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Plating Migration (Diffusion)

I work for a company the does precision machining. A good portion of our machined parts require some type of plating. Recently there have been concerns that some parts we made out of brass and then had nickel plated then gold plated were not plated to the thickness specified. All of our documentation and certifications show that the specification was achieved by the plating company. Note, these parts were made 2-1/2 years ago and just now inspected for plating thickness before put into service.

My question is:

Over time will migration occur between gold plating and nickel plating. Could this cause a false reading on thickness if measured 2-1/2 years after the actual plating took place. Also, would extreme temperatures play a part in the plating to migrate and show a false reading on thickness?

Please asdvise on any literature I may find on plating migration.

Regards,

Greg Cde Baca
machine shop owner - Clackamas, Oregon, USA
February 11, 2008

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Nickel is usually a good diffusion barrier, but that is not to say that gold will not diffuse into it. However, diffusion is normally a very slow process and unless you are operating at high temperatures or have very sensitive equipment, I would not consider it to be measurable by most common methods.
I would suggest you check the thicknesses by doing a microsection and see what is actually going on. You can easily do this by simple metallography and either optical or SEM microscopy. It will also allow you to see whether the nickel or the gold is under spec. Of course, it could just be that your brass substrate was undersized to start with....

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
February 17, 2008