Materials for SCWO processes for the oxidation of chlorine containing residues in supercritical water

The oxidation of harmful residues in supercritical water (SCWO - Supercritical Water Oxidation) is an efficient procedure for converting hazardous substances into environmentally friendly products. Due to the usually high chlorine concentrations, however, the reactor material is subjected to a highly corrosive medium at high pressures and temperatures. For the corrosion studies an experimental set-up was used, that allows to subject the materials to these conditions. The nickel base materials Alloy 625 and Alloy 686 were studied at 500 degrees C and 46.5 MPa up to 320 hours. The model fluid consisted of HCl+H2O+NaCl+methanol with H2O2 as oxidant. Alloy 625 forms complex scales consisting of alternating Cr-Mo-Nb-O mixed oxide layers and chloride layers, mostly NiCl2. Alloy 686 forms similar corrosion scales and shows after 320 h additionally stress corrosion cracking. The ICP-AES analysis of the fluid in function of time shows time intervals with high Ni concentrations alternating with time intervals with high Mo and Cr concentrations, which indicates that alternating mechanisms control the corrosion process.