Abstract:Zr-Nb alloys are known to perform better in corrosion and hydrogen pick-up than other Zr alloys but the mechanism by which this happens is not well understood. Atomistic simulations using density functional theory of both tetragonal and monoclinic ZrO2 were performed, with intrinsic defects and Nb dopants. The overall defect populations with respect to oxygen partial pressure were calculated and presented in the form of Brouwer diagrams. Nb is found to favour 5 + in monoclinic ZrO2 at all partial pressures, bu… Show more
“…Given these rate-limiting factors, one could rationalize the effect of alloying Zr with Nb which empirically is known to reduce the hydrogen pickup fraction [67][68][69]. Comparing Figure 2(a) and (c), one can see that the oxide films in both of the Zr-Nb samples, Figure 2(a) and (b) have a lower concentration of nano-porosity than the oxide on Zircaloy-4, Figure 2(c), which would correlate directly with the measured lower hydrogen pick up fraction because fewer H atoms could reach the metal-oxide boundary.…”
Oxidation of metals by water generates hydrogen which can enter the solid causing serious degradation of its mechanical properties and may also influence the corrosion rate. The present work focuses on hydrogen pickup during the corrosion of zirconium alloys in an aqueous environment. Transmission electron microscopy using Fresnel imaging on three different samples of oxidized Zr has been used to study the type, distribution, concentration and connectivity of nano-porosity as a function of depth through the oxide layer. Extensive interconnected nano-pipes are found in the non-protective outer part of the oxide, while in the protective barrier layer closer to the metal-oxide interface, continuous nano-pipes turn into individual nano-pores. Ab initio calculations show that molecular hydrogen is formed spontaneously by the reaction of water with oxygen vacancies in zirconium oxide. Molecular dynamics simulations reveal that these H2 molecules can diffuse rapidly through nano-pores and nano-pipes as small as 0.5 nm in the oxide layer. Calculations demonstrate that molecular hydrogen dissociates spontaneously on surfaces of suboxides found experimentally at the metal-oxide interface. Oxygen vacancies in ZrO enable the ingress and diffusion of H atoms with an energy barrier of approximately 65 kJ/mol. Further diffusion of hydrogen through oxygen-saturated α-Zr metal is fast, leading to the formation of thermodynamically stable zirconium hydrides. Thus, formation and diffusion of molecular hydrogen through nano-pores in the bulk oxide and ingress of H atoms via suboxides is a possible mechanism of hydrogen pickup in any metal or alloy covered by an oxide scale that contains nano-porosity.
“…Given these rate-limiting factors, one could rationalize the effect of alloying Zr with Nb which empirically is known to reduce the hydrogen pickup fraction [67][68][69]. Comparing Figure 2(a) and (c), one can see that the oxide films in both of the Zr-Nb samples, Figure 2(a) and (b) have a lower concentration of nano-porosity than the oxide on Zircaloy-4, Figure 2(c), which would correlate directly with the measured lower hydrogen pick up fraction because fewer H atoms could reach the metal-oxide boundary.…”
Oxidation of metals by water generates hydrogen which can enter the solid causing serious degradation of its mechanical properties and may also influence the corrosion rate. The present work focuses on hydrogen pickup during the corrosion of zirconium alloys in an aqueous environment. Transmission electron microscopy using Fresnel imaging on three different samples of oxidized Zr has been used to study the type, distribution, concentration and connectivity of nano-porosity as a function of depth through the oxide layer. Extensive interconnected nano-pipes are found in the non-protective outer part of the oxide, while in the protective barrier layer closer to the metal-oxide interface, continuous nano-pipes turn into individual nano-pores. Ab initio calculations show that molecular hydrogen is formed spontaneously by the reaction of water with oxygen vacancies in zirconium oxide. Molecular dynamics simulations reveal that these H2 molecules can diffuse rapidly through nano-pores and nano-pipes as small as 0.5 nm in the oxide layer. Calculations demonstrate that molecular hydrogen dissociates spontaneously on surfaces of suboxides found experimentally at the metal-oxide interface. Oxygen vacancies in ZrO enable the ingress and diffusion of H atoms with an energy barrier of approximately 65 kJ/mol. Further diffusion of hydrogen through oxygen-saturated α-Zr metal is fast, leading to the formation of thermodynamically stable zirconium hydrides. Thus, formation and diffusion of molecular hydrogen through nano-pores in the bulk oxide and ingress of H atoms via suboxides is a possible mechanism of hydrogen pickup in any metal or alloy covered by an oxide scale that contains nano-porosity.
“…This oxidation process commonly consists of pre-transition and transition cycles [3], and while the oxide film grows slowly in the pre-transition regime, obeying 'parabolic' or 'sub-parabolic' growth rate laws [4] [5], 'break-away' growth occurs in the transition regime with much higher oxidation rates [3]. The onset of the detrimental 'break-away' phenomenon of accelerated oxide film growth has been shown to be related to micro-structural and micro-chemical properties of the metal and oxide [6], including the choice of major alloying elements like Nb or Sn [7][8] [9] and the size and chemistry of second phase particles (SPPs) [10]. Under inreactor conditions, the harsh working environment can also affect the corrosion resistance.…”
The stability of the β-Nb Second Phase Particles (SPPs) in two types of Zr-Nb alloys (recrystallised Zr-1.0Nb and Zr-2.5Nb) was studied by in-situ heavy ion irradiation in a transmission electron microscope (TEM), combined with ex-situ analysis by energy dispersive x-ray spectroscopy (EDX). TEM thin foils were irradiated by 1 MeV Kr + ions at four different temperatures from 50 K to 873 K, and by 350 keV Kr + ions at different doses up to 39dpa. The change in size of individual β-Nb SPPs has been measured quantitatively, and the degradation mechanisms under irradiation at different temperatures discussed. It has been shown that the Nb redistribution between the SPPs and the Zr matrix is governed both by radiation induced mixing and local diffusion in the surrounding Zr matrix. Under the radiation conditions reported in this study, the β-Nb SPPs have shown remarkably stability against irradiation, and the extent of Nb redistribution between the SPPs and Zr matrix is very limited under all experimental conditions.
“…The effects of iron additions on the types of defects and their concentrations in ZrO 2 are investigated by carrying out density functional theory (DFT) simulations of iron defects in isolation and in clusters with other iron and intrinsic defects. The need to consider the effect of bound defects on oxidation state, defect populations and therefore overall oxide behaviour was shown by Bell et al for both tin and niobium [38,39]. It was shown for tin that including paired defects significantly increased the oxygen partial pressure at which the 2+ state was stable [39].…”
Simulations based on density functional theory (DFT) were used to investigate the behaviour of substitutional iron in both tetragonal and monoclinic ZrO 2. Brouwer diagrams of predicted defect concentrations, as a function of oxygen partial pressure, suggest that iron behaves as a p-type dopant in monoclinic ZrO 2 while it binds strongly to oxygen vacancies in tetragonal ZrO 2. Analysis of defect relaxation volumes suggest that these results should hold true in thermally grown oxides on zirconium, which is under compressive stresses. X-ray absorption near edge structure (XANES) measurements, performed to determine the oxidation state of iron in Zircaloy-4 oxide samples, revealed that 3+ is the favourable oxidation state but with between a third and half of the iron, still in the metallic Fe 0 state. The DFT calculations on bulk zirconia agree with the preferred oxidation state of iron if it is a substitutional species but do not predict the presence of metallic iron in the oxide. The implications of these results with respect to the corrosion and hydrogen pickup of zirconium cladding are discussed.
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