Silver was widely used in the production of ancient artifacts because of its excellent ductility, malleability and aesthetic appearance. However, some archaeological silver artifacts or, strictly speaking, silver-copper alloys are found to be brittle, due to long-term corrosion, mainly caused by intergranular corrosion. The mechanism is local galvanic attack between the Ag-enriched matrix (α phase) and the Cu-enriched grain boundary (β phase) but it is a rather complex process involving the differences in local electrochemical activities, difficult to be fully understood with the existing techniques.
In the paper Scanning electrochemical cell microscopy: A powerful method to study the intergranular corrosions of archaeological silver artifacts, by Shengyu Liu at al. published on the Journal of Cultural Heritage, Vol. 46, 2020, Pages 176-183, Scanning Electrochemical Cell Microscopy (SECCM) is proposed as a method to visualize local electrochemical activities at high spatial resolution.
Scanning electrochemical cell microscopy (SECCM) is a pipette-based imaging technique, able to perform at the same time electrochemical, conductance, and topographical visualization of surfaces and interfaces. A tiny meniscus or droplet, at the end of a double-barreled pipette is used for high-resolution functional imaging and nanoscale electrochemical measurements. It has recently been applied to the many fields and it has the advantage that the electrochemical data can be obtained directly. Meanwhile, by using the "hopping mode", the topography of the substrate surface does not affect the results of the electrochemical activity measured. Thus, SECCM allows to overcome the limits of Scanning electrochemical microscopy (SECM), a SPM (scanning probe microscopy) method that has been widely used in pitting corrosions and surface heterogeneities of Fe , Al , Mg , Ti and their alloys.
In this paper, the intergranular corrosion mechanism of archaeological silver artifacts in neutral solution is investigated by SECCM combined with structure observation, composition determination and macroscopic corrosion experiments. Single-channel SECCM is used to map the local electrochemical activity of a simulated silver-copper alloy sample, specifically prepared. With the comprehensive results of scanning electron microscope with energy dispersive spectroscopy (SEM-EDS) and the droplet corrosion test, the mechanism of intergranular corrosions of alloys with composition similar to ancient artifacts is illustrated by heterogeneities of thermodynamics, kinetics, microstructures, components and macroscopic corrosion behaviors. This is the first time that direct quantitative data on intergranular corrosions of ancient metal artifacts, or actually of all metal alloys, are obtained. The principal cause of intergranular corrosions are the heterogeneities of microstructure and component caused by discontinuous precipitation. Quantitative results by SECCM reveal that these heterogeneities can lead to the differences in local electrochemical activities, and eventually the occurrence of corrosion.
The change of intergranular corrosion behavior in the presence of Cl- and SO42- is also studied by changing the electrolyte solution in the micropipette. The macroscopic corrosion behavior according to the droplet corrosion tests is highly consistent with the results of local electrochemical activities acquired from SECCM. Meanwhile, the possibility of studying intergranular corrosion behavior in different soil environments by SECCM is demonstrated through a corrosion test under standardized ion conditions, which indicates a bright future of this method in the field of corrosion.
This paper demonstrates the application of SECCM, a quite innovative methodology, as a powerful method for the electrochemical analysis of silver-copper samples at high resolution and in general for the study of metallic artifacts.