【作者】 魏欣；

【导师】 柯伟； 雷明凯； 董俊华；

【作者基本信息】 大连理工大学 ， 材料学， 2013， 博士

【摘要】 高放废物地质处置罐是将高放废物与地质处置环境隔离的第一道人工屏障,在地质处置环境中将面临长期而又严峻的腐蚀科学问题。选择环保耐蚀材料作为高放废物地质处置罐的候选材料具有重要意义。钝态金属不锈钢和钛表面均能够自发生成致密、稳定的钝化膜,具有优良的耐均匀腐蚀性能,被广泛应用于航空航天、深海开发、石油化工及核能核电等众多工业领域。但在实际服役环境中,钝态金属往往因温度、压力、离子种类及其浓度、溶解氧浓度等环境因素的影响而发生局部腐蚀。为满足我国高放废物地质处置工程的需求,本文依据北山高放废物地质处置环境的特征围绕典型钝态金属可能发生的局部腐蚀问题展开相关工作,为我国高放废物处置罐罐体材料的选择提供重要的理论依据。采用动电位极化曲线、电化学阻抗谱、Mott-Schottky曲线、循环极化及恒电位极化等电化学方法研究了温度及Cl-浓度对超纯高铬铁素体不锈钢Cr26Mo1和Cr30Mo2耐点蚀性能的影响。结果表明：25℃时,超纯铁素体不锈钢Cr26Mo1和Cr30Mo2在Cl-浓度为0.01-2M的体系中处于钝化状态,点蚀电位较高。在低温条件发生点蚀后,不锈钢钝化膜具有自修复能力。当温度分别超过45和60℃,Cr26Mo1和Cr30Mo2不锈钢在0.6MCl-溶液中的抗点蚀性能显著降低。在高温条件发生点蚀后,不锈钢钝化膜基本不能自修复。相同环境体系中,Cr30Mo2不锈钢的耐点蚀性能要好于Cr26Mo1不锈钢。依据处置罐温度变化和北山60个地下水样中的Cl-浓度范围,Cr26Mo1不锈钢在地下水中将面临点蚀的威胁。而Cr30Mo2不锈钢在Cl-浓度低于0.06M的环境中相对安全。但在Cl-浓度为0.06-1M时,当温度高于60℃时Cr30Mo2不锈钢可能发生点蚀。处置罐温度升高或渗入地下水中Cl-的富集均能够促进不锈钢点蚀的萌生和扩展。通过对比点蚀电位发现超纯高铬铁素体不锈钢在温度低于45℃时的耐点蚀性能优于316L不锈钢和Hastelloy C-4。高含量Cr在Cr30Mo2和Cr26Mo1不锈钢中起着重要作用。但在高温度条件下,Cr30Mo2和Cr26Mo1不锈钢的抗点蚀性能低于316L和Hastelloy C-4。这主要与316L不锈钢和Hastelloy C-4中的Mo含量较高有关,增加Mo含量可提高材料的抗点蚀能力,尤其在温度较高条件下Mo的作用更加显著。研究发现,在浸泡初期,带缝隙钛电极的维钝电流密度随着温度升高而增大,钝化膜阻抗降低,但在25-90℃内均处于钝化状态。随浸泡时间延长,带缝隙钛电极在温度和Cl-共同作用下发生缝隙腐蚀并扩展,且腐蚀主要集中于缝隙口处。在地质处置环境中,当温度高于85℃,钛处置罐缝隙腐蚀持续扩展的临界Cl-浓度需大于0.6M。当温度为70℃,Cl-浓度需达到2M。当温度低于55℃,提高Cl-浓度对增加钛缝隙腐蚀敏感性并不明显。采用恒电位加速缝隙腐蚀试验方法研究得到,诱发钛缝隙腐蚀各阶段转变的临界温度随体系中Cl-浓度或外加电位升高而下降。这种方法是通过外加电位来改变缝隙内外的电位差,以反映实际缝隙内外面积比、几何形状等因素引起缝隙内外电位不同所带来的影响,具有一定的可行性。在95℃的模拟缝隙溶液中,当H+浓度为0.01M时,阳极极化曲线直接进入钝化区,钛表面钝化膜处于钝化状态,Cl-浓度的影响不明显。当H+浓度为0.1M,且Cl-浓度达到1M时,阳极极化曲线开始呈现出明显的活化-钝化转变特征,Rf和Rp均有所下降。当H+浓度高于0.5M时,致钝电流密度ic和维钝电流密度ip显著增大,Rf和Rp急剧下降,可促进钛表面钝化膜的破裂溶解。在1M HCl+1M NaCl模拟缝隙溶液中,阳极极化曲线呈现活化-钝化转变的临界温度为75℃。研究发现钛表面钝化膜在50-95℃的模拟缝隙酸化溶液中迅速溶解,自腐蚀电位降至-570～-670mV (SCE),远低于析氢反应平衡电位。钛在缝隙腐蚀过程中的吸氢行为是存在的。在高温或较低的外加阴极电位条件下,H+的阴极还原反应速率增加,试样内吸收氢的含量升高,但吸氢效率有所下降。氢化物聚集分布在试样表层,厚度随温度升高或控制阴极电位降低而增加,且表面腐蚀程度加重。采用二次离子质谱深度剖面分析进一步说明了工业纯钛在模拟缝隙溶液中的吸氢行为,氢含量沿深度方向逐渐降低。综合分析认为：钝态金属处置罐在高放废物地质处置过程中能否发生局部腐蚀问题很大程度上取决于环境介质条件的变化。在一定的温度或氯离子浓度范围内,超纯高铬铁素体不锈钢将面临着点蚀的威胁。工业纯钛在低温或低氯离子浓度条件下是相对安全的,但在高温和高氯离子浓度的处置环境中倾向发生缝隙腐蚀,同时伴随着吸氢问题。钛处置罐在结构设计、加工制造过程中应尽量避免缝隙的出现,可减少缝隙腐蚀的发生。更多还原

【Abstract】 High-Level Radioactive Waste (HLW) container is the first important artificial barrier which can separates the HLW from the disposal surroundings. The container in disposal surroundings will be faced with rigorous corrosion problem. Therefore, it is significant to selecte the corrosion resitance material to be used as the HLW container. Passive metals (stainless steels and Ti) are highly corrosion resistant due to their stable passive films spontaneously formed on surface and widely used in aerospace, deep-sea, petroleum chemistry, nuclear power industry, etc. However, passive metals are susceptible to localized corrosion in actual environment. Because of the influences of temperature, chloride concentration, hydrostatic pressure and dissolved oxygen, passive metals usually suffer the localized corrosion in service. In view of the application of passive metals in Beishan Groundwater for Geological Disposal of High-level Radioactive Nuclear Waste, this paper had investigated the