Stress corrosion cracking of Alloy 625 in pH2 aqueous solution at high temperature and pressure

Stress corrosion cracking (SCC) of Alloy 625 (UNS N06625) has been investigated in pH 2 aqueous solution at high temperatures (300°C to 426°C) and high pressure (24.1 MPa) to understand the corrosion behavior in supercritical water oxidation (SCWO) systems, which can destroy aqueous organic wastes with high efficiency with no harmful byproducts. Alloy 625 was exposed to 11 operational (chemical, thermal, pressure) cycles. SCC at subcrittcal temperature comes from the clvemical 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. As a result of the defective dealloyed oxide layer structure, the dtffustvity of Ni is fast, intermediate between the surface and grain boundary dtffusivities. SCC at supercritical temperature comes from the direct chemical attack of associated hydrochloric acid (HCI) molecules.