Oxidation kinetics of ZrNbO in steam: Differences between the pre- and post-transition stages

Abstract

Zirconium based alloys (and particularly Zircaloy-4) are widely used as cladding material of fuel rods in water-cooled nuclear reactors. However, advanced alloys are now used for longer operation time in more severe conditions. One of these new alloys is a ZrNbO, which contains 1% of Nb as alloying element (instead of Sn in the case of Zircaloy-4).This ZrNbO alloy, like many zirconium alloys, undergoes a kinetic transition, which is a sharp increase in the oxidation rate when the oxide thickness exceeds a critical value. In the pre-transition stage, the kinetic oxidation curves are approximately parabolic (which is generally interpreted by a diffusion controlling step), and tend towards a quasi-linear behaviour after the transition. Recent studies have shown that the oxygen pressure has an accelerating effect on the oxidation rate of ZrNbO; a platinum layer deposited on the oxide surface has the same effect, both in oxygen and in water vapour. Thus it has been suggested that the oxidation in the pre-transition stage could be described by a mixed reaction-diffusion kinetics. Due to the lack of consistent data on the effect of water vapour on the oxidation rate of ZrNbO before and after the transition, and to confirm (or not) these interpretations, we have studied the oxidation of ZrNbO by isothermal thermogravimetry at 550°C, under controlled partial pressures of water vapour (13 to 80 hPa) and hydrogen (10 hPa). The aim is to verify if the oxidation proceeds in a steady state and if there is a rate-limiting step in one (or both) stages, and to clearly evidence the differences between the pre-and post transition stages from the kinetic point of view.TG-DSC experiments have shown that the system is in a steady state from the beginning of the oxidation, in the pre-and post-transition stages. Then, the existence of a ratelimiting step was verified using an experimental method based on temperature or pressure jumps: it has been concluded that this assumption is valid in both stages, which is not compatible with the assumption of a mixed reaction-diffusion controlled rate). Moreover, it comes out that the kinetic behaviour is different before and after the transition: the influence of temperature jumps is greater in pre-transition, whereas the effect of water vapour partial pressure is more pronounced in post-transition (nevertheless, an accelerating effect is also observed before the transition). No influence of hydrogen partial pressure has been observed. Besides, the higher the Nb content in the alloy, the higher the oxidation rate (in pre-transition). A mechanism has been proposed to account for the results obtained in pretransition, involving the diffusion of adsorbed species in the porous part of the oxide layer as rate-determining step.Keywords: ZrNbO ; oxidation ; kinetics ; water vapour ; steady state ; rate-limiting step. 1 IntroductionDespite numerous works dedicated to the oxidation kinetics of zirconium alloys by pressurized water, water vapour or oxygen, the mechanisms and the rate-limiting steps...

Air oxidation of Zircaloy-4 in the 600–1000°C temperature range: Modeling for ASTEC code application

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Mechanism of corrosion of zirconium hydride and impact of precipitated hydrides on the Zircaloy-4 corrosion behaviour

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Experimental study of oxidation in oxygen, nitrogen and steam mixtures at 850∘C of pre-oxidized Zircaloy-4

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