The Effect Of Thermal Processing And Alloying Element Cu On The Corrosion Resistance Of Zirconium Alloys

Zirconium alloys are used commonly as structural components and fuel cladding materials in nuclear fuel assemblies in water cooled power reacters, which have low thermal neutron absorption cross-section and good corrosion resistance. The corrosion resistance of zirconium alloy is one of the main factors limiting the service life of fuel assemblies, and can be improved effectively by optimizing the thermal processing and adding new elements. In this work, N18(Zr-1Sn-0.35Nb-0.3Fe-0.1Cr) and M5(Zr-1Nb-0.16O) and S5(Zr-0.8Sn-0.4Nb-0.4Fe-0.l Cr), alloys were prepared. In order to obtain specimens with different microstructure, different processing technology or adding different contents of Cu was adopted. The corrosion behavior of these specimens were investigated after autoclave testing in 360?C/18.6MPa deionized water, in 0.01 M Li OH aqueous solution at 360?C/18.6MPa, and in superheated steam at 400?C/10.3MPa, respectively. The microstructure of the alloys and oxide films were observed by high resolution transmission electron microscope(HRTEM) and scanning electron microscope(SEM). To provide experimental and theoretical basis for the development of new zirconium alloys, the effect of thermal processing and adding Cu on the corrosion resistance of zirconium alloys was discussed from the viewpoint of the microstructural evolution of oxide film. The main results are as follows:(1) The thermal processing has a great influence on the corrosion resistance of N18 zirconium alloy. The corrosion resistance of specimens is improved obviously by ? phase quenching before cold rolling and annealing due to the nano second phase particles(SPPs) precipitated dispersively. The existence of ?-Zr phase in the microstructures of specimens treated at 780 ℃ and 800 ℃ in dual phase region is harmful to the corrosion resistance. But the corrosion resistance returns to a better level after the decomposition of ?-Zr phase during the following processes of cold rolling and annealing at 580 ℃ to obtain fine SPPs. When the intermediate annealing temperature increased to 740 ℃ before cold rolling and annealing, the SPPs coarsen to several hundred nanometers, which is harmful to the corrosion resistance in 0.01 M Li OH aqueous solution at 360?C/18.6MPa, but has little effect on the corrosion resistance in superheated steam at 400?C/10.3MPa and in 360?C/18.6MPa deionized water.(2) When the addition of Cu in the M5 alloys is below 0.2%, the main SPPs are β-Nb in smaller size, and Cu mainly dissolved in the α-Zr matrix, and the corrosion resistance of the alloys is improved markedly with the increase of Cu content. When the addition of Cu is above 0.2%, the SPPs of Zr2 Cu are precipitated. The size and amount of Zr2 Cu particles become larger with the increase of Cu content. The M5+0.35 Cu alloy, in which the SPPs of Zr2 Cu are moderate in size and amount, shows the best corrosion resistance. However, the M5+0.5Cu alloy, in which the SPPs of Zr2 Cu are larger and more, shows the worst corrosion resistance and nodular corrosion appeared during the autoclave tests at 400 ℃ superheated steam.(3) M5 alloy has poor corrosion resistance in 0.01 M Li OH aqueous solution at 360?C/18.6MPa, and even adding 0.05%~0.5% Cu cannot improve the corrosion resistance of M5 alloy in 0.01 M Li OH aqueous solution at 360?C/18.6MPa.(4) The addition of Cu is harmful to the corrosion resistance of S5 alloy in 0.01 M Li OH aqueous solution at 360 ℃/l8.6 MPa. When the addition of Cu content is less 0.2%, the corrosion resistance of S5 alloy decreases linearly with the increase of Cu content. When the addition of Cu content is above 0.2%, the corrosion resistance of S5+x Cu alloys will become worse sharply with the increase of Cu content. The SPPs of Zr2 Cu become more and bigger with the increase of Cu content, which is harmful to the corrosion resistance of S5+x Cu alloys in 0.01 M Li OH aqueous solution at 360 ℃/ l8.6 MPa.(5) The addition of Cu with 0.05%~0.35% has little effect on the corrosion resistance of S5+x Cu alloys in superheated steam at 400?C/10.3MPa, but when the addition of Cu reaches 0.5%, the corrosion resistance of the S5+x Cu alloy decreases due to the SPPs of Zr2 Cu become larger and more.(6) The microstructure observation of the oxide films shows that: 1) Near the interface of the oxide/metal, there are metastable Zr O2 including amorphous, tetragonal and cubic phases besides stable monoclinic phase. Meanwhile, there are many defects in Zr O2 crystals such as vacancies and interstitials. The diffusion, annihilation and condensation of vacancies and interstitials occur under the interaction of stress, temperature and time. The vacancies absorbed by grain boundaries form pores to weaken the bonding strength between grains, and the pores develop to micro-cracks, and the metastable phases transformed into stable phase. This is the inevitable process of microstructural evolution of oxide film when the zirconium alloys corroded. 2) When the Zr2 Cu SPPs are oxidized, it will be first incorporated in the oxide film, and then oxidized gradually. As the P. B. ratio of Cu is 1.72, a great compressive stress is produced in the surrounding oxide film where the Zr2 Cu SPPs were oxidized. And this will accelerate the evolution of the local microstructure of the oxide film. This maybe the reason why M5+0.5Cu appeared nodular corrosion during the autoclave tests at 400 ℃ superheated steam for 300 d.