Corrosion Properties and Mechanisms of Austenitic Stainless Steels and Ni-Base Alloys in Supercritical Water Containing Phosphate, Sulfate, Chloride and Oxygen

Abstract: The corrosion behaviors of 316 stainless steel (SS), 316L SS, Inconel 625, Incoloy 825 and Hastelloy C276 were studied in supercritical water (SCW) containing phosphate, sulfate, chloride and/or oxygen. The results show that the alloy corrosion rate in SCW containing oxygen and phosphate was increased by adding sulfate and/or chloride. A two-layer structure of oxide film on 316 SS consisted of an inner layer with Cr-rich oxides (i.e., Cr₂O₃) and an outer layer rich in phosphates (e.g., FePO₄ and Ni₃(PO₄)₂). A three-layer oxide film formed on Incoloy 825 including corrosion products Fe₂O₃, NiO, FePO₄, Ni₃(PO₄)₂ and NiCr₂O₄, with phosphates in each layer. A two-layer oxide film on Hastelloy C276 was composed of metal oxides in the inner layer and metal oxides as well as phosphates in the outer layer, including Fe₂O₃, Cr₂O₃, NiO, MoO₂, FePO₄, CrPO₄, Ni₃(PO₄)₂ and NiCr₂O₄. The existence of sulfate, chloride, and especially both of them destroyed the formation of water-insoluble phosphates on the oxide film of tested alloy in SCW containing oxygen and phosphate. Graphical Abstract: [Figure not available: see fulltext.].