Cathodic Degradation of Passive Films on Ti in High-Level Radioactive Waste Disposal Environments

为了更好地了解用于高放废物地下处置的Ti包层的腐蚀行为，研究了Ti表面氧化膜的降解过程和金属的应力腐蚀开裂（SCC）行为。本研究的主要结论总结如下：1。在含0-0.5M NaCl的膨润土接触水中，在50℃和95℃下对Ti进行了约3年的浸泡腐蚀试验。研究了腐蚀电位和表面氧化膜性能随浸泡时间的变化。结果表明，腐蚀电位基本保持恒定，且与NaCl浓度无关。氧化膜的厚度没有明显变化，但膜的组成随时间的变化而变化：随着浸泡时间的增加，膜中OH键的比例增加到1.5年，然后随着浸泡时间的增加而保持不变。在与膨润土接触的水中，在电化学阴极极化作用下，分析了钛表面氧化膜的厚度、光学常数和化学成分的变化。在腐蚀电位至-0.8V的电位范围内，膜的性能几乎没有变化。当电位低于-0.8V时，随着电位的减小，薄膜的厚度减小，折射率减小，消光系数增大。与膨润土接触的水的SCC试验结果表明，氢脆诱发的开裂发生在析氢电位区，而与废物处置环境对应的电位区没有发生开裂。

In order to better understand the corrosion behavior of Ti over-packs used for the underground disposal of high-level radioactive wastes, the degradation processes of surface oxide films on Ti and the stress corrosion cracking (SCC) behavior of the metal have been examined.The principal findings of this study are summarized as follows :1. The immersion corrosion tests of Ti were performed for about 3 years in water in contact with bentonite containing 0-0.5M NaCl at 50 and 95℃. Changes in the corrosion potential and properties of surface oxide films were examined as a function of immersion time. It was found that the corrosion potential stays approximately constant and was almost independent of the concentration of NaCl. There was no significant change in the thickness of the oxide films, while the composition of the films changes with time : the ratio of OH bonds in the films increased with increasing immersion time up to 1.5 years, and then became constant as the immersion time was increased further.2. The changes in the thickness, optical constants and chemical composition of surface oxide films on Ti were analyzed under electrochemical cathodic polarization in water in contact with bentonite. There was little or no change in the properties of the films in the potential range between the corrosion potential and -0.8V.At potentials lower than -0.8V, the changes in film properties became evident : the thickness decreased, the refractive index decreased, and the extinction coefficient increased with decreasing potential.3. The results of SCC tests in water in contact with bentonite showed that hydrogen embrittlement-induced cracking initiated in the potential region where hydrogen evolution takes place, while no cracking occurred in the potential region corresponding to the waste disposal environments.