GRAIN BOUNDARY OXIDATION AND STRESS CORROSION CRACKING IN NICKEL-BASE ALLOYS STRAINED IN SUPERCRITICAL WATER

摘要

The objective of this study is to link grain boundary oxidation to grain boundary cracking in nickel-base alloys using the stability of Ni and NiO as a frame of reference. Accelerated stress corrosion cracking tests and exposures were conducted on alloy 600, Ni-9Fe, and Ni-9Fe-5Cr (LCr) in constant extension rate mode in supercritical water (SCW) at 400°C using dissolved hydrogen concentrations of 47 cc/kg and 200 cc/kg to control the stability of NiO and Ni respectively. Unstressed samples of Ni-9Fe exposed in the NiO stable regime and LCr exposed in both the Ni and NiO stable regimes show grain boundary oxides extending several microns below the sample surface. Constant extension rate tensile test results showed that cracking was more pronounced in samples where intergranular oxides were able to form, except in alloy 600 where no intergranular oxides formed. Comparison with oxide penetration from 400°C hydrogenated steam revealed that the supercritical water environment was more aggressive, but does not suggest a different mechanism of cracking is operating.