The corrosion and microstructural characteristics of archaeological brass artifacts from the Byzantine period in Jordan

This study aimed to investigate the corrosion and microstructural characteristics of a group of archaeological costume jewelry and garment accessories made of brass and excavated from the Byzantine period at the Khirbet Yajuz archaeological site near Amman, Jordan. The Scanning electron microscopy/ energy-dispersive x-ray spectroscopy (SEM–EDX) analyses revealed that those artifacts were mainly made of an α-brass alloy that contained a high zinc content, with the presence of smaller amounts of tin and lead. The x-ray diffraction (XRD) analysis of selected corrosion product samples revealed the presence of cuprite (Cu₂O), sampleite (NaCaCu₅(PO₄)₄Cl·5H₂O), paratacamite (γ-Cu₂(OH)₃Cl), nantokite (CuCl), spencerite (Zn₄(PO₄)₂(OH)₂·3H₂O), and zinc chloride hydroxide Zn(OH)Cl. The elemental distribution of Cu and Zn in the x-ray maps showed patterns of selective leaching for the more active metal, zinc, from the alloy, i.e. the presence of dezincification corrosion phenomenon. The x-ray mapping and elemental analysis suggested that the internal zinc depleted zones were likely containing copper metal, cuprous oxide, and cuprous chloride. The metallographic examination was used to investigate the microstructural characteristics of the internal alloys. It showed an as-cast condition for some artifacts as indicated by their dendritic microstructure, while other artifacts were annealed as indicated by their recrystallized microstructure. The metallographic examination also showed intergranular cracks because of the selective corrosion process. The decomposition of the adjacent buried bodies in the graves where the artifacts were found resulted in the release of phosphate ions, which induced the formation of stable phosphate corrosion products. These corrosion products had a beneficial effect on the stability of the artifacts despite the presence of chloride ions that initially induced the dezincification corrosion process in the burial soil. They also helped in reducing the corrosion rate of the artifacts in the poor environmental conditions of their storage area.