Doping in the Valley of Hydrogen Solubility: A Route to Designing Hydrogen-Resistant Zirconium Alloys

Abstract

Hydrogen pickup and embrittlement pose a challenging safety limit for structural alloys used in a wide range of infrastructure applications, including zirconium alloys in nuclear reactors. Previous experimental observations guide the empirical design of hydrogen-resistant zirconium alloys, but the underlying mechanisms remain undecipherable. Here, we assess two critical prongs of hydrogen pickup through the ZrO 2 passive film that serves as a surface barrier of zirconium alloys; the solubility of hydrogen in it-a detrimental process-and the ease of H 2 gas evolution from its surface-a desirable process. By combining statistical thermodynamics and density-functional-theory calculations, we show that hydrogen solubility in ZrO 2 exhibits a valley shape as a function of the chemical potential of electrons, μ e . Here, μ e , which is tunable by doping, serves as a physical descriptor of hydrogen resistance based on the electronic structure of ZrO 2 . For designing zirconium alloys resistant against hydrogen pickup, we target either a dopant that thermodynamically minimizes the solubility of hydrogen in ZrO 2 at the bottom of this valley (such as Cr) or a dopant that maximizes μ e and kinetically accelerates proton reduction and H 2 evolution at the surface of ZrO 2 (such as Nb, Ta, Mo, W, or P). Maximizing μ e also promotes the predomination of a less-mobile form of hydrogen defect, which can reduce the flux of hydrogen uptake. The analysis presented here for the case of ZrO 2 passive film on Zr alloys serves as a broadly applicable and physically informed framework to uncover doping strategies to mitigate hydrogen embrittlement also in other alloys, such as austenitic steels or nickel alloys, which absorb hydrogen through their surface oxide films.

Oxygen self-diffusion mechanisms in monoclinic ZrO2 revealed and quantified by density functional theory, random walk analysis, and kinetic Monte Carlo calculations

“…In previous work, by combining first-principles-based pointdefect calculations with statistical thermodynamics, we were able to predict equilibrium defect concentrations at various temperatures and oxygen partial pressures both in tetragonal [24] and monoclinic ZrO 2 [16]. Here we calculated the migration barriers of different oxygen defect types and migration paths by first-principles calculations.…”

“…In particular, such knowledge could guide design by, e.g., aliovalent doping and controlling operating environmental conditions. Experimental [12][13][14] and computational [15][16][17][18][19][20] studies have been carried out to atomistically resolve the structure, valence states, and defect chemistry in ZrO 2 . Zirconium-oxygen system phase stability has been examined by first-principles studies, and a range of suboxide structures with oxygen dissolved into the metal phase have been identified [20].…”

Integrated Computational Modeling of Water Side Corrosion in Zirconium Metal Clad Under Nominal LWR Operating Conditions

“…In order to understand how hydrogen is incorporated into the oxide passive film and how it enters the zirconium alloy, we have studied hydrogen-related defects in tetragonal (t-) ZrO 2 35 and monoclinic (m-) ZrO 2 . 30 Both in t-ZrO 2 and under oxygen-poor conditions, hydrogen is found to be more energetically favorable as a substitutional H + occupying an oxygen site. As such, the concentration of substitutional hydrogen increases as one approaches the metal/oxide interface.…”

“…30 We identified two doping strategies that could potentially suppress hydrogen uptake in the oxide. Dopants such as Cr could reduce the hydrogen solubility, while dopants such as Nb, Ta, Mo, W or P could maximize l e and accelerate the H 2 evolution kinetics at the surface.…”

“…In fact, the oxide layer is constantly exposed to a gradient in the chemical potential of oxygen from the oxygen-poor zone at the metal/oxide interface to the oxygen-rich zone at oxide/water interface. We assessed all possible intrinsic point defects in both tetragonal 29 and monoclinic 30 phases of ZrO 2 . Our results revealed that, for the tetragonal phase and under oxygen-poor conditions, the dominant defects are doubly-charged oxygen vacancies compensated by electrons.…”