低碳钢/膨润土界面腐蚀产物的演化过程及其作用机制

The feasibility of low carbon steel used for manufacturing the canister of HLW geological disposal is determined by its long-term corrosion mode and corrosion rate. During the geological disposal process, Coupled thermal-hydro-mechanic-chemical (THMC) conditions will be produced in the multiple barrier system. The electric double layer structure of low carbon steel and the interaction between corrosion products and bentonite will be largely influenced by THMC condition, which could change the corrosion evolution mechanism of low carbon steel in the geological disposal environment. Simultaneously, the migration and accumulation of the corrosion products in the bentonite will also change the barrier performance of bentonite, which would threaten the security of the disposal repository. This present investigation aims to simulate the coupled THMC environments consist of GMZ Na-bentonite and simulated Beishan groundwater, which is used to investigate the corrosion mode and corrosion rate of low carbon steel in THMC coupled conditions at different temperatures by electrochemical in-situ monitoring and potentiostatic method. XRD, SEM, XPS and HLS will be used for analyzing the composition and structure of corrosion products. Besides, the effects of bentonite on the migration and adsorption process of corrosion products will be studied. Simultaneously, the effects of corrosion products on thethe barrier performance of bentonite will be investigated. Also, the interaction mechanism of the corrosion evolution of low carbon steel and bentonite will be proposed.

低碳钢能否用作高放废物地质处置容器材料主要取决于它在地质处置环境中的腐蚀模式和腐蚀速率。在地质处置过程中，多重屏障系统内将产生热-水-力-化学多场耦合作用，低碳钢/膨润土界面双电层结构和腐蚀产物的演化迁移过程均将受到极大影响，进而改变低碳钢长期的腐蚀演化机制及腐蚀速率。同时，腐蚀产物在膨润土中的迁移、富集等过程也将改变膨润土的缓冲屏障性能，使处置库的安全保障受到威胁。本项目拟采用高庙子钠基膨润土和模拟北山地下水构成THMC腐蚀环境体系，通过长期原位电化学监测和短期加速腐蚀试验方法研究低碳钢在缓冲材料膨润土中的腐蚀演化过程；采用XRD、SEM、XPS和同步辐射等方法分析腐蚀进程中低碳钢/膨润土界面处腐蚀产物的组成和结构；深入探讨膨润土对界面处腐蚀产物迁移、吸附等过程的影响，以及腐蚀产物对膨润土屏障性能的影响；提出低碳钢腐蚀演化与膨润土之间的相互作用机制。

在高放废物地质处置多重屏障系统中，金属处置容器是防止核素泄露的关键人工屏障。低碳钢作为处置容器候选材料能否满足容器材料要求主要取决于它在地质处置环境中的耐蚀性能。本项目结合我国高放废物地质处置预选场址环境特征，采用电化学原位监测方法研究了温度、含水条件等因素对低碳钢腐蚀行为的影响；提出了低碳钢在热-水-力-化学多场耦合作用下的腐蚀演化模型。在试验台架China-Mock-up中，随着地下水的渗透，水在膨润土中逐渐以吸湿水形式存在，且低碳钢发生轻微腐蚀。当膨润土中有自由水时，低碳钢的腐蚀加速。在水饱和膨润土环境中，低碳钢的腐蚀速率随温度升高而加快。在模拟膨润土胶体环境中，附着的膨润土胶体颗粒可有效抑制溶解氧在低碳钢电极表面的阴极还原，腐蚀产物以Fe (Ⅱ)腐蚀产物为主，腐蚀速率显著下降。采用恒电位极化方法在膨润土中预制低碳钢腐蚀产物。在低电位极化条件下，腐蚀产物以Fe (Ⅱ)为主。而在高电位条件下，铁腐蚀产物主要为Fe (Ⅲ)。恒电位极化后，低碳钢在模拟环境中的阳极溶解电阻随极化电位的升高而降低，腐蚀速率加快，这主要归因于高电位极化生成Fe (Ⅲ) 腐蚀产物的阴极去极化作用。同时，初步探索了铁腐蚀产物对膨润土结构及膨胀性、阳离子交换能力等性能的影响。结果显示，随着铁粉/膨润土比例和含水率增大，蒙脱石衍射峰的2θ值增大，且膨润土的膨胀力下降。这可能是由于铁腐蚀产物Fe3+/2+阳离子与膨润土中的Na+阳离子发生了置换反应，导致层间吸附水减少，从而使蒙脱石层间间距减小，导致膨润土膨胀性能降低。采用电化学噪声方法研究发现，膨润土对低碳钢在模拟溶液中的早期点蚀具有一定的抑制作用。研究结果对我国高放废物地质处置容器材料的选择及多重屏障系统的安全评价具有重要的科学意义和工程应用价值。