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This article is devoted to the qualitative analysis of the Zircaloy‐4 degradation mechanism at 850 °C in oxygen/nitrogen partial pressure atmospheres. Thermogravimetry, optical microscopy, scanning electron microscopy, and energy dispersive X‐ray spectrometry, are used to provide some information regarding the oxidation kinetics and the various phases involved along the process. The kinetic curves reveal two stages: a pre‐transition and a post‐transition one. Oxide growth during the pre‐transition stage is controlled by oxygen vacancy diffusion in the oxide layer, since neither oxygen nor nitrogen partial pressure influences the kinetics. In the post‐transition stage, nitrogen has an accelerating effect in the corrosion reaction. The kinetic curves reveal two distinct behaviors after the transition according to the oxygen and nitrogen partial pressures: in the first case the corrosion rate rises substantially and leads to a rapid degradation of the metal at high oxygen and nitrogen partial pressures, in the second case the corrosion rate rises dramatically and reaches a plateau at low oxygen and nitrogen partial pressures. This paper describes the influence of both gases on the corrosion kinetics in relation with morphological observations, analyses the discrepancies found between high and low oxygen and nitrogen partial pressures; it also exhibits the complexity of the solid state transformations due to three distinct reactions that appear to take place during the corrosion process.

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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.