Sulfate corrosion and phosphate passivation of Ni-based alloy in supercritical water

Corrosion behavior of Ni-based alloy Inconel 625 in sulfate- and phosphate-containing SCW, and the mechanism of phosphate passivation acting as a corrosion inhibitor were investigated in this work. Sodium phosphate passivation treatment reduced the corrosion weight gains in oxygenated and pure SCW by 68.4% and 77.4%, respectively. The synergistic effect of high concentration of sulfate ions and oxygen on the alloy surface caused occurrence and expansion of pitting corrosion. NiCr₂O₄, NiO, Cr₂O₃, divalent and trivalent iron compounds could stably exist on the surface of sodium phosphate-passivated sample before and after exposure to oxygenated SCW. CrPO₄, FePO₄ and Ni₃(PO₄)₂, which were closely and uniformly distributed on the alloy surface, stably existed in SCW and served as a barrier layer for the alloy matrix. These oxides could inhibit the diffusion of metal cations and oxygen anions near the interface to improve corrosion resistance.