Effect of diffusive Nb redistribution on the pitting susceptibility of U-5.5Nb alloys
Introduction
Uranium is widely used as a nuclear fuel material. However, the poor oxidation and corrosion resistance of unalloyed uranium are detrimental to the utilization of the metal, particularly under the conditions present in a reactor where enhanced corrosion properties are necessary during thermal cycling and irradiation. As a result, the corrosion of uranium and improvement of its corrosion resistance has been the subject of a large number of investigations [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]] motivated by a variety of industrial, environmental, and academic activities [[11], [12], [13], [14], [15], [16], [17], [18]]. The U–Nb binary system is of particular interest because of its excellent corrosion resistance relative to unalloyed uranium. It can be considered to be a form of stainless uranium analogous to the Fe–Cr system that serves as the basis for stainless steel [19,20]. For example, U–6 wt%Nb alloys can have corrosion rates that are orders of magnitude lower than that of unalloyed uranium and be considered as potential metallic kernels in dispersed metallic inert matrix fuels [13]. Good corrosion resistance can be obtained for this binary alloy when a sufficient concentration of solute Nb is uniformly distributed throughout the martensite structure (termed α′ or α″), which can be achieved by solution annealing the high-temperature austenite phase (termed γ) followed by cooling, typically by quenching with water, at a rate greater than the critical quenching rate [1,21]. Nevertheless, this homogeneous microstructure is a Nb-supersaturated metastable phase owing to the significant difference in Nb solid solubility between γ-uranium (infinite solid solution) and α-uranium (<0.5 %). Thus, thermodynamically, formation of this phase is expected to result in Nb redistribution, eventually yielding a Nb-depleted α′-phase and a Nb-rich γ1-2-phase in the matrix structure during long-term storage or other service conditions involving thermal excursions [22,23]. In addition, these heat treatment characteristics of U–Nb alloys are sometimes utilized to artificially tune the two-phase pearlite microstructure to achieve high strength and favourable processing performance [24,25]. The chemical redistribution of Nb will undoubtedly deteriorate the corrosion resistance of U–Nb alloys. However, the extent and mechanism of degradation remain unclear and have not been verified experimentally.
Previous works have typically focused on the general corrosion of homogeneous U–Nb alloys rather than local corrosion attacks resulting from Nb segregation. The mechanism by which adding Nb improves the oxidation resistance of U is a major research focus [3,19,26,27], and two basic consensuses have been reached: (1) The outermost oxide layer of U–Nb alloys are composed of UO2 under thermal oxidation conditions and UO3(•nH2O) under electrochemical oxidation conditions. Moreover, Nb enrichment at the oxide–metal interface in the form of NbO, NbO2, or Nb2O5 will enhance corrosion resistance by preventing the diffusion of oxidizing ions. (2) The thickness of the oxides in atmospheric environments and the corrosion rate in aqueous environments decrease approximately linearly as the Nb content increases in homogeneous U–Nb alloys. In recent years, investigations of the correlation between Nb segregation and hydrogen attack in high-temperature-aged U-13 at.% Nb alloys have suggested that Nb-depleted zones are preferential nucleation sites for hydrides [28,29], similar to the Cr-depleted zones in stainless steels, which are susceptible to pitting corrosion in a corrosive aqueous solution [30,31].
In the present work, the pearlite microstructure containing segregated Nb was tailored in the matrix structure of quenched U–5.5 wt.% Nb (U-5.5Nb) alloys by isothermal aging at 400 °C, and the overall kinetics of the transformation was quantified using the quantitative metallography method and the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation [32]. Then, potentiodynamic polarization tests were used to determine the relationship between the pitting potentials and the transformation fractions. An immersion test was conducted on the aged U-5.5Nb alloy with a transformation fraction of 60 % to allow real-time monitoring of the evolution of pit initiation and growth by an in situ optical microscopy. Furthermore, the corrosive pit features were characterized by focused ion beam (FIB)-scanning electron microscopy (SEM). The local Volta potentials in the pearlite zones were determined by scanning Kelvin probe force microscopy (SKPFM) to explain the electrochemical nature of the pitting. The aim of this study was to provide direct micrographic insight into the effects of diffusive Nb redistribution on the pitting susceptibility of U-5.5Nb alloys, which will aid in the development of accident tolerant fuel.
