Long-term corrosion of 2,000-year-old ancient iron sword

Mitsui, Seiichiro  ; Fujii, Atsuhiro*; Higuchi, Megumi*; Nishimura, Kosuke*

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, we studied relationship between burial environment and corrosion. As environmental conditions, we investigated groundwater chemistry and corrosion rate with iron probe monitor, etc. As for corrosion state, we analysed corrosion depths with a X-ray CT and corrosion products with a portable XRD/XRF. As results, we found that the redox potential and dissolved oxygen level as environmental conditions were very low, and that the maximum corrosion rate evaluated from measured corrosion depths was smaller than the probe corrosion rate by two orders of magnitude and identified siderite as a corrosion product. The results suggested that the siderite precipitated on the surface of the iron sword inhibited corrosion reaction.