www.nature.com/npjmatdeg ARTICLE OPEN Mechanism of surface uranium hydride formation during corrosion of uranium Hefei Ji1, Haoxi Wu1, Qifa Pan2, Dingzhou Cai1, Xiandong Meng1, Xianglin Chen1, Peng Shi1 and Xiaolin Wang1 Uranium is widely used in the nuclear industry and it is well known that uranium hydride, UH3, forms when uranium is exposed to air. The associated volume change during this transition can cause the surface region to crack, compromising structural integrity. Here, hydriding regions beneath hydride surface craters are studied by secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS). Our results indicate that strain transition regions exist, which are induced by hydrogen with a certain thickness between hydride craters and the uranium bulk. The SIMS and XPS results suggest that hydrogen exists covalently with uranium and oxygen in these transition regions. A micro-scale induction period model based on the transition region and previous hydriding models is developed. Within the so-called micro-scale induction period, hydrogen diffuses and accumulates at particular sites before reaching the critical concentration required to form stoichiometric UH3 in the transition regions. npj Materials Degradation (2019) 3:9 ; https://doi.org/10.1038/s41529-019-0073-6 INTRODUCTION critical concentration at isolated locations. Thus, H-induced U Uranium hydride (UH ), a radioactive intermediate-level waste corrosion follows a localized, spatially heterogeneous, and random 3 3,4,10,11 material with pyrophoricity when exposed to air, is an unwanted pattern of hydride initiation. Metallic U is in the orthor- corrosion product of metallic uranium (U). Moreover, because of hombic α-phase with significant anisotropy, and the bulk material the difference in density between the metallic U (19.1 g/cm3) and contains many defects including macro-scale inclusions and hydride (10.95 g/cm3), the regional attack of hydrogen results in micro-scale boundaries, vacancies, and impurities. Investigations robust volume expansion underneath the oxide at the metal-oxide of H-induced metallic U corrosion and its alloys have provided interface. The volume expansion breaks open the overlayer, evidences that each of the above-mentioned defect types 1,5,6,12,13 reduces the surface integrity and even leads to material failure. provides a corrosion initiation site for hydride. Electron Hence, U hydrogenation problems have been generally studied in backscatter diffraction (EBSD) has been utilized by Bingert et al.5 the past several decades. The reaction between U and H are to prove the significant finding that grain and twin boundaries widely accepted to be governed by four stages: the induction strongly influence initial hydride site locations.14 Harker et al.15 period, nucleation and growth period, bulk reaction, and have found that inclusions have an effect on the initial location of termination period.1,2 The first two are thought to be the key the H attacks, based on focused ion beam (FIB) analysis. Further, periods, during which H aggregates and hydride sites nucleate Arkush et al.4 and Moreno et al.12 have observed hydride corrosion and grow. In the induction period, hydrogen penetrates the at twin and grain boundaries using scanning electron microscopy surface passivation layer of oxide then gathers at the oxide-metal (SEM). Atomic force microscopy (AFM) has been employed by 16 interface. For the nucleation and growth period, once the hydride Balooch and Hamza to directly observe UH3 growth at grain nucleated and subsequently grew, because of the volume boundaries. Based on the above findings, it has been universally expansion the surface oxide layer will be broken through, concluded that some particular positions on or in the U near- meanwhile hydrogen keeps on penetrating into and reacting surface region can be investigated as initial nucleation sites, with with the bulk material. The hydride sites always grow along the the aim of determining the hydriding characteristics. Previous 11,17–19 surface and into the bulk with the progression of the reaction results also indicate that hydride nucleates at the α-U-U2Ti front to form discrete hydride craters. boundary, and that H reacts preferentially with α-U. With regards Hydriding pitting locations have been one of the key concerned to the nucleation and growth stage, Hill et al.13 and Scott et al.20 issues of many studies.1,3–7 Previous work indicates that, the H have found the phenomenon of hydride expansion in a particular solubility limit in α-U at room temperature (293 K, 20 °C) and 1-atm direction (i.e., twinning). Further, the authors of this article have pressure is approximately 0.04 wppm (weight parts per million, discovered that this hydride expansion occurs preferentially 1 μg/g).8 The critical concentration required to form stoichiometric towards the metal bulk or surface (work unpublished). 