Under-deposit corrosion of Cr-coated Ni-based alloys in supercritical water oxidation environment at 500 °C

In this study, a Cr coating was fabricated on Ni-based alloys to prevent under-deposit corrosion. The effect of salt deposition on the structural integrity of the Cr coating in SCW environment with/without oxygen was investigated. Results show that the Cr coating kept its integrity after exposure in SCW environment, successfully avoided the metallic ions leaching from Ni-based alloys. The structure of the oxide layer after exposure is a thin Fe₂O₃ or FeCr₂O₄ layer and a Cr₂O₃ layer. The oxidation rate of the Cr coating decreased to one third of the sample after exposure in the same environment without salt deposit layer, indicating that the inward mass-transfer of oxygen to the alloy surface was inhibited by the salt deposit layer. The volume expansion of the sample due to the growth of Cr oxide layer was restrained by the salt deposit layer, forming a growth stress in the coating.