Due to the complex corrosion mechanism of the bronze objects, explaining the structure of different layers of corrosion in these objects at microscopic levels is possible. Typically copper oxide(I) is the first corrosion layer on surface of bronze objects that are excavated of historical sites. However, depending on the environmental conditions، various forms of corrosion layer of tin or copper-rich oxide are formed that are effective in corrosion resistance of objects. Identification of tin oxides or copper-tin corrosion products is not simple by using of routine methods of instrumental analysis.However in this research the formation process of copper and tin oxides of some of historical bronze objects of Iran were studied by using the optical microscope with polarized light that show colored corrosion products and in some cases the method of scanning electron microscopy equipped with elemental analysis (SEM-EDX) was used to complete the identification process of oxide layers. According to the results, identify of evidence of differences in distribution of copper and tin in the microstructure that is result of macroscopic and microscopic segregation, formation of a solid solution of copper besides the copper-copper oxide eutectic mixture during the solidification process, the appearance of copper oxide inclusions in the microstructure of metal, moving The tin-rich layer of copper oxide layer to the outside, formation of copper oxide-tin oxide sandwiching layers, selective dissolution of tin in the outer layers, decuprification phenomenon in corrosion layers and condensation of tin oxides in the center of sample and finally the usual formation or degradation of copper oxide layer under certain conditions have been studied with microscopic imagesof selected samples of bronze objects which confirmed the ability of microscopic techniques in the study of oxide layers of bronze objects.
The study of corrosion mechanisms in ancient bronze artifacts, particularly those from the salty and humid region of Hormuz, is crucial due to severe corrosion. This article examines a bronze dagger attributed to excavations in Hormuz to analyze its corrosion behavior influenced by the environment. Techniques such as X-ray imaging, stereomicroscopy, X-ray diffraction (XRD), polarized light microscopy (PLM), and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) were employed. Results show that most of the dagger has transformed into corrosion products, including copper and tin oxides, as well as basic chloride compounds like atacamite and paratacamite, forming a three-layered corrosion structure. The corrosion mechanism involves selective dissolution of copper, its migration to outer layers, and the concentration of tin oxide compounds in the central regions, accompanied by chloride ion penetration. This process has preserved patterns of grain structure and thermal twinning in the inner oxide layers, with periodic deposition of copper and tin oxides indicating the formation of a colloidal solution of copper and tin salts beneath the soil (Scott, 2002; Robbiola et al., 1998).
