Corrosion behavior of Ni base alloys and 316 stainless steel in supercritical water under alkaline conditions

Supercritical water (SCW) process is an attractive method for decompositon of hazardous 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 difficulty to put the SCW processes into industrial applications. It is important to select suitable structural materials, which are compatible with the SCW environment of each particular process, to ensure long term integrity of the reactor components. The objectives of this study are to evaluate the corrosion behavior of candidate corrosion-resistant alloys in SCW environments under reducing atmosphere and to understand the corrosion mechanisms. 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 this study. Corrosion experiments were carried out to investigate the influences of environmental variables such as temperature, anion, pH and the hydrogen partial pressure, on corrosion rate and corrosion morphology of the alloys. 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 the alloys to stress corrosion cracking (SCC) was also evaluated by slow strain rate technique (SSRT). Corrosion rate of type 316 stainless steel was always higher than that of any of the Ni base alloys in SCW containing NaOH. While weight change of type 316 stainless steel was small in deionized water, weight gain was sighnificantly increased with increasing NaOH concentrations. In reducing atomsphere containing NaOH, the weight loss was increased with increasing hydrogen partial pressure. In SCW under reducing atmosphere containing NaOH, Ni base alloys containing both Cr and Mo, such as Alloy C-276 and MAT21, would be more resistant to corrosion than Ni-Cr binary alloys. SCC did not occur in the deionized water at 400°C/25MPa. With 0.001mol/L HCl or 0.01 mol/l NaOH, cracking occured in type 316 stainless steel at 400°C/25MPa. With 0.001mol/L HCl or NaOH, cracking occured in Alloy C-276 at 400°C/25MPa. With 0.01mol/L HCl or NaOH, cracking occured on Alloy 625 at 400°C/25MPa. Cracking susceptibility of Ni base alloys decreased with increased Cr content of alloys in SCW either containing HCl or NaOH.