Corrosion behavior of nickel base alloys, stainless steel and titanium alloy in supercritical water containing chloride, phosphate and oxygen

• The chloride and oxygen caused the synergistic effect to alloy corrosion.
• A three layer stable and protective oxide film formed on the surface of TA 10.
• Phosphate can form a passivating metal salt film, acting as a corrosion inhibitor.
• The corrosion resistance of the Mo alloy increased as the amount of Cr increased.

The corrosion behaviors of 316 stainless steel (316 SS), Inconel 600, Inconel 625, Hastelloy C-276 and TA10 coupons exposed in supercritical water (400 °C/25 MPa) with sodium chloride, sodium triphosphate and hydrogen peroxide were investigated by using SEM, XRD and XPS. 316 SS shows the most severe spallation of oxide scales and Inconel 600 exhibited the most severe pitting corrosion due to a lack of molybdenum in the alloy. TA10 showed the lowest corrosion rate and the slightest corrosion morphology. TA10 had corrosion rates less than 1.8 mmpy in the four experiments and a triplex oxide layer structure formed which consisted of TiO2/Ti2O3/TiO from outer to inner layer. The formation of (TiO)2P2O7 was observed on TA10 in oxidizing supercritical water containing phosphate. Phosphate shows significant corrosion inhibition for metal alloys in SCWO due to the formation of a passivating metal phosphate film. An oxide film growth mechanism, metal dissolution mechanism and metal salt precipitation mechanism are discussed in order to explain the corrosion behavior of the specimens in this research.