Contribution to the Understanding of the Effect of the Water Chemistry on the Oxidation Kinetics of Zircaloy-4 Cladding

Abstract

Zircaloy-4 cladding has been oxidized in autoclave at 633 K with various chemistry conditions ([Li+] = 0, 10, 70, 700 ppm and [B] = 0, 650 ppm as boric acid) to quantify the effect of lithium hydroxide and boric acid on the oxidation kinetics and to provide oxide films that have been analyzed by SIMS, SEM, and RAMAN spectroscopy to improve our knowledge of the oxidation process in a lithiated environment. Additional specific tests consisting of isotopic exchanges (6Li+/7Li+) have also been conducted to study the incorporation of lithium in the oxide films. The main results are that: • Depending on the water chemistry, the lithium hydroxide has two effects on the oxidation kinetics: a decrease of the time to transition and a strong enhancement of the post-transition oxidation rate. • The boric acid strongly reduces these two effects of lithium hydroxide. • In the oxide film, the lithium is located mainly in the pores (or adsorbed on their walls). • The lithium has a quick access to the bulk of the oxide film, even in the inner barrier layer (considered as not completely impervious). • The enhancement of the oxidation rate due to lithium hydroxide is linked to the degradual alteration of the inner barrier layer, up to its quasi-disappearance. These experimental results are described extensively, and several factors involved in the alteration of the inner barrier layer, such as lithium pickup in the ZrO2 film, evolution of the morphology of the oxide grains, tetragonal zirconia phase transformation, and hydriding, are then discussed.

The Distribution of Li Ions in the Oxide Film Formed on Zircaloy-4 Corroded in Lithiated Water at 633 K

“…Kim et al [9] suggested that the Li ions can accelerate t-ZrO 2 transformation to m-ZrO 2 since the Li + can substitute the Zr 4+ in the oxide lattice; however, it supports the view proposed by Jeong [5,6]. In 2000, Pêcheur et al [8] found that the concentration of Li ions at the interface between the oxide and matrix (O/M) dropped sharply, and they suggested that the Li ions could not enter in the dense oxide film. Thus, we can infer that the effect of Li ions on the transformation of t-ZrO 2 to m-ZrO 2 is limited.…”

“…However, it is well known that the energy for the process of Li + to replace Zr 4+ in the oxide film is high; there is no way of knowing where the energy comes from. Pêcheur et al [7,8] considered that the dense oxide layer could be modified and eliminated by Li ions and that this could be the reason for the enhancement of the corrosion rate of Zircaloy-4 corroded in lithiated water. Kim et al [9] suggested that the Li ions can accelerate t-ZrO 2 transformation to m-ZrO 2 since the Li + can substitute the Zr 4+ in the oxide lattice; however, it supports the view proposed by Jeong [5,6].…”

The measurement of stress and phase fraction distributions in pre and post-transition Zircaloy oxides using nano-beam synchrotron X-ray diffraction

“…This phase transformation has been associated with increasingly compressive stresses in the monoclinic phase at the point of transition [5], with cracking in the oxide [2], and with the linking-up of nano-porosity [15] thereby aiding the transport of the environment to the metal-oxide interface [1,15,20,21]. Pêcheur et al [22] associated the accelerated corrosion rate to the gradual transformation of the tetragonal phase to the monoclinic phase in the protective 'barrier layer'.…”

Effect of surface nanocrystallization on the corrosion behavior of Zircaloy-4

“…It is well known that the corrosion resistance of Zircaloy-4 mainly depends on the compaction and integrity of the oxide films during the initial corrosion stages. The oxide films were composed of t-ZrO 2 [19,20] . Simultaneously, Cox [21] found that the stability of the t-ZrO 2 was dependent on the compressive stress of oxide/metal interface.…”