Protection of silver surfaces against tarnishing by means of alumina/titania-nanolayers

Tarnishing of Ag is due to the fact that the surface layer containing sulfides is not protective at all as, for instance, the oxide layers which are formed on stainless steel.Transparent ceramic coatings obtained by physical vapor deposition (PVD) intended as corrosion barriers, were studied in the past but their intrinsic defectiveness (residual porosity of the order of 1%) has been the major obstacle in order to guarantee a long-lasting protection. Therefore, organic varnishes are currently employed as protection.Recent nanotechnologies like Atomic Layer Deposition (ALD) are suitable to deposit single molecular layers which, by growing sequentially, turn out to be almost free of defects, displaying therefore a remarkable corrosion protection.In this work, Ag surfaces were coated with ALD alumina/titania nanolayers. Their microstructure and composition were studied with scanning electron microscopy (SEM) and glow discharge optical emission spectroscopy (GDOES) depth profiling. An evaluation of the corrosion behavior of the coatings was made through tarnishing tests and it was shown that the protection given by the ALD layers is at least 10 times superior to conventional organic varnishes. Such result is supported by electrochemical impedance spectroscopy (EIS), which indicates that the initial porosity of the ALD layers is of the order of 0.003%. It was also possible to follow as a function of time, the evolution of the electrical parameters characteristic of the protection system without damaging seriously the structure of the protecting layer.