Qualitative analysis of Zircaloy‐4 cladding air degradation in O₂–N₂ mixtures at high temperature

Abstract

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.

In-situ digital image correlation for fracture analysis of oxides formed on zirconium alloys

“…In order to understand oxidation during loss of coolant accidents (LOCA) many authors have researched oxidation of zirconium alloys in the 600-1200°C temperature range [21][22][23][24][25].…”

“…Whether oxidation occurs in air or steam zirconium alloys typically demonstrate a transition to accelerated breakaway corrosion kinetics [21][22][23]. Research has indicated this acceleration is associated with the formation of large cracks perpendicular to the metal-oxide interface, and the formation of a defective, heavily-fractured oxide layer [21,24,25].…”

High-Temperature Oxidation of Zircaloy-4 in Oxygen–Nitrogen Mixtures

“…This nonhomogeneity could be caused by different thermal-hydraulic boundary conditions at the external and internal surfaces of the tube segments, by changing gas composition along the sample due to consumption of oxygen, and by the inherent local initiation of nitrogen attack as described e.g. in [12,15]. Although the experiments did not exhibit ideal parabolic to linear transitions, the rate constants of all tests were determined based on parabolic and linear reaction kinetics, i.e.…”

“…Most samples reveal a non-homogeneous surface. This is caused by the local initiation of nitrogen attack as it was observed in earlier experiments on air oxidation of zirconium alloys [12,15]. Furthermore, and especially for gas mixtures with low concentration of one gas and increasing with temperature, the composition and therefore the oxidation conditions may change with the height of the sample.…”

“…Hence, air oxidation of zirconium alloys was investigated in numerous studies as well, e.g. [10][11][12][13][14][15]. It was found that the formation of zirconium nitride at the metal-oxide interface and its re-oxidation with continuing reaction causes the formation of very porous oxide scales and hence significantly accelerates oxidation kinetics.…”