| Zirconium alloys are used as the fuel cladding materials in water-cooled nuclear power reactors. Corrosion and hydrogen uptake are two important issues in the application of zirconium alloys, which involves the lifetime of fuel assembles, and the safety and reliability of the operation for nuclear power reactors. Hence, how to improve the corrosion resistance and reduce the amount of hydrogen uptake of the fuel cladding for high burn-up of fuel assemblies need to be further investigated. In this study, Zr-2 (Zr-1.5Sn-0.2Fe-0.1Cr-0.05Ni), Zr-4 (Zr-1.5Sn-0.2Fe-0.1Cr), N36 (Zr-1Sn-1Nb-0.3Fe) and N18 (Zr-1Sn-0.35Nb-0.3Fe-0.1Cr) alloys were prepared to obtain specimens with different second phase particles (SPPs) in size and number by different heat treatments, respectively. The corrosion and hydrogen uptake behaviors of these specimens were investigated after autoclave testing in superheated steam at 400℃/10.3MPa and in 0.01 M LiOH aqueous solution at 360℃/18.6MPa, respectively. The mechanism on the impact of SPPs on hydrogen uptake behavior during corrosion tests was discussed. The main experimental results and conclusions are as follows:1. Theβ-quenched Zr-4 specimens possess superior corrosion resistance in lithiated water at 360℃, the weight gain of which is consistently comparable to that of ZIRLO and N18 alloys during 529 days exposure. The supersaturated solid solution contents of Fe and Cr inα-Zr matrix amount to about 700μg/g after P-quenching respectively. This is responsible for the improvement of corrosion resistance by P-quenching treatment. It provides a new thought in developing advanced zirconium alloys.2. The impact of heat treatment on the corrosion resistance of Zr-2 and Zr-4 alloys in superheated steam at 400℃is different from that in lithiated water at 360℃. The heat treatments by fast cooling from P-phase and upper temperature in a-phase are of benefit to improve the corrosion resistance in lithiated water at 360℃, but degrade the corrosion resistance in superheated steam at 400℃. This illustrates that the increase of supersaturated solid solution contents of Fe, Cr and Ni inα-Zr matrix is detrimental to the corrosion resistance of Zr-2 and Zr-4 testing in superheated steam at 400℃.3. In the case of N36 alloy with higher Nb content, the residualβ-Zr inα-Zr grains, which is formed during 820℃/2h treatment, doesn't occur to decompose after ageing treatment at 580℃for 50h. However, the cold-rolling before the ageing treatment promotes its decomposition to form a banding distribution of SPPs. Andβ-quenching treatment before cold-rolling can obtain the SPPs with nano-meter size and uniform distribution. Corrosion testing shows that the impact of the microstructure on the corrosion resistance of Zr-Sn-Nb alloys in superheated steam at 400℃is similar to in lithiated water at 360℃, i.e., residualβ-Zr inα-Zr matrix is detrimental to the corrosion resistance; the decomposition ofβZr and the SPPs with nano-meter size and uniform distribution can significantly improve their corrosion resistance.4. The corrosion resistance of N18 alloy is superior to N36 alloy, whether corroded in super-heated steam at 400℃or in lithiated water at 360℃. After N18 and N36 alloys are heat-treated by 700-1020℃, cold rolling and 580℃/50h aging, the Nb contents inα-Zr matrix are 0.15-0.3% and 0.55-0.57%, respectively, the former of which is closer to the equilibrium solubility at the corrosion testing temperature. This is responsible for the superior corrosion resistance of N18 to N36. From the standpoint of improving corrosion resistance, the addition content of Nb shouldn't be too high.5. When corroded in superheated steam at 400℃, the corrosion resistance of N18 and N36 alloys is inferior to Zr-4 alloy prepared by conventional procedure, but when corroded in lithiated water at 360℃, the corrosion resistance of the two former alloys is superior to the latter one. This further confirms that the impact of alloying composition on the corrosion resistance is different in different water chemistry.6. The composition of alloying elements