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"Zinc migration mechanism"

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I wish to understand from a chemical point of view, what is the process of zinc migration.

The product:

CA 260 Brass relay terminal

underplate (2 - 4 micrometre Cu)(78 - 157 microinches)
overplate (2 - 4 micrometre Sn)
No soldering is required on this part, just welding to
another CA 260 Brass metal part.

I have consulted ASTM B545 [affil link] -92 which recommends Min 2.5 micrometre Copper or 1.3 micrometre Nickel underplate on metals which contain > 5% zinc to prevent zinc migration. I have received varying recommendations of what underplate of Copper is required to prevent zinc migration. Therefore if I could receive some recommendations and a description of the mechanism or where I can learn more of the process this would be of help to me.

Thank-you very much,

Andrew Ysselmuiden
- OAKVILLE, ON, CANADA

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If the ASTM specification calls for copper, I guess that is okay, but it doesn't make much sense to me, since the zinc is already alloyed with copper in the brass. I would use nickel as a barrier.

  
Tom Pullizzi
Falls Township, Pennsylvania