Long-Term Corrosion of 2,000-Year-Old Ancient Iron Sword
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Long-Term Corrosion of 2,000-Year-Old Ancient Iron Sword Seiichiro Mitsui1, Atsuhiro Fujii2, Megumi Higuchi3 and Kohsuke Nishimura4 1 Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency (JAEA), 4-33 Muramatsu, Tokai, Ibaraki, 319-1194, Japan 2 Yao Board of Education, 1-1-1 Hon-machi, Yao-city, Osaka 581-0003, Japan 3 Yao City Museum, 3-180-1 Chizuka, Yao-city, Osaka 581-0862, Japan 4 Cultural Property Research Association of Yao City, 4-58-2 Saiwai-cho, Yao-city, Osaka 5810821, Japan ABSTRACT An ancient iron sword (ca 2,000 year old) excavated from Ohtake-nishi archaeological site in Osaka Prefecture, is one of the oldest cast iron swords in Japan. Because of a good state of preservation, such as the sharply-defined "Shinogi" (the longitudinal ridge) on the surface, we studied relationship between the burial environment and corrosion behavior. As environmental conditions, we investigated groundwater chemistry and corrosion rate with iron probe monitor, etc. Regarding corrosion state, we analyzed corrosion depths with an X-ray computed tomography and corrosion products with a portable X-ray diffractometer equipped with X-ray fluorescence spectrometer. As results, we found that the redox potential and dissolved oxygen level as environmental conditions were very low, and that the corrosion rate (7.5×10-4 mm/y) evaluated from measured corrosion depths was smaller than the probe corrosion rate (3.2– 5.2×10-2 mm/y) by two orders of magnitude. The results suggested that the corrosion layer of siderite formed on the iron sword surface inhibited corrosion reaction. INTRODUCTION Carbon steel is one of the candidate overpack materials for geological disposal of highlevel radioactive waste in Japan. The overpack is expected to prevent contact between the vitrified waste and the groundwater for at least 1,000 years, which is the amount of time required to avoid problems due to radiogenic heat production of short-lived radionuclides [1]. To assess the lifetime of the overpack, it is necessary to understand the long-term corrosion behavior of carbon steel in the anaerobic disposal environment. The results of laboratory studies under anaerobic conditions indicate the corrosion rate of carbon steel in compacted bentonite decreases with time because of the formation of a protective corrosion product layer [2-6]. The longest tests in compacted bentonite under anaerobic conditions for a 10-year duration were performed by Taniguchi et al. [4]. They reported that the corrosion rate decreased with time with approximately power-law equation depending on the protectiveness of surface layer consisting of FeCO3, and other carbonate compounds. However, it is not certain that the corrosion product layer is protective for the long term over the test duration. From this perspective, it is advantageous to use evidences from archaeological analog studies [7-9]. For example, Neff et al. [7] studied corrosion rates of archaeological iron artifacts excavated from five different sites ranging from the 2nd to the 20th c
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