has a significant impact on hydrogen uptake behavior during corrosion testing. Comparing the hydrogen content in the specimens normalized by weight gains after corrosion testing, the hydrogen uptake amount of Zr-2 alloy is the highest, followed by Zr-4 and N18 alloys, and that of N36 alloy is the lowest, which is closely related to the kinds and composition of SPPs. Zr-2 alloy contains Ni, Fe, Cr, which will form Zr2(Fe, Ni) and Zr(Fe,Cr)2 SPPs with Zr; Zr-4 alloy contains Fe, Cr, which will form Zr(Fe,Cr)2 SPPs with Zr; N18 alloy is an advanced alloy by adding Nb to Zr-4, which also contains Zr(Fe,Cr)2 SPPs; Compared with N18, N36 alloy doesn't contain Cr, but the Nb content is higher, in which the SPPs areβ-Nb and Nb-Zr-Fe. It is well known that Zr2(Fe,Ni) and Zr(Fe,Cr)2 intermetallics are extremely reactive with hydrogen in their metallic state and their hydrogen absorption are faster than that of Zr. This indicates that if the SPPs inα-Zr matrix are more reactive with hydrogen than Zr does, they will have a significant impact on the hydrogen uptake behavior of zirconium alloys during corrosion testing. From the standpoint of reducing hydrogen uptake during corrosion testing, the contents of alloying elements precipitated as such kind of SPPs should be as low as possible. This result provides a useful reference for the selection of alloying elements in developing advanced zirconium alloys.7. The impact degree of heat treatment on the hydrogen uptake behavior during corrosion testing is different for different zirconium alloys. It is the largest for Zr-2 alloy, followed by Zr-4, N18 and 36 alloys. This is related to the size and number of SPPs, besides the kind of SPPs. From the standpoint of reducing hydrogen uptake during corrosion testing, when the SPPs in zirconium alloys are strong hydrogen absorption materials, heat treatment should be used to minimize the size of this kind of SPPs. This result provides a guide for the optimization of heat treatment procedure, control of the microstructure to obtain excellent comprehensive properties of zirconium alloys during their processing.8. The corrosion temperature has a very significant impact on the hydrogen uptake behavior of zirconium alloys. The hydrogen uptake fraction of zirconium alloys corroded in superheated steam at 400℃(20-40%) is twofold as high as that corroded in lithiatd water at 360℃(10-20%). This indicates that if the development of advanced zirconium alloy for fuel cladding used in supercritical water reactors is considered, the issue of hydrogen uptake should be given more attention, in addition to improving corrosion resistance of zirconium alloys.9. The results of SIMS analysis verify that H+ and OH- exist in Zr-4 oxide film formed whether corroded in superheated steam at 400℃or in lithiated water at 360℃. Based on this phenomena, a mechanism on hydrogen uptake of zirconium alloys during corrosion testing is proposed as follows: the water molecules in corrosion mediums get electrons diffusing from metal / oxide film interface to the outer surface of oxide film and occur to the reaction of H2O+e→H +OH+; OH- goes through the oxide film to metal / oxide film interface and react with zirconium ( 2OH- + Zr→ZrO2 +2H+2e) to generate hydrogen, a part of which can be absorbed byα-Zr matrix. Based on such hydrogen uptake mechanism, the mechanism on the impact of SPPs on hydrogen uptake behavior of zirconium alloys during corrosion testing is proposed as follows: when the SPPs are more reactive with hydrogen than Zr does, the kind of SPPs embedded inα-Zr matrix and exposed at the metal/oxide interface could act as a preferred path for hydrogen uptake, thus their size and number will have a significant impact on the hydrogen uptake behavior during corrosion testing; when the SPPs are less reactive with hydrogen than Zr does, the hydrogen uptake behavior depends largely on matrix ofα-Zr itself, and the size and number of this kind of SPPs have a little impact on the hydrogen behavior during corrosion testing.
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