Section snippets
Sample preparation
The nominal U-5.5Nb alloys were vacuum-induction melted from high-purity niobium and depleted uranium, and then cast into a Ø20 mm × 20 mm graphite crucible coated with yttria. The cylinder ingot was machined to a diameter of 15 mm, then wrapped in tantalum foils and encapsulated in an evacuated quartz tube. The quartz capsule was placed inside a resistance furnace for homogenization at 1000 °C for 24 h followed by quenching in water by shattering the tube immediately. Subsequently, the
Microstructural evolution during isothermal aging
The microstructural evolution of the samples isothermally aged at 400 °C is illustrated in Fig. 2(a–e). It can be clearly observed that pearlitic colonies preferentially nucleated along prior γ-phase grain boundaries and grew into grains by consuming the parent matrix phase until the overall decomposition reaction was completed. Hackenberg et al. [20,36] reported that the transformation mechanism is discontinuous precipitation (DP) which is characterised by a discontinuous or abrupt change in
Conclusion
Under thermal activation, niobium redistribution occurred in U-5.5Nb alloys, with excellent corrosion resistance which originated from the homogeneous Nb-supersaturated α″ microstructure, resulting in the emergence of Nb-depleted lamellae. The Nb-depleted lamellae were more susceptible to pitting corrosion and were preferentially corroded in aggressive aqueous environments containing chloride ions because they had the lowest Volta potential within the matrix microstructure. However, the pitting
Declarations of interest
Nothing declared.
CRediT authorship contribution statement
Xianglin Chen: Conceptualization, Writing - original draft, Writing - review & editing. Hefei Ji: Formal analysis, Investigation. Yawen Zhao: Investigation. Yanping Wu: Investigation. Chao Lu: Investigation. Yanzhi Zhang: Investigation. Peng Shi: Resources. Daqiao Meng: Conceptualization, Supervision, Project administration.
Acknowledgements
The author would like to thank D. Cai, Z. Pu, and S. He for fruitful discussions on the electrochemical results. This investigation was supported by the Science Challenge Project of China (No. TZ2016004) and the National Natural Science Foundation of China (No. 51671176).
References (40)
- et al.
Auger electron spectroscopic study of the surface oxidation of uranium–niobium alloy {U–6wt.% Nb} in a UHV environment containing primarily H2, H2O and CO
Surf. Sci.
(2007) - et al.
Uranium and U–Zr and U–Ru alloy corrosion rates in the transpassive state
J. Alloys Compd.
(2007) - et al.
Effect of alloyed Ti on the microstructure and corrosion characteristics of a U–Ti alloy in a hydrogen environment
Corros. Sci.
(2015) - et al.
Hydrogen accumulation in and at the perimeter of U–C–N–O inclusions in uranium – a SIMS analysis
J. Alloys Compd.
(2015) - et al.
The crystallographic structure of the air-grown oxide on depleted uranium metal
Corros. Sci.
(2016) - et al.
Effect of carbo-nitride-rich and oxide-rich inclusions on the pitting susceptibility of depleted uranium
Corros. Sci.
(2017) - et al.
A review of uranium corrosion by hydrogen and the formation of uranium hydride
Corros. Sci.
(2018) - et al.
The effect of work-hardening and thermal annealing on the early stages of the uranium-hydrogen corrosion reaction
Corros. Sci.
(2018) Preshock-induced phase transition in spalled U–0.75 wt% Ti
J. Nucl. Mater.
(1999)Civil use of depleted uranium
J. Environ. Radioact.
(2003)
Rapidly solidified U–6wt%Nb powders for dispersion-type nuclear fuels
J. Nucl. Mater.
Why is weapons grade plutonium more hazardous to work with than highly enriched uranium?
J. Chem. Health Saf.
The influence of impurities on the crystal structure and mechanical properties of additive manufactured U–14at.% Nb
Scr. Mater.
Light impurity atoms as the probes for the electronic structures of actinide dioxides
Comput. Mater. Sci.
Corrosion of uranium in liquid water under vacuum contained conditions. Part 1: The initial binary U + H2O(l) system
Corros. Sci.
Corrosion of uranium in liquid water under contained conditions with a headspace deuterium overpressure. Part 2: The ternary U + H2O(l) + D2 system
Corros. Sci.
An examination of the initial oxidation of a uranium-base alloy (U–14.1 at.% Nb) by O2 and D2O using surface-sensitive techniques
Appl. Surf. Sci.
Crystallographic and kinetic origins of acicular and banded microstructures in U–Nb alloys
J. Nucl. Mater.
Low temperature age hardening in U–13 at.% Nb: an assessment of chemical redistribution mechanisms
J. Nucl. Mater.
Straightforward understanding of the structures of metastable α’’ and possible ordered phases in uranium–niobium alloys from crystallographic simulation
J. Alloys Compd.
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2024, Journal of Nuclear MaterialsEffect of diffusive niobium on the oxidative kinetics of U-2 wt%Nb alloy at different stages
2023, Journal of Alloys and CompoundsCitation Excerpt :Therefore, a great deal techniques have been applied to improve the corrosion resistance of uranium such as alloyed, organic coatings, chemical plating and electroplating coatings, and ion-plating protective layers [2,3]. Ultimately, the U-Nb binary system has gained wide attention for its excellent corrosion resistance [4–6]. For example, the U-2 wt%Nb (shortening as U-2 Nb) alloy is potential metallic kernels in dispersed metallic inert matrix fuels for its excellent mechanical properties and corrosion resistance [7–9].
Research progress in control of structure and performance of uranium-niobium alloys
2020, Cailiao Kexue yu Gongyi/Material Science and Technology