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.