Corrosion Behavior of Titanium in Deaerated Aqueous Solutions

钛是发电厂产生的高放射性废物玻璃原木的过度包装材料之一。超包装材料必须保护其内容物不受环境的影响，如含有膨润土粘土溶解的化学成分的地下水和由于核裂变反应产生的热而产生的高温高达100度。在本研究中，将钛试件浸入含有膨润土粉末、硼酸盐缓冲溶液或碳酸盐溶液的除氧水中，得到以下结果：(1)在机械抛光的钛表面，即使在除氧水溶液中也能生长出氧化层。(2)长期浸泡，电障性能下降。(3)长期浸泡后，浸泡电位变得不稳定。(4)浸泡20周后可见溶蚀痕迹。(5)在与膨润土粉接触的水溶液中观察到局部腐蚀的发生。(6)80℃比25℃腐蚀痕迹和膜层劣化更明显。从阻抗结果可以看出，硫酸盐离子的存在对氧化膜的腐蚀速率和退化有很大的影响。从阻抗结果和原子力显微镜观察，钛在膨润土接触液等含硫酸盐的溶液中形成的阳极氧化膜具有多孔结构和高密度的电性缺陷，更容易降解和腐蚀。在本研究中，钛在浸泡20周后观察到深度为10 nm的裂缝。因此，重要的是评估缝隙扩展到更严重的局部腐蚀的可能性。

Titanium is one of the candidates for overpacking material of glass log containing high level radioactive waste produced in power plants. The overpacking material has to protect its contents over 1000 years from environment such as underground water containing chemical composition dissolved from bentonite clay and high temperature up to 100 degree due to heat production of nuclear fission reaction. In this study Ti specimens were immersed in deaerated water in contact with bentonite powder, borate buffer solution, or carbonate solution, and obtained following results. (1) Oxide layer grows on Ti mechanically polished even in deaerated aqueous solution. (2) Electric barrier property degraded during long term immersion. (3) Immersion potential became unstable after long term immersion. (4) Dissolution traces of general corrosion were observed after 20 weeks immersion. (5) Initiation of localized corrosion was observed in aqueous solution in contact with bentonite powder. (6) Corrosion traces and degradation of the film was more obvious at 80 degree than 25 degree. From impedance results, the corrosion rate and degradation of the oxide film were mostly affected by the presence of sulfate ion. From impedance results and AFM observation the anodic oxide film formed on Ti in the solutions containing sulfate ion such as bentonite contact solution has porous structure and electric defects at high density, resulting in more susceptible to the degradation and corrosion. In the present study crevices with a few tenth nm in depth were observed on Ti after 20 weeks immersion. Therefore, it is important to evaluate the possibility that the crevice grows to more serious localized corrosion.