Microstructure of Oxide Films Formed during the Waterside Corrosion of the Zircaloy-4 Cladding in Lithiated Environment

Zircaloy-4 cladding materials have been oxidized in a lithiated environment in autoclave and in out-of-pile loop tests. In such oxidation tests, a strong enhancement of the oxidation rate can occur depending on the water chemistry conditions and on the oxidation time. In this work, in order to improve our understanding of the detrimental effect of lithium on the corrosion behavior of the Zircaloy-4 cladding, the microstructure of oxide films has been characterized by TEM. Simultaneously, the lithium profiles and concentrations in the oxide layers have been determined using the SIMS technique, special attention being paid to the metal-oxide interface.

These analyses revealed the existence of a correlation between the oxidation rate, the oxide microstructure, and the lithium profiles in the oxide films. Before the occurrence of the strong acceleration of the oxidation rate, the whole oxide layer is mainly composed of columnar grains and the lithium has no access to the metal-oxide interface. On the contrary, after the occurrence of such a strong accelerated corrosion rate, the inner part of the oxide layer is composed mainly of equiaxed grains (up to the metal-oxide interface), lithium then having access to the metaloxide interface.

These experimental results are described extensively and analyzed, and their contribution to the understanding of the influence of lithium on the oxidation process of Zircaloy-4 cladding material is discussed as a function of an existing thin inner barrier layer at the metal-oxide interface.