Stress corrosion cracking of Alloy 625 in pH 2 aqueous solution at high temperature and pressure

Stress corrosion cracking (SCC) of Alloy 625 (N06625) has been investigated in pH 2 aqueous solution at high temperatures (300-426^oC) and high pressure (24.1MPa) to understand the corrosion behavior in supercritical water oxidation (SCWO) systems, which can destroy aqueous organic waste water with high efficiency with no harmful byproducts. Alloy 625 was exposed to 11 operational (chemical, thermal, pressure) cycles. SCC at subcritical temperature comes from the chemical stability of the elements, which produces a dealloyed oxide layer where Ni is selectively dissolved and Cr forms stable oxides. Its growth is accelerated along the grain boundaries, where SCC develops during the operational cycles. Due to the defective dealloyed oxide layer structure, the diffusivity of Ni is fast, intermediate between the surface and grain boundary diffusivities. SCC at supercritical temperature comes from the direct chemical attack of associated HCl molecules.