A number of hydride sites could be observed after hydriding experiments, these hydride sites grew in a specific direction or simply stochastically alongside the craters, as shown in Fig. 1a. This photograph illustrates the hydride expansion, focusing on the sample surface. To reveal the under-surface information, the post-hydriding sample was then abraded and polished slightly layer by layer. The corresponding results after approximately 5 μm are shown in Fig. 1b, c, in which the black features are hydride craters that were not fully abraded. The bulk material could also be distinguished by the color difference. It can be seen that, each black crater is surrounded by a large number of line-type or plate-like gray transition regions, which lie between the black craters. Figure 2b shows a cross-section outline corresponding to the yellow line in Fig. 2a, located on the crater center. In the Fig. 2b plot, material above the blue line was removed by the polishing process. The remaining transition regions and shallow crater are distributed on the pre-crater position. Further, line-type or plate-like gray cracks are distributed in the transition region (Fig. 1c). H atoms had diffused into these cracks, which can be easily distinguished by their gray color, since the craters are black and the bulk color is lighter.

Fig. 1 Laser scanning confocal microscope (LSCM) photographs of uranium (U) surface a after hydriding and b, c after slight surface polishing Full size image

Fig. 2 a Laser scanning confocal microscope (LSCM) photograph of a hydride crater. b Plot corresponding to the yellow line of the same crater Full size image

The cross-section morphology of the hydride craters was acquired by polishing the reaction sample in the direction perpendicular to the reaction surface. Figure 3 illustrates the cross-section morphology of one hydride crater. From Fig. 3a, we can see that the hydrogen attacked region shows a semi-elliptical shape. A number of line-type gray cracks could be observed near the reaction front, this is identical to the surface morphology around the large hydriding craters, as shown in Fig. 1c.

Fig. 3 a Laser scanning confocal microscope (LSCM) photograph focusing on a well-polished mechanically sectioned hydride crater and b a locally magnified photograph focusing on the transition region Full size image

These line-type gray cracks are possibly related with the transformation twinning region. First, regardless of the degree of surface polishing and cross-section abrasion, the line-type gray cracks are always present in or adjacent to the hydride craters. Further, the color discrimination between the bulk and these regions must be attributed to the H action on these regions. Second, a previous EBSD study reported extensive twinning within the metal grains in the vicinity of hydride craters.7 It is believed that transformation twinning occurred in that case because of the generation and accumulation of internal stress during the reaction, accompanied by volume expansion due to the hydride formation.7,21 EBSD analyses have proven that twinning in U occurs at nucleation sites, and hydride grows in conjunction with twinning.5,13 Under these conditions, the twinning may act as a kind of diffusion channel and H may congregate in these internal channels beneath the hydride craters. However, in the examined samples, the H concentration had not reached the critical concentration required to cause phase transformation for UH 3 formation.9

Furthermore, the cross-sections of the reacted regions on the micro-scale were acquired via FIB, as shown in Fig. 4. Figure 4a shows the cross-section of a smaller hydride crater and Fig. 4b shows a H-damaged area where the surface oxide layer had not cracked. The obvious contrast differences in the SEM photographs indicate the H activity regions. The regions between the loose hydride or surface layer and the bulk are the so-called transition regions.

Fig. 4 a Scanning electron microscopy (SEM) images of cross-section cut through hydride pit. The hydride is clearly distinguished by the loose material. There is an obvious boundary between the hydride and bulk uranium (U), indicated by the contrast difference. b SEM image of cross-section cut through a hydrided region, there is an obvious boundary between hydride and bulk U Full size image

The H distribution in the so-called transition regions was analyzed using TOF-SIMS, as described in the third paragraph of the Methods section. Figure 5 shows the different clustered ion contents, the U+ and UO+ ions had the strongest intensity, whereas the UH+ and UO 2 + ion intensities were relatively weak. This clearly indicated that H atoms were detected in the target area on the reaction sample surface after the processing described above. Figure 6 shows SE (secondary electron) and SIMS images of a 200 μm × 200 μm region of hydrogenated U metal, focusing on a hydride crater with a slightly abraded surface. Figure 6a–d are an SE image and secondary ion UO+, UH+, and UO 2 + images, respectively. In the four photographs, the hydride crater is distinguished by color difference, being notably darker. In the previous section, the transition region was distinguished by line-type and plate-like gray regions. From comparison of the similar regions in Fig. 6b, c, bright regions are visible around the dark craters. This implies higher H and O content in those regions than in the bulk; in other words, H was present in the transition regions. From Fig. 1 and 3a, it can be deduced that the line-type and plate-like regions formed as a result of the diffusion of H. However, in the secondary ion UH+ image (Fig. 6c), the H-diffused regions around the carters can be clearly distinguished relative to the bulk. Thus, these regions can be identified as transition regions. However, it should be noted that the gray line-type and plate-like regions must have some differences from the transition field. Further, the O and H enrichment fields have almost identical location, which indicates that the fields into which the H diffused had poorer corrosion resistance to O when subjected to wet abrasion and polishing.

