Abstract
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.
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