【作者】 刘瑞岩；

【导师】 张俊善；

【作者基本信息】 大连理工大学 ， 材料学， 2004， 博士

【摘要】 使用后的核燃料-乏燃料的锂化还原处理技术是能够有效地减小乏燃料的体积和辐射性的一种新技术,便于其储存和管理。但是在乏燃料的处理过程中,结构材料受到熔融LiCl-Li2O严重的腐蚀,从而阻碍了这种新技术的工业化的应用。目前,材料在熔融LiCl-Li2O中的腐蚀行为的研究报道甚少。本文采用浸没法模拟材料在乏燃料处理过程中的服役条件,利用X射线衍射(XRD),金相,带能谱的扫描电镜(SEM/EDAX),电子探针(EPMA)等手段研究了纯金属Fe、Ni和Cr,含不同Cr含量的Fe-Ni-Cr合金,金属间化合物以及Ni-B化合物渗层在熔融LiCl-Li2O中的腐蚀行为,探讨了金属在熔盐中的腐蚀机制,为乏燃料锂化还原处理技术所需结构材料的选材和防护涂层提供有益的研究结果和实验数据。纯金属Fe和Cr在熔融LiCl-Li2O中的腐蚀速度均随着Li2O浓度的升高而加快,而且腐蚀产物也随Li2O的浓度升高发生了转变:纯Fe的腐蚀产物由LiFe5O8向LiFeO2转变,纯Cr的腐蚀产物由固态的LiCrO2向液态的Li2CrO4转变。通过本实验条件和传统碱性溶解一再析出的对比,初步探讨了材料在熔融LiCl-Li2O中的腐蚀机制。通过热力学计算建立了Li-Fe-O和Li-Cr-O相稳定图,讨论了熔盐中Li2O的活度和氧压对两种金属在熔盐中腐蚀的影响。纯Ni在熔融LiCl-Li2O中发生快速腐蚀,腐蚀产物随时间的增加,依次为NiO(5小时)、Li2Ni8O10(15小时)、LiNiO2(25小时)。纯镍在熔盐中的腐蚀动力学曲线呈直线,这归因于腐蚀层/熔盐的界面反应生成的LiNiO2破坏了原有腐蚀层的完整性。Fe-Ni-Cr合金经LiCl和LiCl-Li2O不同熔融介质腐蚀后形成相同的腐蚀产物LiCrO2(或LiFeO2) 。在750℃以上温度时,合金在熔融LiCl-Li2O中的腐蚀速度明显高于在熔融LiCl中的腐蚀速度,即Li2O的加入使合金腐蚀显著加剧。在熔融LiCl-Li2O中,当合金的Cr含量大于7wt%时,Cr被选择腐蚀,合金的腐蚀速度随着Cr含量的增加而升高;当Cr含量小于7wt%时,Fe被选择腐蚀,腐蚀速度随Fe含量的增加而升高。几种金属间化合物在熔融LiCl-Li2O都发生了加速腐蚀,形成的Al的氧化物LiAlO2不具有保护性。从FeAl和两种NiAl基金属间化合物表面形成腐蚀层均产生了严重的脱落;只有由Ti的氧化物构成的TiAl-5Nb的腐蚀层内层形成了具有保护性的腐蚀膜,但由LiAlO2构成的外层并不具有保护性。采用固体粉末包装法在纯镍表面制备了致密、连续的渗硼层,研究了渗硼处理对纯镍在750℃熔融LiCl-Li2O中腐蚀行为的影响。渗硼层在熔融LiCl-Li2O中硼元素的优先腐蚀有效的抑制了镍的氧化物的形成,而且其腐蚀速度比纯镍较慢,从而改善了纯镍在熔融LiCl-Li2O中的抗腐蚀性能。更多还原

【Abstract】 A lithium reduction technique has been developed as an effective method for reducing the volume and radiation of the spent nuclear fuel, which can benefit to the disposal and management of the spent nuclear fuel. In this process, structural materials used in the technique undertake serious corrosion in molten LiCl-Li2O, which delayed the application of the new technique. To date, there have been few studies on the corrosion behavior of materials in molten LiCl-Li2O. In this paper, immersion experiments were used to simulate service environment of structural material in the lithium reduction process. X-ray diffraction(XRD), optical microscopy(OM), Scanning electron microscopy with coupled with energy dispersive X-ray microanalysis (SEM/EDAX), and Electron probe microanalysis (EPMA) were used to investigate the corrosion behaviors of pure metal Fe、Cr and Ni, Fe-Ni-Cr alloys with different Cr content, several intermetallics, Ni-B compound coating in molten LiCl-Li2O. It primarily discussed the corrosion mechanism and provided useful results and testing data for selecting materials and protective coatings under these conditions.Corrosion rates of pure Fe and Cr in molten LiCl-Li2O increase with the increase of concentration of Li2O in the melt. The corrosion products of pure Fe and Cr also changed with the increase of concentration of Li2O in the melt: corrosion products of pure Fe changed from LiFe5O8 to LiFeO2; corrosion products of pure Cr changed from solid LiCrO2 to liquid Li2CrO4. Based on the comparison of this experiment environments with the traditional basic fluxing-reprecipitation, it is primarily discusses the corrosion mechanism of materials in molten LiCl-Li2O. According to both the establishment of the Li-Fe-0 and Li-Cr-0 phase stability diagrams and the experimental results, the effect of activity of Li2O and pressure of O2 on the corrosion of material in molten LiCl-Li2O were discussed. Nickel shows very fast corrosion in the melt under formation of layer of NiO (5h) after short corrosion time (5h), of NiO and Li2Ni8O10 after corroded for 15h, and of NiO and LiNiO2 after longer times (25h-50h). The weigh loss curve shows linear, which is mainly related to harmful reaction between the oxides and melt at the interface of melt/scale accompanied by the formation of non-protective scale.The corrosion products of Fe-Cr-Ni alloys after corroded in molten LiCl and LiCl-Li2O are the same: LiCrO2 (or LiFeO2). The alloys suffered more severe attack in the molten LiCl-Li2O mixture than in the molten LiCl at temperature higher than 750℃. In Fe-Ni-Cr alloys containing more than 7%Cr, Cr was selectively corroded, and the corrosion rates decrease with increase of Cr content in the alloy in the molten salts; but for the alloys containing less than 7wt%Cr, Fewas selectively corroded, and the corrosion rates increased with increase Fe content.The intermetallics investigated in this paper all exhibited accelerated corrosion in molten LiCl-Li2O, and their corrosion products of LiAlO2 were non-protective. Serious spallations of corrosion scale were observed on the surface of FeAl and tow NiAl intermetallics. Only inner layer of titanium oxides formed on TiAl-5Nb was protective, but the out layer of TiAl-5Nb was non-protective.The pack bonding process of pure nickel produced a dense and continuous borided layer which is composed of Ni-B compound. The effect of borided hot treatment on the corrosion resistance of pure nickel in molten LiCl-Li2O at 750℃ was studied. The experimental results indicate that the borided coating can greatly improve the corrosion resistance of pure nickel in molten LiCl-Li2O, which is ascribed to the preferential corrosion of B in the borided layer that prevent the severe corrosion of nickel.更多还原