Long-Term Corrosion of 2,000-Year-Old Ancient Iron Sword

Abstract

ABSTRACTAn 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.