Protection of silver electrodeposit surfaces against accelerated and natural atmospheric corrosion by Ge incorporation in Ag structure; a microstructural investigation

Abstract

Silver, as the most reactive noble metal, tarnishes immediately after exposure to industrial atmosphere or indoor S-bearing air. In this study, tarnish resisting and long term microstructural stability of silver-germanium (Ag-Ge) coating in industrial atmosphere is investigated and compared to silver electrodeposits. Ag-Ge coatings were electrodeposited on copper from cyanide base bath by DC current application. Coatings consisting about 0.5 to 6wt.% Ge have been studied and the optimum content of Ge in the coating was found to be 6 wt.% regarding dense and uniform microstructure along with best tarnish resistance and long term microstructural stability that has been discussed in this work. Ge forms a thin protective oxide layer, so a small incorporation of Ge in Ag structure, could improve tarnish resistance to a large extent. Tarnish resistance of samples have been evaluated according to ISO 4538 standard and considering color and reflectivity change detected by UV-Visible spectrometer. In addition, X-ray Diffraction (XRD) results were used to determine the crystalline structure of coatings before and after tarnish resistance and atmospheric corrosion. XRD patterns in grazing mode indicated the formation of Ge-Oxide passive layer on the Ag-6Ge surface which is known as the key factor for this sample's best performance. Scanning Electron Microscopy (SEM) along with Field Emission Scanning Electron Microscopy (FESEM) equipped with Energy Dispersive Spectrometry (EDS) were used for microscopic analysis and surface characterization of samples. The various tarnish behavior of the coatings were evaluated and discussed via correlation to their microstructural characteristics in both severe and low aggressive conditions.