Correlations between Oxide Structure, Iron Distributions, and Zirconium Oxide Growth

Abstract

Waterside corrosion and associated hydrogen generation remain a major limiting factor for the use of zirconium alloys as cladding materials under severe fuel duty conditions. Although it has been observed that minor alloying additions might significantly affect the corrosion response of Zr alloys, the origins for these differences remain unclear. To develop alloys with greater corrosion resistance, it is important to understand the correlations between alloying additions, oxide structure and the oxidation behavior of zirconium alloys.

Three alloys were examined: pure Zr (0.009 wt.% Fe), Zircaloy-4 (1.32 wt.% Sn, 0.19 wt.% Fe, and 0.09 wt.% Cr) , and a ZrFeCr alloy (0.38 wt.% Fe and 0.22 wt.% Cr). All three alloys were corroded in autoclaves in 360°C water. Pure Zr loses protectiveness relatively early (<55 days). Zircaloy-4 exhibits multiple oxide transitions and oxide regains its protectiveness after each transition. The oxide in ZrFeCr alloy remains protective for the whole duration (500 days) of the test. The corroded alloy samples were analyzed through a multi-scale characterization approach combining X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and atom probe tomography.

The sequence of phases observed from the oxide/water interface to the metal was the same in all alloys, starting with ZrO2 and followed by a series of oxygen rich phases (suboxide ZrO, suboxide Zr2O, and solid solution). The thickness of the suboxides inversely correlated to the oxidation rate. It is hypothesized that the thickening of suboxide phases is a consequence rather than the cause of the oxidation rate reduction.

The three alloys exhibited different oxide growth behaviors. Unstable oxide growth and the preferential advance of oxide along the grain boundaries were observed only in pure Zr. Since Fe segregation to grain boundaries is common to all three alloys, grain boundary chemistry alone cannot explain the differences in oxidation behavior. Diffraction analysis revealed evidence for an orientation relationship between oxide grains and pure Zr metal grains. In the Zr-Fe-Cr alloy, oxide grains aligned their (200)m pole parallel to the oxide growth direction to form a fiber texture. Enhanced Fe enrichments were detected on various microstructural features (dislocations, oxide grain boundaries, etc.) near oxide scales in both pure Zr and ZrFeCr alloy, however ZrFeCr alloy exhibited much less dataset-to-dataset variation in Fe segregations and overall higher Fe content. It is therefore hypothesized that Fe contributes to the formation of the fiber texture, which can induce minimum growth stresses and contributes to stable oxide growth.