localized corrosion behavior of ultra pure high chromium ferrite stainless steel and Grade-2Ti in simulated chloride environments which is very important to improve the localized corrosion and prolong their service life.The effects of temperature and Cl-concentration on the pitting corrosion of Cr26Mo1and Cr30Mo2ultra pure high chromium ferritic stainless steel had been investigated using different electrochemical techniques of potentiodynamic polarization, electrochemical impedance spectroscopy, capacitance measurements (Mott-Schottky approach) and potentiostatic polarization measurements. The results revealed that Cr26Mo1and Cr30Mo2stainless steels exhibited the passive characteristics in0.01-2M NaCl solutions at25℃, and the pitting potential all were positive. In the condition of lower temperature, the passive films could be self-repaired after pitting corrosion. When the temperature increased to45℃, the pitting corrosion resitance of Cr26Mo1stainless steel decreased evidently, and the critical temperature of Cr30Mo2stainless steel was60℃. The passive films can not be self repaired after pitting corrosion at higher temperature.In the same environment, the pitting corrosion resistance of Cr30Mo2stainless steel is better than Cr26Mo1. According to the temperature of container and the range of Cl-concentration in Beishan groundwater, Cr26Mo1stainless steel is dangerous. Cr30Mo2stainless steel will be secure when Cl-concentration is lower than0.06M, but will suffer pitting corrosion in0.06-1M when the temperature increase to60℃. Therefore, increasing temperature and Cl" concentration in groundwater all can promote the initiation and propagation of piiting corrosion of stainless steels HLW disposal container.According to the comparison of the pitting corrosion potential, the pitting corrosion resistances of Cr26Mo1and Cr30Mo2ultra pure ferritic stainless steels are better than316L austenitic stainless steel and Hastelloy C-4when the temperature is lower than45℃. Cr content plays an important role in Cr26Mo1and Cr30Mo2stainless steels. When the temperature is higher (60-90℃), the pitting corrosion resistances of Cr26Mo1and Cr30Mo2stainless steel are not as good as316L and Hastelloy C-4.This is because of the effect of Mo, especially at higher temperatures.The results showed that all the creviced specimens of Grade-2Ti exhibited the passive characteristics in the initial immersion period at25-95℃. As increasing the temperature, the galvanic current increased and the resistance of passive film decreased. With extending the immersion time, the crevice corrosion of Grade-2Ti initiated and propagated as a result of the gradual aggressive environment in the crevice. The damage caused by anodic active dissolution in the crevice mainly located near the crevice mouth. It can be speculated that the critical Cl" concentration of propagation of crevice corrosion for Grade-2Ti is0.6M in geological disposal environment at80-95℃. At70℃, the critical Cl" concentration is2M. When the temperature is lower than55℃, the effect of