A STUDY ON THE OXIDATION BEHAVIOUR OF NICKEL ALLOYS AND THE LIKELY EFFECT ON PWSCC SUSCEPTIBILITY

摘要

Internal oxidation as the possible mechanism of primary water stress corrosion cracking (PWSCC) in Alloy 600 was studied. Alloy 600 (Ni-16Cr-9Fe), in the solution annealed (SA) and thermally treated (TT) conditions, was exposed to a hydrogenated steam environment considered to simulate primary water in nuclear plants.Atom probe tomography (APT) was used to study the oxidation behaviour in the 600 alloy at a sub-nanometer scale. Internal oxidation, both intra- and intergranularly, was observed leading to the expulsion of metallic Ni to the surface. Intergranular oxidation in Alloy 600SA resulted in embrittlement and fracture under stress. Ni was displaced away from internal oxides and the network used for expulsion of the solvent metal, Ni, to the surface was discernable at a sub-nanometer level of resolution. APT on grain boundary oxides provided further evidence for the embrittlement of Alloy 600SA by intergranular internal oxidation and lent insight into the beneficial effects of thermal treatment. Final comparisons are made between the hydrogenated steam environment and primary water with discussion on the viability of internal oxidation as the likely mechanism of PWSCC in Alloy 600.