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Electro-tin coming out black; but not every time

I run a high speed stannous tin process here in Georgia and from time to time the parts come out nearly black. On the majority they come out a nicer gray color but sometimes its not so nice. I have a feeling that it has to do with the pH in the tank and that adding more pot ash seems to help; but I need more input. Someone please lend a brain on this one.

Gregg Matti
- Hampton, Georgia, USA
2007

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1st of two simultaneous responses --

It sounds like you are plating at too high a current density. The coat should be metallic white, not gray. do you use a brightener? Do you have an operations sheet for the product, or is this a home brew?

James Watts
- Navarre, Florida
2007

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2nd of two simultaneous responses --

Difficult to answer this one, because all we know is that a stannous tin bath sometimes gives a black deposit....Causes could be: 1) bath out of specification; 2) temperature too high; 3) pH too high; 4) current density too low; 5) current density to high; 5) brighteners out of balance; 6)passive anodes; 7) metal concentration too high; 8)anode area too low.
Perhaps with more detail of the bath spec and operating conditions the problem can be narrowed down. Is there any chloride in the bath? If so, it could be in excess. Your comment about adding potash improving it suggests the pH is too low, but without knowing what the bath should be, suggesting any additions or changes could be disastrous to the bath.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2007

I've come to a facility where there were no standards set for the tin bath and no testing was ever done. Seriously. I've been trying to set standards for this tank but cannot find any that are consistent, so I take an average. I know this is not scientific; but at least I'm making an effort. This has been a voodoo chemistry operation for many, many years and I have most of the process in control now; but this one bath still leaves me with questions. I'll post most of my information this morning and see if that helps. I've got to get to a meeting. Thanks for your responses.

Gregg Matti
- Hampton Georgia USA
2007