Effect of hydrochemistry and alloy composition on microstructure evolution of oxide film during corrosion of zirconium alloys

During the operation of nuclear power plant, H3BO3 was added to the primary water, and 10B was used as a combustible poison to control and regulate the excess nuclear reactivity. Alkaline water (pH=7.1~7.2) is needed to reduce the release of corrosion products from various steel components and the migration of radioactive materials in the primary circuit in order to reduce the radiation dose level received by workers. For this reason, LiOH should be added to adjust the pH value while adding H3BO3 in the primary water. The addition of LiOH has a harmful effect on the corrosion resistance of fuel cladding zirconium alloy, which reduces the time of corrosion transition and increases the corrosion rate after the transition. At present, although some mechanisms have been proposed to explain this accelerated corrosion process, no unified understanding has been formed.

In the research and development of new zirconium alloy, it was found that after adding Nb to the composition of Zr-4 alloy, Zirlo alloy of Zr-1Sn-1Nb-0.1Fe, E635 alloy of Zr-1.2Sn-1Nb-0.4Fe and N18(or NZ2) alloy of Zr-1Sn-0.4Nb-0.3Fe-0.1Cr were obtained by inhibiting the harmful effect of LiOH solution on the corrosion resistance of zirconium alloy A variety of alloys. The corrosion resistance of these zirconium alloys in aqueous solution of LiOH is obviously better than that of Zr-4, but the mechanism of this effect is still not clear. The further development of new zirconium alloys will surely be promoted by studying this law and mechanism.

The cracks and pores in the oxide film after the corrosion transition of zirconium alloy were measured by mercury pressure porometer. According to the change of mercury pressure, the pores were divided into the cracks and micro-cracks with the size of more than 20 nm and the pores with the diameter of 2nm~6nm. The cracks and pores in the oxide film were classified. The International Atomic Energy Agency (IAEA) organized a number of experts to summarize and evaluate the research results of water side corrosion of zirconium alloys used in nuclear power plants, and formed the IAEA-TECDOC-966 document, which also pointed out that the nature of the holes in the oxide film and the formation mechanism should be further studied.

The evolution of the microstructure and crystal structure of the oxide film during the corrosion process of Zr-4 alloy was studied, and the close relationship between them and the change of corrosion kinetics was pointed out. When the oxide film is formed on the surface of the alloy, due to the volume expansion and the constraint of the metal matrix, a great compressive stress will be formed inside the oxide film, which causes many defects in the zirconia crystal and stabilizes some metastable phases. Under the action of temperature, stress and time, defects such as vacancies and interstitial atoms diffuse, annihilate and agglomerate, and the vacancies are absorbed by grain boundaries to form nano-sized clusters of pores, which weaken the binding force between grains. The pore clusters further develop into cracks, so that the oxide film loses its original good protection, and the corrosion rate turns. Since the compressive stress generated during the formation of the oxide film is unavoidable, the evolution of the microstructure of the oxide film during the corrosion process is also inevitable. If environmental factors or the composition and structure of the alloy itself precipitate or delay this evolution, the corrosion resistance of zirconium alloys may also deteriorate or improve. Based on this concept, the study of the influence of hydrochemistry and alloy composition on the microstructure evolution of zirconium alloy oxide film and the relationship between it and corrosion kinetics will be helpful to understand some essential problems in corrosion process and provide some new ideas for the research and development of new zirconium alloys.