An Electrochemical Model for Oxide Growth on Zircaloy‐2

Abstract

The kinetics of Zircaloy‐2 oxidation were followed at 573 K in oxygen‐free steam and dry air. At various stages during the oxidation polarization measurements were made in molten alkali nitrates and nitrite at 573 K. These measurements provided a comparison of the ionic conductivity of the zirconia and the electrical conductivity of the oxide on the intermetallics, grown in steam and air with those of the oxides grown in the molten salts. A model is proposed for thermal oxide growth on Zircaloy‐2. During oxidation in steam, protons are conducted in the adsorbed water phase and reduced by the electron transport occurring at the conducting intermetallic sites. Thus, the oxidation behavior is similar to that in the molten salts, where nitronium and nitrosonium ions are reduced to complete the oxidation. The low rates of oxidation observed in air are attributed to a high resistance for the surface conduction of electrons, and the absence of a conduction and reduction step involving cationic species. Therefore, the rest potential is not lowered as effectively as that during oxidation in steam and the molten salts, in spite of the electron conduction at the intermetallic sites. The molten salts are suitable media for studying, in general, the mechanism of oxide growth on Zircaloy‐2; the information obtained is relevant either directly or indirectly.