An Electrochemical Simulation and Treatment Study for the Carbonate and Chloride Based Corrosion of Heritage Silver-Copper Alloys

• Published in: Studies in conservation 2021-05, Vol.66 (4), p.190-210
• Main Authors: Abu-Baker, Ahmad N., MacLeod, Ian D.
• Format: Article
• Language: English
• Subjects: chronoamperometry; coins; linear sweep voltammetry; SEM-EDX; Silver-copper alloy; XRD
• ISSN: 0039-3630; 2047-0584
• DOI: 10.1080/00393630.2020.1796020

This study aimed to investigate the corrosion of a group of historic silver-copper alloy coins and their conservation treatment on defined parameters by potentiostatic methods. Basic copper carbonates and silver chloride were identified on the surface of the coins by a combination of scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and X-ray diffraction (XRD) analyses. The carbonate and chloride-based corrosion mechanisms were synthetically simulated on modern silver-copper alloy samples (Ag72/Cu28) by anodic polarisation measured by linear sweep voltammetry (LSV) in sodium bicarbonate (NaHCO 3 ) and sodium chloride (NaCl) solutions. Cathodic polarisation curves, measured by LSV, were used to identify the working conditions to reduce the corrosion products on the corroded samples and to treat the actual coins. The reduction process was then monitored by chronoamperometry. Following the potentiostatic reduction process, the metallic copper spots on the treated samples were wiped with cotton swabs moistened with a diluted solution of silver nitrate (AgNO 3 ) to obtain a silvery appearance, and the treatment was completed by a gentle rubbing of the redeposited silver with a cotton swab moistened with a calcium carbonate (CaCO 3 ) suspension followed by rinsing and drying. The corrosion products identified on the synthetically corroded samples indicated that the deterioration mechanisms were dominated by selective attack on the copper-rich phases of the (Ag72/Cu28) alloy, which is consistent with an internal galvanic corrosion phenomenon. The potentiostatic methods showed excellent effectiveness in determining treatment parameters and monitoring the reduction process of the corrosion products on the coins, which helped to preserve surface details in the cleaning process and to prevent over-treatment.