 stainless steel increased with increasing HCl concentration. In reducing SCW containing HCl, Ni-based alloys containing both Cr and Mo, such as Alloy C-276 and MAT21, were found to be more resistant to corrosion than Ni-Cr binary alloys.