土壌中の考古学的金属製品の腐食に関する調査(IV)

Inspection about the Corrosion of Metallic Archaeological Artifacts in Ground (IV)

本田 卓*; 山口 新吾*

Honda, Takashi*; Yamaguchi, Shingo*

宮城県内の4遺跡及び青森県内の3遺跡から出土した鉄器8点の腐食状況と埋蔵環境について調査した。埋蔵期間は10001700年間であった。地下式横穴墓から出土した3点は,長期間,弱酸化雰囲気に曝されていたと考えられ,1500年間の腐食量は2㎜以下で,表面は主にマグネタイトに覆われていた。青森県内で出土した試料は破壊評価に供し,材質の影響についても評価した。これらの試料はいずれも酸化雰囲気に曝され,ゲーサイトとマグネタイトの2層皮膜に覆われており,10001200年間の腐食量は3㎜で,これまでの調査結果と同じレベルであった。鍛鋼と鋳鉄を比べると,炭素(C)以外の元素組成に差は見られないが,孔職電位は後者の方が卑な値を示し,耐局部腐食性に劣ることが分かった。調査した鉄器の大半は鍛鋼であり,材質的には今日のものと優位な差はない。本調査では,これまでの40点に及ぶ鉄器の調査結果をもとに,長期間の土壌腐食モデルについても言及し,オーバーパック材料の長期健全性を評価した。

In general, it is difficult to evaluate non-destructively the corroded states of iron-based archaeological remains, as they are fully covered by thick rust formed under ground during a long period over a hundred years. The purpose of this study is to estimate the corrosion amounts of such remains with using X-ray CT and summarize the longevity of iron in soil. It has been clarified that rust and residual metallic iron can be quantitatively divided by this technique. Therefore, it is supposed that the amounts of corrosion can be figured out on the basis of thickness and density of rust. Eight remains dug out at seven relics were analyzed. The burial periods in soil were estimated to be from 1000 to 1500 years. Metallic iron remained in six remains, and the corrosion amounts were figured out to be from 0.5 to 3 mm in these periods. In addition, the soil environments of relics were analyzed, and the relation between corrosion behaviors and environmental factors was discussed. The rust was composed of outer goethite and inner magnetite layers in normally oxidizing conditions. On the other hand, a few samples were buried in slightly oxidizing environments, and these were covered by magnetite single layers. The corrosion amount of remains in such an environment was small compared to the others.