Fig. 5 Secondary ion mass spectrum of hydriding region after polishing and Au+ sputter cleaning for 5 min Full size image

Fig. 6 a SE image of 200 μm × 200 μm region focusing on hydriding craters of reacted sample after polishing. b–d Secondary ion mass spectroscopy (SIMS) images for UO+, UH+, and UO 2 + Full size image

The surface potential on a H-damaged area of 0.8 mm × 0.8 mm was acquired by the SKP (scanning Kelvin probe) system, as shown in Fig. 7. Before SKP analysis, the hydriding sample surface had been polished slightly to remove the loose material caused by volume expansion and to reveal the so-called transition regions. In Fig. 7, the surface potential fluctuations are obvious, the maximum potential contrast was approximately 400 mV, which is sufficient to distinguish different regions by the surface potential distribution. Greater corrosion occurs in a lower-potential area19,22,23,24,25; thus, the fluctuating surface potentials indicate areas with different levels of corrosion resistance. From Fig. 7, we can deduce that, first, during the hydriding reaction, the formation of UH 3 craters caused stress field and plastic deformation, leading to the easy diffusion of H into these regions along certain transition path, which rendered these areas susceptible to be attacked by the subsequent H. Second, after hydriding and during the polishing process, these areas also exhibited poor corrosion resistance to H, as confirmed by the secondary ion images in Fig. 6. Therefore, considering the surface potential fluctuation, the perfect surface had the highest potential, the transition region had lower potential, whereas the craters corresponded to the lowest-potential region because of the missing material.

Fig. 7 SKP 3D-potential image of well-polished hydrided uranium (U) Full size image

High-resolution X-ray photoelectron spectra of O1s and U4f for the transition regions are shown in Fig. 8a, b, respectively. Prior to spectrum acquisition, sputter cleaning was performed for pure U without hydriding for 10 min, so as to obtain a clean U surface. Then, the spectra corresponding to the clean U surface were obtained for the parameters discussed in the fifth paragraph of the Methods section. Next, the spectrum acquisition operation was applied to a hydriding sample with a surface that was slightly polished to reveal the similar transition areas. Focusing on a transition region near a large hydride crater, a series of high-resolution XPS spectra were obtained for the surface in a step-by-step manner, from non-sputtered to 5-min sputtering, and then to 10-min sputtering. XPS spectra collected from a sputtering-cleaned U specimen were used as reference. For the hydriding sample, the U4f-oxide signals in Fig. 8 exhibit an obvious tendency to shift toward a higher binding energy (BE) as the sputtering progressed. In addition, single low-intensity satellites positioned at 6.5 and 6.8 eV to the high-BE side of U4f7/2 are apparent for the hydriding sample before and after sputtering, respectively. These results are in agreement with those of Pireaux et al.26 and Allen et al.27 who reported that the UO 2 surface possesses satellite structures observed at 6.4 and 8.2 eV, respectively. In the transition area, the U4f7/2 peak increases to 381.4 eV, due to H+ cation bind the oxyanion together, which agreed with a reported result.28

Fig. 8 a U4f, b O1s, and c valence band X-ray photoelectron spectroscopy (XPS) spectra of sample surface Full size image

Differences before and after sputtering are apparent for the O1s peaks, in terms of the peak positions and shapes. The O1s peak shifted to a higher BE after the sputtering process. From the U4f peak discussed above, the component corresponding to the three O1s-U peaks was determined to be UO 2 . Note that a O1s-H peak positioned at 532.2 eV could be clearly observed for the non-sputtered surface. After sputtering, O1s-H peaks positioned at 531.8 eV were discovered through peak assignment using XPS Peak 4.1 Software, as shown in Fig. 9. These peaks shifted slightly to a higher BE. These results prove the presence of H atoms located in the transformation regions, from a different approach. In addition, the data are in good agreement with those acquired from the SIMS analysis.

Fig. 9 Fitting spectra for a O1s of hydriding sample before sputtering, b O1s of hydriding sample sputtered for 5 min, c O1s of hydriding sample sputtered for 10 min, d U4f of hydriding sample before sputtering, e U4f of hydriding sample sputtered for 5 min and f U4f of hydriding sample sputtered for 10 min Full size image

The relative sensitivity factor method was applied to calculate the O/U concentration ratios for each situation, where

$$C_O/C_U = I_O/S_O/I_U/S_U.$$ (1)