Metal finishing Q&As since 1989

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Staining of tin plated brass

I have a problem with a tin plated brass cover. Brownish blotches appear in the tin. The manufacturer states it was a problem with the rinse of the copper before the tin plating. Will this lead to weaker corrosion resistance? I need to determine difference between an asthetic or mechanical problem. Any input or reference material would be helpful.

Todd Reffey
Engineer - Bucyrus, Ohio
2005

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2005

Hello Todd,

I believe you are missing a strike of Nickel plating between the copper and Tin, your patches are from Migration of metal.

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

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2005

Todd,

Unless you do FTIR or some other surface chemistry analysis on the spot itself, one cannot be definitive as to the nature of the stain. Even then, the source of the offending chemical can be difficult to isolate. The gentleman from India is correct, nickel barrier over brass substrate is the preferred system. It has a slower intermetallic growth rate than copper underplate and is an excellent barrier to diffusion of zinc from brass.

There is also general acceptance in the non-high reliability connector industry that nickel under lead free tin will mitigate tin wisker growth.

There are new barrier systems available that provide superior performance vs. standard copper or nickel. I would be happy to discuss these if you have need.

Bill Ward
alloy wire - Hawthorne, New Jersey

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2005

If the brown stains appear within a few hours, then it is NOT copper migration thru the tin.

Tin Plating solution will NOT rinse off in 40 rinses, you have to clean it off with a dip in a milf alkaline cleaner. Most of the time that STOPS the brown stain.

Also, the plater should enter bare copper parts into the tin tank "HOT". That is, make contact before entering the tin solution and go in hot so that copper does no have time to contaminate the tin.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina