Dissolution of oxygen-enriched Zircaloy-2

When metal is removed from oxidised Zircaloy specimens in order to study the morphology of the oxide at the metal–oxide interface needle-like features are frequently observed. Since similar features are not observed in cross-section TEM examinations it has been questioned whether or not the needles are a result of the dissolution process. In particular it has been proposed that reprecipitation of oxide may take place when the metal is enriched with oxygen. In the present work oxygen-enriched Zircaloy has been dissolved and the resulting structures examined. The results indicate that the needles are in fact artefacts of the specimen preparation procedure. However, there are no significant differences between oxygen-enriched and normal Zircaloy below an oxide layer grown in steam at about 400 °C. In view of the differences between the needle structure observed after metal removal with bromine or by electropolishing and removal by HF–HNO3 pickling solution it is speculated that the needles consist of a hydrous zirconium oxide which is unstable in the pickling solution.