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Electromotive Force potential of chromate coating

I am a Signal Integrity Engineer that specifies surface finishes to both aluminum chassis and printed circuit boards(PCBs). With the current lead-free initiatives in PCBs a common surface finish is immersion silver. When silver is directly tied to zinc, the Electromotive forces (EMF)are significant and we see corrosion within 6 months. Does anyone know the EMF of typical chromates (not chromium) used on aluminum chassis? If so, please indicate if gold or hydrogen was used as the zero point.

Shawn X. Arnold
High-Speed-Solutions and Design - San Jose, California, USA
2005

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I will just say that chromates are a whole 'nother different animal. There are hexavalent chromates, chrome free chromates(!), clear and yellow and iridescent, olive drab. black chromates (some of which used silver, and other metals to get the black color), so why not pick one you want to use and tie silver to it to get some numbers on your own. You may be able to contact the vendors of conversion coatings to see what they recommend.

  
Tom Pullizzi
Falls Township, Pennsylvania
2005