Electrochemical and Microstructural Characterization of Alloy 600 in Low Pressure H2-Steam

Low pressure superheated H₂-steam system has been extensively used in the past years to accelerate the oxidation kinetics while keeping the conditions representative to PWR primary water. One of the most important requirements of this environment is that needs to replicate the Ni/NiO transition. However, despite several studies have been carried out by different research groups in H₂-steam environment, there is still some level of uncertainty over the thermodynamic of the oxidation process. In this study, the Ni/NiO transition in hydrogenated steam was investigated via electrochemical potential measurements using a Ni/NiO solid state reference electrode. Furthermore, solution annealed Alloy 600 coupons were exposed to H₂-steam at 480 ℃ in order to examine the effect of oxidizing conditions with respect to the Ni/NiO transition on the preferential intergranular oxidation. The effect of the redox potential on the preferential intergranular oxidation is discussed in the context of the precursor stages of stress corrosion cracking for Alloy 600.