Analysis of the Excavated Archaeological Iron Using Xray-CT

In order to evaluate the long-term corrosion behavior of carbon steel, we investigated the rust of archaeological iron buried in soil. It is difficult to obtain experimental data of the long-term corrosion in the laboratory. However, it is possible to obtain corrosion data over several hundred years by using archaeological iron and to develop a reliable model for the long-term corrosion behavior by using such natural analogue data. Since these archaeological samples are very rare and important, we can not get agreement to destroy it for analysis. The rust of the sample has been analyzed no-destructively and quantitatively using high-power X-ray computed tomography. The X-ray strength was developed in the two-demensional image. We observed a rust layer distinct from the inner iron metal as a main body by using X-ray map element concentration. A mass-balance quantity of rust calculation was performed from the amount of corroded layer. A sample of axe which was excavated in the Izumo-Taisha-ruin (Shimane prefecture) was analyzed by using the method. The region in Izumo is famous as the production area of the iron from ancient times in Japan. The axe is traditional Japanese type, made of iron, and probably used for a foundation ceremony of the building. The sample has been buried under the column of the shrine and enveloped by clay. It is assumed that the axe remained under reducing conditions until its discovery in 2001 for about 750 years. We have also investigated the corrosion of the gas pipe buried in the soil in several decades as natural analogue study. By the comparison of these data with the corrosion data of water pipe (cast iron) buried in clay soil at most for 100 year, the results of this study do not exceed the extrapolated pitting corrosion depth based on the corrosion depth of the cast iron pipe.