9 UH3 throughout the metal bulk is approximately 12,700 wppm, The U hydriding could be divided several stages as mentioned being six orders of magnitude higher than the dissolved H above, and until now, the controlling factors for hydride pitting concentration. Therefore, UH3 transformation must occur locally sites in the initial stage are still the key points of related study. rather than uniformly across the surface, under conditions in However, relatively less attention was given on the hydride which the dissolved H atoms can accumulate and exceed the growth stage except for the reaction kinetic studies. In fact, how 1Institute of Materials Research, China Academy of Engineering Physics, 621900 Mianyang, China and 2Science and Technology on Surface Physics and Chemistry Laboratory, P.O. Box Nos. 9–35, Huafengxincun, 621908 Jiangyou City, China Correspondence: Peng Shi ([email protected]) or Xiaolin Wang ([email protected]) Received: 18 September 2018 Accepted: 1 February 2019 Published in partnership with CSCP and USTB H. Ji et al. 2 Fig. 1 Laser scanning confocal microscope (LSCM) photographs of uranium (U) surface a after hydriding and b, c after slight surface polishing 1234567890():,; Fig. 2 a Laser scanning confocal microscope (LSCM) photograph of a hydride crater. b Plot corresponding to the yellow line of the same crater the reaction front extends during the dynamic reaction procedure like gray transition regions, which lie between the black craters. strongly affect the corrosion behavior including the crater shape, Figure 2b shows a cross-section outline corresponding to the which is an important factor for material performance. With regard yellow line in Fig. 2a, located on the crater center. In the Fig. 2b to a hydride crater (surface oxide layer being broken up), plot, material above the blue line was removed by the polishing hydrogen may firstly diffuse and distribute in the bulk near crater process. The remaining transition regions and shallow crater are edge, once the H concentration reaches the critical value in distributed on the pre-crater position. Further, line-type or plate- particular sites, phase transformation happens and reaction like gray cracks are distributed in the transition region (Fig. 1c). H propagates. The internal metal microstructure characteristics atoms had diffused into these cracks, which can be easily may play a vital role in the hydride growth. distinguished by their gray color, since the craters are black and In this work, U hydriding experiments are performed with focus the bulk color is lighter. on the H attacking regions beneath the hydride craters. A strain The cross-section morphology of the hydride craters was transition region was found in the vicinity of the crater edge. acquired by polishing the reaction sample in the direction Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) and X- perpendicular to the reaction surface. Figure 3 illustrates the ray photoelectron spectroscopy (XPS) are utilized to examine the cross-section morphology of one hydride crater. From Fig. 3a, we transition regions beneath the hydride craters. The existence of a can see that the hydrogen attacked region shows a semi-elliptical micro-scale induction period, which is different from the starting shape. A number of line-type gray cracks could be observed near induction period, before UH3 phase transformation during the the reaction front, this is identical to the surface morphology hydriding growth procedure is also deduced. Within this induction around the large hydriding craters, as shown in Fig. 1c. period, the H atoms diffuse and accumulate at some particular These line-type gray cracks are possibly related with the sites before reaching the critical concentration required to form transformation twinning region. First, regardless of the degree of stoichiometric UH3. 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 RESULTS regions must be attributed to the H action on these regions. A number of hydride sites could be observed after hydriding Second, a previous EBSD study reported extensive twinning within experiments, these hydride sites grew in a specific direction or the metal grains in the vicinity of hydride craters.7 It is believed simply stochastically alongside the craters, as shown in Fig. 1a. that transformation twinning occurred in that case because of the This photograph illustrates the hydride expansion, focusing on the generation and accumulation of internal stress during the sample surface. To reveal the under-surface information, the post- reaction, accompanied by volume expansion due to the hydride hydriding sample was then abraded and polished slightly layer by formation.7,21 EBSD analyses have proven that twinning in U layer. The corresponding results after approximately 5 μm are occurs at nucleation sites, and hydride grows in conjunction with shown in Fig. 1b, c, in which the black features are hydride craters twinning.5,13 Under these conditions, the twinning may act as a that were not fully abraded. The bulk material could also be kind of diffusion channel and H may congregate in these internal distinguished by the color difference.