Evaluation of stress corrosion and micro-segregation in copper-based artefacts through X-ray Microscopy

The role of alloying elements in production technology and corrosion process has been investigated in different type of archaeological copper-based artefacts from the Phoenician-Punic site of Motya (Sicily, Italy). For this purpose, a combination of multi-analytical techniques (Multiscale X-ray Microscopy, SEM, HR-FESEM- EDS and EMPA) has been carried out to investigate the inner bulk of the alloy, performing tomographic virtual slices of the objects, and exploring grain boundaries segregation at sub-microscale. All copper-based artefacts were exposed to bronze disease corrosion induced by the presence of the reactive cuprous chloride (CuCl) located at the interface between external corrosion layers and the surviving metal core. The results highlighted micro-segregation in a Cu-nail induced by As, which was forced outwards along inter-granular channels and combined with Fe atoms at Cu grain boundaries, leading to the formation of copper-iron arsenate, due to the high chemical affinity between these two elements. Binary and ternary alloys revealed marked Cu and Sn selective corrosion and thicker patina compared with Cu metal due to the presence of Sn in chlorine-rich environment. The dissolution factor of copper in these alloys showed a great variability. In addition, the occurrence of cracks inside the bronze needle acted as new corrosion interfaces and involved the formation of complex and periodic stratified corrosion layers, leading to a complete mineralized structure.