Comparative study of the corrosion behavior of Inconel 625 in supercritical water containing dissolved and molten phosphates

This paper presents a comparative study of the corrosion behavior of Inconel 625 in supercritical water with different oxygen contents and phosphate phases. The results showed that the dissolved phosphate phase could compete with oxygen and that the addition of oxygen converted Cr₂O₃ to Cr^6 +, resulting in a reduction in the thickness of the oxide film. The oxide film formed consisted of three distinct layers. The outer layer consists of NiO, Fe₂O₃, and NiFe₂O₄, which were formed by the diffusion of metal cations outward. The middle layer was a metal phosphate passivation layer which could prevent the diffusion of Fe, Ni, etc. and retard corrosion. The inner layer was formed by inward oxidation of oxygen ions with Cr to form Cr₂O₃. Phosphate molten salt were highly corrosive and can directly dissolve Cr₂O₃ into soluble Cr³⁺, a process accelerated by oxygen, increasing the Cr³⁺ content from 59.63 % to 77.54 %.