Altering the hydriding behaviour of uranium metal by induced oxide penetration around carbo-nitride inclusions

Secondary ion mass spectrometry (SIMS) and focused ion beam (FIB) milling were used to examine the surfaces of uranium samples, both before and after limited hydride formation to identify surface features that could provide sites for preferential hydrogen attack. Two sets of cast uranium samples containing 600 ppm carbon in the form of carbo-nitride inclusions were examined; one set was prepared by mechanically polishing to a fine grade and the other received a subsequent electropolishing step. SIMS analysis showed that the additional electropolishing resulted in oxide development along the inclusion metal interface that was not present after only mechanical polishing. These samples then underwent hydrogen exposure for a limited period under conditions expected to result in hydride formation. For uranium prepared by only mechanical polishing, the hydride growths were observed to occur in significant numbers and almost exclusively around exposed inclusions. Conversely, preparation involving electropolishing resulted in extremely limited numbers of hydride growth sites not obviously associated with inclusions. These differences in hydride formation behaviour are attributed to the presence of oxide formed along the inclusion–metal interface resulting from electropolishing, and highlight how the observed hydride-forming behaviours exhibited by uranium can be significantly altered by the method of surface preparation.