Corrosion behavior study of bronze objects of Parthian historical period by note on tin segregation phenomena

摘要

The created patina on the historical bronze objects is formed as a result of different corrosion reactions due to the complex and heterogeneity of alloy structure and environmental conditions. Compounds of tin-oxide or copper-tin corrosion products appear with different mechanisms on the surface of object that is effective on corrosion process as well as conservation of bronze objects. The greater tendency of tin to corrosion and its leaving out in form of oxide-compounds is normally expected. On the basis of this research, the corrosion process of four bronze object of the Valyran historical site has been studied focusing on tin corrosion process. The Valyran is in the east of Tehran the capital of tan. Based on the historical period of excavated coins from this site, it is expected that the Dating of these objects is related to second Mehrdad of Parthian Kings (250 BC to 224 AD). In this work, the corrosion and erosion mechanisms are investigated using optical and scanning electron microscopies techniques (SEM-EDX). For this purpose, samples of selected objects are studied with microscopic methods after mounting and preparing process. The existing corrosion products on the surface of the specimens are identified with the X-Ray Diffraction (XRD). The elemental distribution in the corrosion layers was carried out by EDX and PIXE methods. The results of the instrumental analysis of the samples indicate that the tin content of the bronze samples varies from4 to 19 weight percent. The analysis of corrosion products indicates the presence of tin oxides compounds such as Cassiterite with copper oxide in many cases. Some evidences of different copper and tin distribution during solidification process could be seen in microstructure analysis of the samples. The corrosion products of tin formed in the outer layer as a white phase of oxide could be due to inverse segregation during solidification and the corrosion of tin-rich phase then changes to oxide products. This segregation during solidification in bronze casting is known as tin sweat. Of course there is a possibility that copper has dissolved preferentially from the outer layer and tin remained as tin oxide on the external part of the samples. Corrosion of grain boundaries and the presence of tin oxide products in grain boundaries are observed that can be related to the diffusion of tin alloying element in crystal lattice of copper and its substitution during solidification. Therefore the amounts of tin alloying elements in grain boundaries are due to the more resistance of tin-oxides formed on grain boundaries. The studies show also chloride penetration to the inner layers affected the acceleration of corrosion process.