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Switch mfgr asks if sulphate/sulfamate from nickel causes silver oxidation

In a switch containing a stamped sliding metal contacts (contacts made of phosphorous bronze plated with Nickel and then with Silver) we experienced erratic voltage drops over time when the switch was closed. An SEM/FTIR analysis revealed some sulfur contamination. We thought this was causing high resistance and thus the erratic and high voltage drop in the closed position. We discovered that the grease used in this application had some small amounts of sulfur. We switched to a sulfur free grease, and the problem appeared to diminish somewhat. We also found that during the nickel plating process, sulfamate is used. Could this be a contributing factor? Is sulfamate known to cause corrosion in the same manner as sulfur? How about sulphate?

Thanks in advance for considering.

Adnan Abbas
Automotive - Atlanta, Georgia, USA
2003

The "sulfur" in sulfamate nickel is a part of the nickel alloy and will have no effect on the electrical or the corrosion of your part. Grease is a notorious insulator so the fact that you have a problem is not shocking. A sulfur based grease is not an alloy, it is not in solution, it is more of a mixture so is somewhat free to attack your equipment. Silver forms tenacious silver sulfide layers very easily and is a bad conductor compared to the nickel or the silver.

James Watts
- Navarre, Florida
2003

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First of three simultaneous responses --

Sulfamate nickel plating solution will not discolor silver. Silver is soluble in sulfamic acid thus oxide is removed. Divalent sulfur is the form that reacts with silver to produce tarnish. Divalent sulfur is often in the air. It may be worth while to use a very light chromate conversion coating on the silver to prevent sulfur tarnish.

Don Baudrand
Consultant - Poulsbo, Washington
(Don is co-author of "Plating on Plastics" [on Amazon or AbeBooks affil links]
           and "Plating ABS Plastics" [on Amazon or eBay or AbeBooks affil links])
2003

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Second of three simultaneous responses --

Adnan,

First silver will never be oxidized silver react with sulfuric to a sulfide coating. I think for the first, you must use a barrier strike of copper before the nickel to protect the material from diffusion between nickel an phosphorus bronze. Both sulfamate and sulfuric nickel contents sulfuric acid. I think you should go for the grease.

Regard,

Anders Sundman
4th Generation Surface Engineering
Consultant - Arvika,
Sweden
2003

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Third of three simultaneous responses --

Most likely, sulfur might come from brighteners in the final silver bath chemistry. Very unlikely from underlying nickel deposit which had to be very well rinsed before application of silver.

Guillermo Marrufo
Monterrey, NL, Mexico
2003