In article , Harold & Susan Vordos says...

I imagine you've played with gold plated electrical pins in the course of
your hobby or work. Have you ever noticed that they are plated with nickel
before gold is applied? Sir T.K. Rose, in his Metallurgy of Gold,
documents the phenomenon whereby gold and silver, placed in intimate contact
with one another, will migrate from one to the other. Pins without the
nickel barrier slowly absorb the gold plating, leaving behind nothing more
than a copper or brass pin that has a miniscule amount of gold in its
makeup. Not enough to prevent corrosion. That happens at room temps.
No wonder heated lead is so aggressive.

The nickel in this case is called a 'diffusion barrier.' Other
transition metals, notably titanium, can be used that way. This
is common in integrated circuit wiring.

Jim

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