Mechanism of corrosion rate degradation due to tin

The microstructure, crystal structure, and morphology of oxide Alms growing on zirconium alloys have been studied to better understand the effect of tin on the corrosion of Zircaloy. Zr-Sn-Fe-Cr sheets whose tin content varied from 0.99 to 1.41% were prepared and corroded in 400 degrees C static steam for 300 days. The weight gain increased consistently with tin content. When the oxide film thickness was the same among all specimens, glancing angle XRD and TEM examination indicated a thick tetragonal ZrO2 inner layer only on the low-tin content alloy. The distribution of tin in the oxide films was analyzed using high-resolution EDX. Tin was enriched at the boundaries of the oxide crystallites. Tin concentration at the crystallite boundaries increased with the tin content of the alloy. Tin oxide was observed at the crystallite boundaries of the monoclinic ZrO2 layer by a high-resolution TEM image. When the condensed tin is oxidized, it expands, transforming the oxide from tetragonal to monoclinic, which in turn leads to the loss of a dense oxide layer. The low-tin content alloy consequently shows high-corrosion resistance owing to the smaller amount of tin condensing at the oxide grain boundaries.