A Study of Zircaloy-2 Corrosion in High Temperature Water Using Ion Beam Methods

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Abstract
Corrosion experiments have been carried out on Zircaloy-2 specimens in water at 355 C to study the transport of oxygen and hydrogen (as deuterium) in growing corrosion films. The transport behavior was studied by using O18 and deuterium labelled water, and analyzing the corrosion films for these isotopes by the use of ion beam induced nuclear reactions. Analysis for O18 was performed by means of the O18 (p,α)N15 reaction, using incident 850 and 950 keV protons. The analyses for deuterium were made using the D(He3,d)p reaction employing incident 850 and 1300 keV He3 ions, and detecting the scattered α-particles. The composition of the corrosion films was also examined in 2.9 MeV and 3.9 MeV α-particle backscattering experiments. From these studies, it was concluded that the corrosion of Zircaloy-2 in high temperature water occurs predominantly by oxygen diffusion through the corrosion film via grain boundary or similar short circuit diffusion paths, to form fresh oxide at the oxide metal interface. The diffusivity of oxygen through the pre-breakaway films decreased with time as a result of a decrease in the available easy diffusion paths as the oxide aged. This was interpreted as the primary cause of the subparabolic kinetics normally observed prior to the rate transition during high temperature aqueous corrosion of Zircaloy-2. There was also evidence that increasing grain size deeper within thick pre-breakaway films contributes to the decrease in oxygen diffusivity. The oxygen transport behavior in post-breakaway corrosion films indicates that the rate transition results from the generation of new diffusion pathways in previously protective oxide. Unexpectedly high concentrations of deuterium were observed in the corrosion films. These were about 4.5% atomic at the oxide surface, falling to 1 % atomic at depths between 1 and 1.5 μm. The deuterium was also found to be highly mobile within the oxide, much more so than oxygen.

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