Abstract
The evolution of Cr and Fe within the oxide formed on Zircaloy-4 in high-temperature pure water and a high-pH aqueous environment has been followed in detail by electron energy loss spectroscopy (EELS). The oxidation states of Cr, Fe and Zr have been determined as a function of exposure time and location in the oxide film. Much Cr and all the Fe diffuse out of the original Laves phase and precipitate within cracks or on the outer oxide surface. Cr oxidation is complete prior to transition; Fe oxidation occurs mainly after transition. The different hypotheses concerning the mechanisms by which the alloying elements influence oxidation and hydrogen pickup are examined in the light of this information. The mechanisms which agree with these observations include: doping of the bulk ZrO2 by low levels of alliovalent cations; doping ZrO2 grain boundaries with alliovalent cations; injection of holes into near-surface oxide by islands of Fe2O3; reaction of water with islands of metallic Fe (and Ni) on surfaces of internal cracks around transition; control of transition by mechanical properties
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