Cr离子变价对镍基合金氟盐腐蚀行为的影响及机理研究

In high-temperature fluoride molten salt, the corrosion process of Ni-base alloy is mainly controlled by the selective dissolution of the active elements such as Cr. At the first step of corrosion, Cr diffuses into the metal/salt interface from metal matrix. Then it dissolves into the molten salt as Cr2+ ion. The valence change of Cr ion is induced in the next step by disproportionation reaction between Cr2+ and Cr3+. This process of valence change is considered as the key driving force for molten salt corrosion. Previous investigations had reported that the molten salt corrosion was accelerated by the interactions between alloys and other inert materials. However, studies on the influence of dissimilar materials interaction on molten salt corrosion has showed discrepancy. While a few mechanisms, including non-electric transfer, electrochemical reaction and mass transfer were proposed, no systematical and consistent understanding in the corrosion mechanism has been attained. Further, more efforts should be paid on the systematic and comprehensive research on the corrosion in fluoride molten salt under interactions between dissimilar materials. This proposal will investigate the corrosion behavior of GH3535 Alloy in FLiNaK molten salt, with focuses on the effects of Cr ion valence change induced by dissimilar materials interaction on corrosion. In the GH3535-graphite dissimilar material system, three setups will be designed and used for corrosion study. They are the GH3535-GH3535 same material setup, GH3535-graphite insulation setup, and the GH3535-graphite conduction setup. The corrosion evolution with time was compared in the three setups. The relationship between corrosion performance and dissimilar materials interaction will be obtained. In-situ molten salt corrosion instrument using synchrotron X-ray will be used for valence and content measurement of Cr ions. The synchrotron X-ray with high intensity and low damage was suitable for Cr2+ and Cr3+ measurement. The ion content measurement could achieve an accuracy of several ppm by synchrotron X-ray detection. The Cr ion valence and content measurement will clarify the mechanism of dissimilar material interaction influencing fluoride corrosion by affecting the Cr ion valence change. This study will improve and unify the theoretical understanding about the effect of dissimilar material interaction on fluoride corrosion. It also helps to supplement and the research scope of fluoride molten salt corrosion.

氟化物熔盐中镍基合金的腐蚀过程主要是Cr元素的选择溶解过程。Cr离子在腐蚀过程中发生歧化反应导致的变价行为，是腐蚀的主要驱动力。影响氟盐腐蚀的因素，本质上是对歧化反应和Cr离子变价产生影响。异质材料作用已被公认为是另一个影响熔盐腐蚀的重要因素，目前还存在研究不系统和认识不统一等问题。本项目以异质材料作用对镍基合金氟盐腐蚀的影响为研究对象，设计同质合金、异质材料绝缘和异质材料电接触三种典型试验装置。通过对镍基合金在三种装置中的腐蚀对比，不仅能够获得全面系统的异质材料作用与氟盐腐蚀行为之间的定量关系，更为研究Cr离子变价行为提供了典型的熔盐样本。引入了同步辐射研究方法，实现对腐蚀后固态和液态熔盐中Cr离子含量和变价趋势的测量，揭示异质材料体系中Cr离子变价行为影响氟盐腐蚀的机理。本研究不仅对统一异质材料作用影响熔盐腐蚀的理论认识，更为完善氟化物熔盐腐蚀研究提供重要的有益补充。

熔盐堆（MSR）具有更高的安全性、热功率密度和燃料利用率等突出优点，是“第四代核能国际论坛”选中的6个候选堆型之一。MSR选用碱土金属的氟化物作为核燃料载体和冷却剂。氟化物熔盐的强腐蚀性是熔盐堆用结构材料面临的主要挑战之一。美国ORNL在熔盐堆中的研究经验表明镍基合金具有较好的耐氟化物熔盐腐蚀性。经过多年的腐蚀研究发现镍基合金在氟化物熔盐中的腐蚀主要是由熔盐中杂质驱动镍基合金中Cr元素在氟化物熔盐中的扩散溶解。在熔盐堆中除了镍基合金外还存在石墨，石墨与镍基合金的腐蚀电位不同，因此氟化物熔盐中镍基合金与石墨电接触时可能存在电偶腐蚀。在本项研究中主要研究了异质材料作用对镍基合金在熔盐中腐蚀行为的影响。对镍基合金分别在同质材料坩埚中、石墨坩埚（进行绝缘处理）、石墨坩埚（存在电接触）中进行静态LiF-NaF-KF（FLiNaK）熔盐腐蚀实验，研究发现在同批次熔盐中镍基合金的腐蚀程度由强到弱依次为：电接触≈直接接触＞绝缘接触状态。同时研究结果表明镍基合金与石墨的腐蚀电位确实不同。根据腐蚀电位差异判断在高温FLiNaK熔盐中镍基合金充当阳极，石墨充当阴极。熔盐作为电解质溶液，当石墨与镍基合金存在电接触时，石墨会加速镍基合金腐蚀，但是随着时间延长在阳极的反应速率与在阴极的反应速率基本一致时腐蚀进入相对平稳阶段，因此研究结果揭示镍基合金在同质材料坩埚、石墨坩埚（进行绝缘处理）、石墨坩埚（存在电接触）中的腐蚀都会趋于稳态。.截至到2018年12月31日已按照计划完成项目书中的研究内容，并在项目执行期间在Corrosion Science、Journal of Nuclear Materials等国际期刊上发表9篇SCI论文，研究结果得到了国际同行专家的认可。同时，在项目执行期间参加了学术会议，研究结果得到了国内同行专家的认可。本项目中的研究结果对揭示镍基合金在FLiNaK熔盐中的腐蚀及影响腐蚀的因素具有非常重要的指导意义，同时研究结果也揭示了在FLiNaK熔盐中石墨对镍基合金腐蚀的影响，对揭示熔盐堆中的腐蚀具有重要的参考意义。