Niobium speciation at the metal/oxide interface of corroded niobium-doped Zircaloys: A X-ray absorption near-edge structure study

Froideval, Annick; Degueldre, C.; Segre, Carlo U.; Pouchon, Manuel A.; Grolimund, Daniel

doi:10.1016/j.corsci.2008.01.011

Cited by 40 publications

(16 citation statements)

References 14 publications

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“…The assumption that Nb exists only in the 5+ charge state in the oxide layer is supported by previous DFT simulation work [11]. However, several independent x-ray absorption near-edge spectroscopy (XANES) studies have identified oxidation states ranging from 5+ to 2+ (and also metallic Nb) in the oxide layer of Zr-Nb alloys [12,13,14,15,16]. Recently there has been some debate regarding the location of these lower charge state Nb ions (i.e.…”

Section: Introductionmentioning

confidence: 94%

The effect of Nb on the corrosion and hydrogen pick-up of Zr alloys

et al. 2017

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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, but can exist in oxidation states ranging from 5 + to 3 + in the tetragonal phase. Nb5+ is charge balanced by Zr vacancies in both phases, suggesting that contrary to previous assumptions, Nb does not act as an n-type dopant in the oxide layer. Clusters containing oxygen vacancies were considered, Nb2+ was shown to exist in the tetragonal phase with a binding energy of 2.4 eV. This supports the proposed mechanism whereby low oxidation state Nb ions (2 + or 3+) charge balance the build-up of positive space-charge in the oxide layer, increasing oxygen vacancy and electron mobility, leading to near-parabolic corrosion kinetics and a reduced hydrogen pick-up. Previous experimental work has shown that tetragonal ZrO2 transforms to the monoclinic phase during transition, and that during transition a sharp drop in the instantaneous hydrogen pick-up fraction occurs. The oxidation of lower charge state Nb defects to Nb5+ during this phase change, and the consequent temporary n-doping of the oxide layer, is proposed as an explanation for the drop in hydrogen pick-up during transition

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Section: Introductionmentioning

confidence: 94%

The effect of Nb on the corrosion and hydrogen pick-up of Zr alloys

et al. 2017

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“…However, experimental work [23,24] has consistently suggested Nb in ZrO 2 in fact occupies a range of states between 0 and 5+, with one X-ray absorption near edge structure (XANES) study by Froideval et al [25] unable to identify any 5+ Nb at all, thus casting doubt on the previous assumptions regarding the method by which Nb improves corrosion resistance and HPUF. Nb and Sn demonstrate some solid solubility in α-Zr and so an even distribution throughout the alloy is expected; this is an important point as the current investigation considers only single and paired defects isolated in a bulk oxide and thus an even distribution of dopants in the oxide layer is assumed.…”

Section: ••mentioning

confidence: 99%

The influence of alloying elements on the corrosion of Zr alloys

et al. 2016

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Density functional theory (DFT) and autoclave corrosion tests in 360°C water were used to investigate the influence of Sb, Sc, Nb and Sn on the corrosion and hydrogen pick-up (HPU) of Zr-alloys. Sc was shown to have a strongly detrimental effect on alloy corrosion resistance. The Nb-Sb-Zr ternary alloy exhibited significantly improved corrosion resistance over Zr-Nb and ZIRLO, and had little measurable HPU after 195 days. The ratio of Sb Zr /Sb• Zr was shown to transition smoothly with applied space charge, implying Sb can act as a buffer to charge imbalance in the oxide layer.

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“…Nevertheless, this assumption has been challenged recently through XANES work which instead suggests the charge state is between 2+ and 4+ [55,56]. Recent theoretical and experimental work by Couet et al [57]has suggested another possible source of positive charge close to the metal-oxide interface due to a space charge effect caused by the non-equilibrium distribution of electrons in the insulating oxide layer.…”

Section: Tin Interaction With Other Alloying Elementsmentioning

confidence: 99%