Surface Analysis of the Tarnishing Layer in Silver Alloys
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MRS Advances © 2017 Materials Research Society DOI: 10.1557/adv.2017.612
Surface Analysis of the Tarnishing Layer in Silver Alloys Janette Ortíz-Corona1, José L. Ruvalcaba-Sil2, Edgar Casanova-González3, and Francisco J. Rodríguez-Gómez1 Facultad de Química, Depto. Ingeniería Metalúrgica, Universidad Nacional Autónoma de México, México City, C.P. 04510, México.
1
Laboratorio Nacional de Ciencias para la Investigación y Conservación del Patrimonio Cultural LANCIC, Instituto de Física, Universidad Nacional Autónoma de México, México City, C.P. 04510, México. 2
CONACyT - Laboratorio Nacional de Ciencias para la Investigación y Conservación del Patrimonio Cultural LANCIC, Instituto de Física, Universidad Nacional Autónoma de México, México City, C.P. 04510, México. 3
Abstract
Ancient silver artifacts, when exposed to environments that contain sulfides (H2S), become tarnished and a black film is formed on the surface. The current study deals with the role of copper content and oxygen in the formation of tarnishing in the silver alloys 0.925, 0.800 and 0.720. An ammonium sulfide solution was used as an accelerator of the tarnishing process for different immersion conditions. The analysis of the tarnishing layer in silver alloys was performed by Raman Spectroscopy, Scanning Electron Microscopy - Energy Dispersive X-ray spectroscopy (SEM-EDS) and X-Ray Fluorescence (XRF). The formation of the tarnishing layer was found to be influenced by copper and oxygen contents. The corrosion products under the conditions studied were found to be mainly acanthite and jalpaite. Introduction Silver is a precious metal, which was extremely appreciated in the antiquity to elaborate diverse artifacts. However, these artifacts suffer corrosion when they are exposed to environments containing sulfurs (H2S) [1]; this process is commonly known as tarnishing [2]. which distort the aesthetic appearance of the artifacts and also causes a weight loss that eventually could result in severe degradation and the complete destruction of the items. Despite their different chemical composition, the corrosion layer has the same color in ancient artifacts. Such artifacts were made with silver alloys, and commonly contain different amounts of copper as a constituent in the alloy. The copper in the alloy significantly changes the behavior of the corrosion process because it is more active (Cu+ + e- ↔ Cu0 E0= +0.52 V, and Cu2+ + 2e- ↔ Cu0 E0=
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+0.34 V, ENH) than silver (Ag+ + e- ↔ Ag0 E0= +0.80 V, ENH). As a result, the copper presents a localized corrosion. In the conservation field, it is important to identify the composition of the corrosion layer to restore or stabilize the heritage artifacts. The composition of this layer depends on the kind of environment (burial, atmospheric and water), and the time the artifacts
