General corrosion and stress corrosion cracking behaviors of Alloy 825 in high-temperature supercritical water

Abstract

General corrosion and stress corrosion cracking behaviors of Alloy 825 were investigated in supercritical water at 550 °C and 650 °C. The results show that Alloy 825 demonstrates consistent weight gain with prolonged exposure at 650 °C, while exhibits alternating weight gain and weight loss behaviors at 550 °C due to the interplay between uniform corrosion and pitting corrosion caused by the oxidation of TiN precipitates. At 550 °C, a porous and double-layer oxide film is generated, with the outer layer primarily composed of (Cr,Fe)₂O₃ grains and the inner layer consisting of the sub-layer matrix interspersed with polycrystalline spinel oxides. Stress corrosion cracking at this temperature is characterized by intergranular cracks at the edge regions of the fracture surface. At 650 °C, the oxide film is more uniform, compact and protective, comprising polycrystalline spinel oxides and Cr₂O₃. The fracture mode at this elevated temperature transitions to a combination of cleavage and ductile fractures.