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钛外包装材料的腐蚀行为，本文研究了钛表面氧化膜的降解过程和金属的应力腐蚀开裂（SCC）行为。在50 ℃和95℃下，钛在与含0-0.5M NaCl的膨润土接触的水中进行了约3年的浸泡腐蚀试验。在腐蚀电位和表面氧化膜的性能的变化进行了检查作为浸泡时间的函数。结果发现，腐蚀电位保持近似恒定，几乎是独立的NaCl的浓度。氧化膜的厚度没有明显变化，但氧化膜的组成随时间变化：氧化膜中OH键的比例随着浸泡时间的增加而增加，直到1.5年，随着浸泡时间的进一步增加，氧化膜中OH键的比例趋于恒定.研究了钛表面氧化膜的厚度、光学常数和化学组成在水中与膨润土接触的电化学阴极极化过程中的变化。在腐蚀电位至-0.8V范围内，薄膜的性质变化不大或没有变化;在低于-0.8V的电位下，薄膜的性质发生明显变化：随着电位的降低，薄膜厚度减小，折射率减小，消光系数增大.在水中与膨润土接触的SCC试验结果表明，氢脆诱导开裂的潜在区域发生析氢，而没有发生开裂的潜在区域对应的废物处置环境。

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.