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

Abstract The under-deposit corrosion behavior of Ni-based alloys 625, 825 and Fe-based alloy 800 in supercritical water (SCW) at 400 °C and 500 °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 °C, hydrolysis of the metallic ions occurred, forming HCl with chloride ions. At 500 °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 HCl vapor at 500 °C.