How effective are reducing plasma afterglows at atmospheric pressure in removing sulphide layers: Application on tarnished silver, sterling silver and copper

A plasma afterglow from a gas mixture of 5 vol% H2 in He is able to remove tarnish layers on pure silver in a matter of seconds. This dry, localized, and noncontact cleaning technique is a promising method to clean historical objects where silver is combined with organic materials and where traditional cleaning techniques are not recommended. However, historical objects are often manufactured with silver alloys but somehow their tarnish layers are removed less effectively with plasma treatments. To understand the different impact of the afterglow, the surfaces of corroded silver, sterling silver, and copper coupons are characterized before and after plasma treatment by a multianalytical approach combing optical and confocal microscopy, scanning electron microscopy coupled to energy dispersive X-ray analysis and chronopotentiometry. The analyses demonstrate that the few hundred nanometre-thick tarnish layer on pure silver is transformed through the whole thickness resulting in a porous metallic film. On top of that metallic film, some isolated remnant Ag2S particles are found. For sterling silver, a yellowing of the surface occurs. The Ag-rich corrosion products are reduced to a large extent, while the Cu-rich corrosion products are only partially reduced. For corroded copper, no apparent visual change is observed at a macroscopic scale, although the morphology of the surface changed. The results allow an evaluation of the cleaning efficiency and provide a deeper understanding of hydrogen plasma effects on the transformation of sulphide corrosion layers at atmospheric pressure.