Effect of high flow velocity on corrosion behavior of Ni based and Ni-Fe based alloys in supercritical water oxidation environment

In this study, the corrosion behavior of Ni-based alloy 625, 825, and Ni-Fe based alloy 800 was investigated in SCW containing salts and oxygen at 400 °C. The flow velocity of the corrosion medium near the sample surface was around 2 m/s. The surface and cross-section morphologies were observed, and the oxide phase was analyzed. After 100 h exposure, a bi-layer structure was formed on the alloy surface. The Ni and Fe rich outer layer was loose and porous, and the Cr rich inner layer was compact. Weight loss and oxide spallation were observed in the samples after exposure. The flow pattern near the test alloys was turbulent flow, but the outer oxide layer was immersed in the viscous sublayer. Once the Ni and Fe cations reached the alloy surface, some of them were carried away by the fast flowing fluid. Force analysis was done on an oxide particle from the outer oxide layer. The combining effect of the lifting force in the vertical direction and the shear force in the horizontal direction is the major reason for the porous structure of the outer layer and the oxide layer spallation.