Characterization of the corrosion layer on iron archaeological artefacts from K_2 (825-1220 AD), an archaeological site in South Africa

摘要

A study of the composition and phase distribution of the corrosion layers on three ferrous objects, excavated at K_2 (Bambandyanalo), an archaeological site in South Africa, was conducted. The objective of the study was to obtain information that can contribute to conservation procedures to be performed on the iron artefacts from this site. Examination of cross sections by means of energy-dispersive X-ray spectroscopy coupled to a scanning electron microscope (SEM-EDX), X-ray powder diffraction (XRD), and micro-Raman spectroscopy revealed the same corrosion composition and structure for all the objects under study, namely an internal layer adjacent to the metal surface with ghost inclusions and an external layer containing quartz grains. The study also revealed that the presence of magnetite (Fe_3O_4), maghemite (γFe_2O_3), and lepidocrocite (γ-FeOOH) within the internal layer is the only difference between the chemical compositions of iron corrosion products within the two layers. The results also made it possible to retrace the corrosion history during burial and long-term storage.