HR-FESEM-EDS study of corrosion pattern in Cu-Zn based alloy

Corrosion phenomena are processes that can create several problems in archaeological materials. Nanoscale studies of the corrosion mechanism of ancient alloys and the distribution of the damage both on the patina and in the core of metal object represent one of the major challenges that must be implemented to protect archaeological metal findings. In this research, the corrosion pattern of Cu-Zn-based Roman coins was investigated from micro to nanoscale using SEM-EDS and HR-FESEM-EDS (high resolution field emission scanning electron microscope). Both the patinas and the uncorroded cores of the samples were analysed, exploring the nature of the alloy, structures and complex patterns of the external layers. The high magnification imaging highlighted signs of selective corrosion, depending of the Cu-Zn ratio, as well as marks of cold-working of coins. The X-ray maps showed the distribution of the elements that compose the samples, helping to understand the evolution of the dezincification processes. Nanoscale investigation highlighted a heterogeneous composition within the strain lines structures, induced by selective de-alloying. The corroded micro and nanodomains were explored in depth to study the structural constrains which control altered areas inside orichalcum coins. We evaluated also all the parameters that induced corrosion and dealloying. The results suggested that mechanical stressed-areas could be the trigger zone of corrosion processes. Moreover, this research aimed to fill the scientific gap about this ancient alloy and to improve the knowledge on the mechanisms involved in orichalcum degradation.