Characterisation of oxygen and hydrogen migration through oxide scales formed on nickel-base alloys in PWR primary medium conditions

This study aims at providing quantitative data concerning the oxygen and hydrogen diffusion through the oxide scale formed on alloy 600 in PWR primary medium conditions (325 °C) using 2D and 18O isotopes as markers. The diffusion coefficients were calculated according to a short-circuit diffusion of the tracers along the oxide grain boundaries (so-called “C-regime”). The values obtained are respectively Dsc = 4.6 +/− 0.9 × 10− 17 cm2/s for 18O, and Dsc = 5.2 +/− 1.2 × 10− 17 cm2/s for 2D. These experiments confirm that the transport mechanism of hydrogen through the oxide scale as hydroxide ions on the anionic sublattice is highly probable. The effect of hydrogen subsequent absorption by the alloy substrate, notably in stress corrosion cracking phenomenon is discussed.