Microstructure changes of zirconium oxide film

Zirconium alloys are widely used as fuel element cladding and other internal components of nuclear power reactors because of their low neutron absorption cross section, excellent corrosion resistance and mechanical properties.

Zr-2, Zr-4 and Zr-1Nb alloys are currently used as nuclear fuel cladding materials.

In recent years, in order to meet the requirements of long life, high burnup and zero damage of reactor fuel assembly, the existing zirconium alloy has been improved all over the world to improve its comprehensive performance. Therefore, by optimizing zirconium niobium and zirconium stannum series alloys, new ZIRLO, M5, E635, N18, HANA, NDA and other zirconium alloys which have been applied in engineering have been obtained.

But different alloy elements on the corrosion behavior of zirconium alloy influence mechanism is different, at present, the corrosion mechanism of zirconium alloy is unclear, containing niobium zirconium alloy corrosion is mainly related to its microstructure, in order to better understand the corrosion behavior of Zr-Nb binary alloy, you have to research the N content and heat treatment system on Zr-Nb micro structure of the alloy.

Therefore, it is necessary to systematically understand the effects of alloying elements, second phase and heat treatment on the corrosion resistance of zirconium alloys, and the relationship between the changes of microstructure and corrosion resistance of zirconium oxide film, which is of great significance for further understanding the corrosion mechanism of zirconium alloys and the development of new zirconium alloys.

It is found that with the extension of annealing time, the grain growth of zirconium alloy with low niobium content is easy, while the grain growth of zirconium alloy with high niobium content is inhibited by the precipitation of the niobium-containing second phase. This is because the martensite transition temperature of Zr-Nb system decreases with the increase of niobium content and cooling rate. It can be guessed that the type of grain generated is related to the niobium content.

Nb in the oxide film can improve the electron conduction performance of the oxide film and reduce the aggregation of H at the metal/oxide film interface, thus reducing the possibility of local rupture of the oxide film.

Before the corrosion transition, the dense barrier layer near the oxide film/metal interface contains more quadrate zirconia, which has high resistance and has high corrosion resistance. However, after the corrosion transition, there is little or no zirconia near the oxide film/metal interface in the tetragonal structure, mainly monocline zirconia, its resistance is very low, corrosion resistance is very poor. Therefore, it is speculated that what kind of oxide film is generated is related to Nb content.