Corrosion of ancient silver alloys

Precious metal artefacts are characterized by a wide compositional nature and have been produced via different complex manufacturing techniques that have greatly influenced their chemical and metallurgical stability. Furthermore, the Ag or Au based alloys can be characterized by different amount of impurities coming from the extractive processes, which can influence their long-term mechanical and chemical properties. In this work, we try to specify the types of corrosion attack on the silver metal alloys surface in soil. Silver based alloys with chemical composition and metallurgical features similar to those of ancient alloys have been prepared in the laboratory. Coin-like coupons were artificially preaged and then submitted to soil corrosion conditions to simulate the corrosion patina of real artefacts before excavation. An extensive study of the produced patina layers was performed by Glancing Angle X-ray Diffractometry (GAXRD), Scanning Electron Microscopy (SEM) and EDS surface analysis as well as optical microscopy. The silver coupon examination shows the same type of attack as the one reported on original artefacts. © 2009 by NACE International.