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鳥取市良田中道遺跡出土袋状鉄斧の埋蔵環境と腐食

Burial environment and corrosion of socketed iron axe excavated from Yoshida Nakamichi site in Tottori City, Tottori Prefecture

三ツ井 誠一郎

Mitsui, Seiichiro

2012年6月に鳥取市良田中道遺跡の古墳時代以降の水田土壌から出土した袋状鉄斧は、原形を留めるなど遺存状態が良好であった。この要因を調べるため、埋蔵環境と腐食状態に関する調査・分析を実施した。埋蔵環境として、湧水水質(酸化還元電位、溶存酸素濃度等)の分析、鉄電極の腐食速度(自然腐食速度)等の測定を現地で実施した。鉄斧の腐食状態については、X線CT装置を用いた腐食層厚の計測、ポータブルX線回折・蛍光X線分析装置を用いた鉄斧表面の腐食生成物等の分析を実施した。その結果、腐食生成物層の厚さはおおむね1mm前後であり鉄斧内部は金属鉄であること、表面に生成した菱鉄鉱(FeCO$$_{3}$$)が腐食反応を抑制していた可能性があることなどを確認した。

An ancient socketed iron axe was excavated from Yoshida Nakamichi site in Tottori City, Tottori Prefecture. To understand reasons of corrosion state of the axe, 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 (2$$times$$10$$^{-3}$$ mm/y) evaluated from measured corrosion depths was smaller than the probe corrosion rate (5$$times$$10$$^{-2}$$ mm/y) by two orders of magnitude and identified siderite (FeCO$$_{3}$$) as a corrosion product. The results suggested that the siderite precipitated on the surface of the iron sword inhibited corrosion reaction.

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