Effect of salt deposit on corrosion behavior of Ni-based alloys and stainless steels in supercritical water

The under-deposit corrosion behavior of Ni-based alloys 625,825 and Fe-based alloy 800 in supercritical water (SON) at 400 degrees C and 500 degrees C was investigated. Newly designed experimental apparatus was used to ensure that the surface of alloy specimens was covered by NaCl or KCl deposit layers during the corrosion test in SCW environment. Based on the cross-sectional observation and surface chemical analyses, the oxide film on the samples after exposure contains an outer layer containing Fe-Ni-Cr spinel and Fe3O4, and an inner layer of Cr2O3. NiCl2 was also found on the alloy surface. At 400 degrees C, hydrolysis of the metallic ions occurred, forming HCl with chloride ions. At 500 degrees C, the oxides react with the solid-phase salt deposit via the formation and transformation of the HCl vapor. The salt deposit layer acted as a barrier for the diffusion of SCW and HCI vapor at 500 degrees C. (C) 2019 Elsevier B.V. All rights reserved.