Letter 1117 Life expectancy of silver plated contacts --- Need your advice on the following: In the area of Silver plating of contactors (say for Switchgear products), how many years of life one can expect without tarnishing? Plating thickness is in the range of 5-25 microns on copper. How do I protect the silver coating without formation of silver sulfide and without loosing the electrical conductivity. Organic coatings or Rhodium plating has one or other disadvantages. Do you suggest any alloy plating? K. Murugan Crompton Greaves --- I do not know what is your substrate material. I assume it is copper. There are two choices I could think of. For low frequency contact use, low temperature applications, your better bet would be cadmium plating. This material is also better for applications in humid environmets. That is why it is used for connector plating. Be careful about the use of cadmium though because cadmium, being toxic, is not accepted in many countries. The other method would be gold plating. Mandar. Mandar Sunthankar - Fort Collins, CO --- LIFE OF SILVER CONTACTS measured in the terms you use - without tarnish -is virtually nil. But why worry about tarnish? It is generally very conductive and for most electrical contacts where applied voltage is generally quite high(in excess of 10 volts) the tarnish will not be electrically noticed. For electronic contacts, where very low contact resistances may be required, you probably cannot use silver - depends upon the application. Very low contact resistances have to come usually from gold or even unpolished rhodium (very expensive) but for general purpose switchgear tarnished silver is excellent. It is also one of the best finishes for switches in a marine environment. Peter Vivian

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