【作者】 姚美意；

【导师】 周邦新；

【作者基本信息】 上海大学 ， 材料学， 2008， 博士

【摘要】 在水冷核动力反应堆中，锆合金是一种重要的结构材料，用作核燃料的包壳，腐蚀和吸氢是其应用中遇到的两个重要问题，这涉及核燃料元件的寿命和反应堆运行的安全可靠性。随着燃料组件燃耗的进一步提高，如何提高锆合金包壳的耐腐蚀性能和降低腐蚀时的吸氢量是两个值得深入研究的问题。本工作以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）和N18（Zr-1Sn-0．35Nb-0．3Fe-0．1Cr）四种成分锆合金为研究对象，采用不同的热处理制备第二相尺寸和数量不同的样品，研究了这些样品在400℃／10．3MPa过热蒸汽和360℃／18．6MPa/0．01M LiOH水溶液中的腐蚀和吸氢行为，探讨了第二相影响锆合金腐蚀时吸氢行为的机理。得到的主要实验结果和结论如下：1．经β相水淬处理的Zr-4样品在360℃／LiOH水溶液中腐蚀时表现出非常优良的耐腐蚀性能，在长达529d的腐蚀试验中，腐蚀增重一直与ZIRLO和N18合金的相当。β相水淬处理后Zr-4合金基体中过饱和固溶的Fe和Cr含量分别达到700μg/g左右，这是β相水淬处理提高Zr-4合金在360℃／LiOH水溶液中耐腐蚀性能的主要原因，这为开发新锆合金提供了新的思路。2．热处理对Zr-Sn系的Zr-2和Zr04合金在400℃过热蒸汽中耐腐蚀性能的影响规律与在360℃／LiOH水溶液中腐蚀时的不同：从β相快冷和α上限温区处理使Zr-4和Zr-2合金在400℃过热蒸汽中的耐腐蚀性能变坏，但却能改善Zr-4合金在360℃／LiOH水溶液中的耐腐蚀性能。这说明提高α-Zr基体中过饱和固溶的Fe、Cr或Ni合金元素含量会对Zr-4和Zr-2合金在400℃过热蒸汽中的耐腐蚀性能产生有害影响。3．含Nb较高的N36合金在820℃保温2h处理后晶粒内残留的棒状β-Zr在随后偏析温度（610℃）以下580℃时效处理时并未发生分解，但时效处理之前的冷轧变形可促进它的分解，形成条带状分布的第二相；冷轧之前的β相水淬处理可以获得纳米大小、弥散分布的第二相。腐蚀试验表明，显微组织对N18和N36合金在400℃过热蒸汽中耐腐蚀性能的影响规律与360℃／LiOH水溶液中的基本相同：β-Zr的存在对耐腐性能都是有害的，β-Zr分解并获得纳米大小、弥散分布的第二相后可明显改善它们的耐腐蚀性能。4．无论是在400℃过热蒸汽中，还是在360℃／LiOH水溶液中，N18合金的耐腐蚀性能均优于N36合金。N18和N36合金经700～1020℃处理、冷轧和580℃／50h时效处理后，基体中固溶的Nb含量分别为0．15～0．3％和0．55～0．57％，前者更接近腐蚀温度时Nb在α-Zr基体中的平衡固溶度，这是N18合金耐腐蚀性能优于N36合金的原因。从合金元素对锆合金耐腐蚀性能影响的角度来说，添加的Nb含量不宜太高。5．在400℃过热蒸汽中腐蚀时，Zr-Sn-Nb系的N18合金的耐腐蚀性能略低于常规处理的Zr-4合金，但N36合金的耐腐蚀性能明显不如常规处理的Zr-4合金，而在360℃／LiOH水溶液中腐蚀时，N18和N36合金的耐腐蚀性能明显优于常规处理的Zr-4合金，这进一步证实不同水化学条件下合金元素影响锆合金耐腐蚀性能的规律是不同的。6．合金成分对锆合金腐蚀时的吸氢行为有很大影响。用腐蚀增重对样品腐蚀后的氢含量进行归一化处理后，可以看出在相同的腐蚀增重情况下，Zr-2合金腐蚀时的吸氢量最大，其次是Zr-4和N18合金，而N36合金的吸氢量最小，这与不同合金中的第二相种类和成分密切相关。Zr-2合金中含有Ni、Fe、Cr，它们与Zr形成的第二相有Zr₂（Fe,Ni）和Zr（Fe,Cr）₂：Zr-4合金中含有Fe、Cr，它们与Zr形成的第二相有Zr（Fe,Cr）₂；N18合金是在Zr-4合金基础上添加少量Nb的新合金，其中也含有Zr（Fe,Cr）₂第二相；N36合金也是一种含Nb锆合金，与N18合金相比，Nb含量更高但不含Cr，形成的第二相主要为β-Nb和Zr-Nb-Fe第二相。众所周知，Zr₂（Fe,Ni）和Zr（Fe,Cr）₂本身是一种强烈吸氢的金属间化合物，这说明如果添加的合金元素与Zr形成的第二相是一种比Zr吸氢能力更强的物质，那么它对锆合金腐蚀时的吸氢行为会产生显著影响，从减少腐蚀时吸氢的角度来考虑，这种合金元素要尽量少加。这一结果可为开发新锆合金时选择合金元素提供有价值的参考。7．热处理对不同成分锆合金腐蚀时的吸氢行为影响程度不一样，其中对Zr-2合金的影响最大，其次是Zr-4合金，而对N18和N36合金的影响都比较小，这除与第二相的种类有关外，还与第二相的尺寸和数量密切相关。当合金中的第二相本身是一种强烈吸氢的物质时，要尽量减小这种第二相的尺寸，这对减少腐蚀时的吸氢是有利的。这一结果可为优化锆材成型加工工艺，控制显微组织，并获得最佳的使用性能提供依据。8．腐蚀温度对吸氢的影响非常大。锆合金在400℃过热蒸汽中腐蚀时的吸氢分数（20～40％）明显大于360℃／LiOH水溶液中的吸氢分数（10～20％），前者比后者高出一倍。这给我们的启示是如果要发展超临界水堆核燃料包壳用锆合金，那么除了需要解决耐腐蚀性能以外，吸氢问题应给予更大关注。9．二次离子质谱分析证实Zr-4合金样品无论是在400℃过热蒸汽还是360℃／LiOH水溶液中腐蚀后形成的氧化膜中均存在H⁺和OH^-。基于这一事实，提出了锆合金腐蚀时的吸氢机制如下：腐蚀介质中的水分子得到从金属／氧化膜界面处扩散到氧化膜表面的电子发生H₂O+e→H+OH^-的反应，OH^-通过氧化膜扩散到金属／氧化膜界面处与锆发生2OH^-+Zr→ZrO₂+2H+2e的反应直接生成氢，一部分氢可以被α-Zr基体吸收。基于这一吸氢机制，提出了第二相影响锆合金腐蚀时吸氢行为的机理：当第二相是一种比锆吸氢能力更强的物质时，金属／氧化膜界面处未被氧化的这些第二相可作为吸氢的优先通道，这类第二相的尺寸和数量会对锆合金腐蚀时的吸氢行为产生明显影响；当第二相是一种比锆吸氢能力弱的物质时，锆合金腐蚀时的吸氢行为主要取决于α-Zr基体本身，这类第二相的尺寸和数量对锆合金腐蚀时的吸氢行为影响不大。更多还原

【Abstract】 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.更多还原