Investigation on corrosion stratigraphy and morphology in some Iron Age bronze alloys vessels by OM, XRD and SEM–EDS methods

The recently study of the corrosion in some bronze artefacts from the Sangtarashan Iron Age site, western Iran, was established to identify corrosion morphology and mechanism in these objects. The corrosion layers in 22 samples were studied by optical microscopy, scanning electron microscopy–energy-dispersive X-ray spectroscopy and X-ray diffraction methods. The results showed that a thin corrosion crust has formed on the surface of bronzes with a triple-layer structure, including two internal and one external corrosion layers. The formation of these layers is due to copper leaching from the bronze surface. The internal corrosion part has been a compact, tin-rich corrosion/oxidation product (noble patina) with some evidences from original metallurgical aspects of the bronze as well as a very thin layer beneath the tin-rich layer. External corrosion products have been identified as basic copper carbonates, malachite and azurite. Based on the results, the corrosion morphology in the Sangtarashan Iron Age bronzes is due to long-term burial in an appropriate environment in a moderately corrosive soil. Although it is the first time to investigate Iron Age bronzes from Iran, this corrosion morphology is partially similar to type I corrosion morphology observed in archaeological bronze objects; nevertheless, some deviations are visible in comparison with previously established patterns.