increasing Cl-concentration is not obvious to promte the crevice corrosion of titanium container. In addition, the results of potentiostatic polarization measurements revealed that the critical temperature of crevice corrosion of Grade-2Ti decreased with increasing Cl-concentration and the applied potential. This accelerated test method can reflect the influence of potential difference between the inside and outside of crevice on titanium. So this method is useful and feasible.In simulated crevice corrosion solutions, the effects of H+, Cl-and temperature on the active/passive characteristics of Grade-2Ti were investigated in detail. When H+concentration was0.01M, the anodic polarization curves exhibited the typical passive characteristics at95℃, and the effect of Cl" concentration was not obvious. When H+concentration was0.1M, and Cl" concentrations increased to1M, the anodic polarization curves exhibited obvious active/passive characteristics. When H+concentration was0.5M, the critical current density and passive current density increases evidently increased, Rf and Rp decreased. In1M HCl+1M NaCl solutions, the critical temperature of exhibiting obvious active/passive characteristics was75℃.It is found that the passive films on Grade-2Ti dissolved quickly in simulated crevice corrosion solutions at50-95℃, and the corrosion potential decreased to-570～-670mV (SCE) which was more negative than hydrogen evolution potential. So hydrogen absorption accompanied by the crevice corrosion of Grade-2Ti. In order to understand this problem, hydrogen absorption and mechanical performance degradation of Grade-2Titanium in simulated crevice acid solution were studied. The results showed that with the increase of temperature and cathodic potential, the velocity of H+reduction and the cathodic current density increased, and the content of hydrogen in charged specimens increased but the efficiency of absorbed hydrogen decreased. The section morphologies of Grade-2Ti after being cathodically polarized revealed that hydride accumulated on the surface of specimens. With increasing temperature or decreasing cathodic potential, the thickness of hydride layer increased and tended to be uniform. Further, the content of hydrogen decreasing with depth was approved by SIMS.Comprehensive analysis shows that:localized corrosion of passive metals container in the long-term disposal of High-level Radioactive Nuclear Waste mainly depends on the variation of environment. Under a range of temperature and Cl-concentration, ultra pure high chromium ferritic stainless steel may suffer pitting corrosion. Grade-2Ti will be safe in the environment with lower temperature or lower Cl-concentration, but may suffer crevice corrosion in the environment with higher temperature or higher Cl-concentration. Also, hydrogen absorption may occure together with crevice corrosion. In order to decrease the occurrence of crevice corrosion of titanium disposal container, it should try to avoid the gap in the process of structural design and manufacturing.更多还原