Corrosion of Alloy 625 and Pure Chromium in Cl− Containing Fluids During Supercritical Water Oxidation (SCWO)

Abstract Supercritical water oxidation (SCWO) is an efficient procedure for complete degradation of hazardous residues, converting them into acids, salts, and carbon dioxide. The reactor material, however, is subjected to a highly corrosive fluid and to high pressures at high temperatures. An experimental set-up was designed that allows corrosion studies under these conditions. Alloy 625 and chromium of high purity were studied at 500°C and 46.5 MPa up to 300 h with a model fluid consisting of HCl+H2O+NaCl+methanol using H2O2 as oxidant. Alloy 625 forms complex layers with alternating scales consisting of Cr-Mo-Nb-O mixed oxides, and layers containing chlorides, mainly NiCl2. Additionally pitting corrosion and local intergranular corrosion were observed. The analysis of the fluid phase by ICP-AES as a function of time showed periods with a strong transition of Ni into the fluid phase and alternating time periods with high Mo and Cr concentrations indicating that alternating mechanisms are controlling the corrosion procedure. On pure chromium, Cr2O3 scales composed of several layers were formed and in wide regions spallation was observed. The reduction of sample thickness and mass changes indicate greater corrosion rates of Cr than Alloy 625.