Modelling of Zircaloy-4 accelerated degradation kinetics in nitrogen–oxygen mixtures at 850 °C

Abstract

Zirconium-based alloys used in PWR cladding show an acceleration of their oxidation kinetics in air at high temperature compared to their behaviour under oxygen or steam alone. This paper presents an analysis of the oxidation kinetics in order to explain the role of nitrogen during the accelerated corrosion. Isothermal thermogravimetry on alloy thin plates was used to collect kinetic data during the reaction of Zircaloy-4 at 850°C in oxygen and nitrogen mixtures. The influence of oxygen and nitrogen partial pressure on the degradation kinetics was studied by a jump method. The presence of nitrogen in the reacting gas enables the formation of zirconium nitride near the oxide-metal interface which acts as a catalytic phase. A three steps reaction path composed of nitride oxidation, α-Zr(O) nitridation and oxidation is proposed. A detailed mechanism and the rate-determining step of the overall process are proposed that account for the experimentally observed dependence of the kinetic rate with the oxygen and nitrogen partial pressures; a kinetic model based on surface nucleation and growth of regions attacked by nitrogen was successful in describing the mass variations with time of exposure at 850°C.

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

“…Here, it may be anticipated that such a spalling is not observed for nitrogen-free atmospheres, simply because the progress of the reaction is much slower. As already discussed [8,11,18], the presence of these zirconium nitride particles and their subsequent re-oxidation certainly play a role in the kinetic acceleration, as they do for bare Zy-4.…”

“…After approximately 3000 s at 850°C, the mass gain rate reaches a minimum value which corresponds to a "kinetic transition", associated to the formation of first cracks in the dense zirconia layer. During the "post-transition" stage, cracks in the oxide layer spread over the surface of the sample allowing gas phase diffusion into the cracked oxide scale [9] with a catalytic effect of nitrogen [11]. The behavior of the pre-oxidized sample is completely different since the mass gain rate always increases.…”

“…The effect of nitrogen is particularly strong in the 600e1000°C temperature range since it leads to early kinetic transition and subsequent accelerated oxidation. A kinetic model, based on surface nucleation and growth of regions attacked by nitrogen [11], enables to account for the fast oxidation of bare Zy-4 in dry air at 850°C. In steam-air atmospheres, less data are available.…”

“…Thus, in this particular case, isotopic labelling makes it possible to confirm that the rate-limiting steps for the kinetics of formation of the oxide are different before and after the kinetic transition. Actually, it was demonstrated in previous publications that the kinetic mechanism is completely different between the pre and post-transition regimes for as-received Zy-4 or ZrNbO samples oxidized between 500 and 550°C in steam [27,28] and for as-received Zy-4 samples oxidized at 850°C in dry air [11]. In the post-transition regime, a significant influence of steam partial pressure (when Zy-4 or ZrNbO samples are oxidized in steam) or oxygen partial pressure (when Zy-4 samples are oxidized in dry air) has been found, contrarily to the kinetic behavior before the transition.…”

Effect of Nitrogen on the Kinetics of Oxide Scale Growth and of Oxygen Dissolution in the Ti6242S Titanium-Based Alloy

“…Finally, at the oxide-metal interface, the oxygen partial pressure is low enough to favor nitride formation as seen for Ti-Cr and Ti-Si alloys where chromium and titanium nitrides were detected [1,2] or as observed in TiAl and Ti 3 Al alloys [12]. As Zr-O system is close to the Ti-O system, it is worth noting that nitrides and oxynitrides of zirconium were also detected, thanks to optical microscopy and Raman spectroscopy, at the oxide-alloy interface and sometimes in the oxide scale for Zircaloy-4 oxidized at 850°C in oxygen and nitrogen mixtures [13]. However, no work on a fine characterization of the oxide-alloy interface was published in order to explain the effect of nitrogen on the oxidation kinetics.…”