Stress Corrosion Cracking of Ni-Based Alloys in Batch Supercritical Water Oxidation Environments Available to Purchase

The objective of this study is to characterize stress-corrosion cracking in the Ni-based alloys 625 and 617 during exposure to environments encountered in the batch supercritical water oxidation cycle. A fracture-mechanics approach was applied to quantify the stress-corrosion cracking susceptibility of alloys 625 and 617 in two environmental conditions related to batch supercritical water oxidation processing: thermal cycling into the supercritical range and isothermal soaking in the subcritical range. Lower-bound values of K_ISCC were measured for solution heat treated alloys 625 and 617 during exposure to the subcritical environment. The K_ISCC values were identical in the two alloys (24 MPa√m). Aging alloys 625 and 617 for 200 hours at 600°C promoted more severe stress-corrosion cracking in the subcritical environment, as evidenced by greater amounts of stress-corrosion crack extension in the aged alloys compared to the solution heat treated alloys. Values of K_ISCC were not established for aged alloys in the subcritical environment. Corrosion damage in alloys 625 and 617 exposed to thermal cycling into the supercritical range more closely resembled intergranular corrosion as opposed to stress-corrosion cracking. The extent of intergranular corrosion depended on specimen location in the test reactor and exposure time.