Ex situ and in situ characterization of stress corrosion cracking of nickel-base alloys at high temperature

Abstract

Recent ex situ and in situ characterization of the major factors influencing primary water stress corrosion cracking (PWSCC) and outside diameter stress corrosion cracking (ODSCC) of nickel-base alloys of Alloy 600 and Alloy 690 as the structural materials in pressurized water reactors (PWRs) was introduced to understand cracking mechanisms. In the primary water environment of PWRs, the effect of stress on PWSCC was analyzed using the in situ direct-current potential drop method combined with electrochemical noise measurement. The compositional and microstructural changes around a crack tip including the oxide film formed during crack propagation were evaluated by using ex situ microscopic methods with energy-dispersive spectroscopy (EDS). The equivalent local strain was also evaluated by using the electron backscatter diffraction method as ex situ technique. With the aid of ex situ and in situ characterization, it was proposed that the intergranular precipitation of Cr oxides ahead of the crack tip by O penetration along the grain boundary that make the “oxidized grain boundary” can significantly increase the susceptibility to PWSCC propagation of the alloys. In the secondary water environment of PWRs, the properties of oxide film formed on the surface of the alloys were characterized as a function of the Pb content as impurity by using in situ methods of potentiodynamic polarization measurement and electrochemical impedance spectroscopy and ex situ techniques of EDS and X-ray photoelectron spectroscopy. From these ex situ and in situ characterizations, the degradation model of the passivity of the surface oxide film by Pb incorporation was proposed to understand the ODSCC mechanism.