Tarnishing (Ag2S) Layer on Silver-Plated Electrical Contacts: Its Influence on Electrical Contact Resistance

Silver is ubiquitously used in electrical contacts due to its exceptional electrical conductivity. Although silver does not oxidize, given time it does sulfurize under atmospheric conditions forming silver sulfide. This tarnishing film is often the cause of an electrical contact’s malfunction or failure. In this research work, we analyzed three alternate methods, which could reduce the effect of the tarnishing film on the contact, therefore minimizing the chances of component malfunction. The alternatives analyzed included: puncturing through the tarnishing film, mechanical removal by scratch and fretting tests, and resistive heating removal by applying various currents. These methods were compared to chemically cleaned silver contacts used as a reference. A careful analysis of the studied methods proved that normal loads above 5 N are enough to puncture the tarnishing film, thus reducing its effect on the contact material’s conductivity. Furthermore, although mechanical removal is not a viable option, it proves that the silver sulfide film does not significantly affect the conductivity of the system when piled-up (as is the case with tin oxide in tin-plated contacts). Finally, the resistance of tarnished contacts is considerably higher than the chemically cleaned contacts for loads at or below 5 N (over one order of magnitude).