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An X-ray absorption near-edge structure (XANES) study of the oxidation state of tin in zirconium alloy oxide films

Hulme, Helen Victoria

[Thesis]. Manchester, UK: The University of Manchester; 2016.

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Abstract

X-ray absorption near-edge structure (XANES) spectroscopy has been used to determine the oxidation state of alloying additions of tin (1 - 1.2wt%) in oxide layers grown on tin-containing nuclear grade zirconium alloys, Zircaloy-4 and ZIRLOTM. The aim of this project is to determine whether the oxidation state of alloying additions of tin can be identified in oxide films grown on zirconium alloys by studying the Sn L3 edge in order to determine the role that tin plays during the corrosion process. This represents the first ever attempt to identify the oxide state of tin in thin zirconium oxide films using this technique. Following successful identification of tin in a Zircaloy-4 oxide film, Zircaloy-4 and ZIRLOTM oxides removed from autoclave corrosion studies at different stages of the corrosion process were compared to see whether there was a correlation between the corrosion kinetics and the oxidation state(s) of tin detected. Data obtained for metallic and corroded ZIRLOTM (1wt% Sn) and Zircaloy-4 (1.2wt% Sn) indicate tin has a similar chemical speciation in both metal alloys but this differs in the oxidised surface layers. Finally, when recording XANES at various incident angles to vary the photon penetration depth and amount of the oxide layer probed in the measurement, there is evidence to suggest that the oxidation of tin progresses with increasing oxide thickness. Comparison with Sn, SnO and SnO2 reference as well as Zircaloy-4 and ZIRLOTM base metal samples suggest tin in oxidation states Sn(II) and Sn(IV) are present within the oxide film and the ratio of Sn(II) to Sn(IV) changes as a function of oxide depth. This indicates Sn(0) is first oxidised to Sn(II) before gradually being oxidised further to Sn(IV) during the pre-transition corrosion cycle. Similar observations have been seen for both alloys.Results from this work are discussed within context of the overall role tin plays in the corrosion process of tin-containing zirconium alloys to suggest a possible mechanism for the role tin play during zirconium alloy corrosion.