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Tin Cobalt going Black

(-----) February 18, 2008

Hi I put in a tin - cobalt bath
it was plating ok
I then raised the Tin content by 30% to whiten the bath.
Now it is plating black.

What plates first Cobalt ( What colour is Cobalt? ) or Tin.

How can I get the Tin to plate more of the Alloy ?

Warren Castle
- South Africa

First of three simultaneous responses -- February 24, 2008

I plate Tin/Cobalt a couple of years back. I hope your bath uses proprietary additives and is not a home brew type.
Although there may be other types, the Tin/cobalt system I used then, the constituents as below :

Tin pyrophosphate - 25 g/l
Cobalt pyrophosphate - 20 g/l
Potassium pyrophosphate - 200 g/l

Original bath make up uses starters, then on, regular analysis for above 3 salts on a regular basis and addition of MU ( make up ) and BR ( Brighteners ) according to every 100 A-hr ( MU - 100 ml, BR - 400 ml ).Tin pyrophosphate is added by mixing in potassium pyrophosphate slurry. The same goes to cobalt pyrophosphate.I hope you do not add tin salts directly into your plating bath.30 % increase of tin content seems excessive and your bath is out of balance with the brighteners.You need a Hull cell test.Try testing your present solution at 50 deg C, 1 A at 2 mins.The hull cell plate should show the whole area of the plate black and grey. Then slowly increase your make up and brightener addition until you see at least half the hull cell plate bright. You should be able to plate normally at this condition. Good Luck.

SK Cheah
- Penang, Malaysia

Second of three simultaneous responses -- February 25, 2008

Dear Warren,

First of all, are you plating pure Cobalt, or an alloy of Cobalt and Tin?

Based on Standard Electrode/Redox potentials, Tin is more noble than Cobalt and should deposit in preference to Cobalt. However, if you are trying to deposit Tin and Cobalt together in an alloy, there is usually an additive to enable the codeposition of both metals in the alloy.

If you are using a commercial solution, it is best to check with the manufacturer/supplier of the solution.

My response is based on my experience of plating alloys.

I await your response, and best of luck.

Maurice Mason, Plating Engineer.

Maurice Mason
metal deposition - Danbury, Connecticut, USA

Third of three simultaneous responses -- February 25, 2008

Hi There,

Could you please give us further more detail of the kind of bath you are using , is it a sulphate based bath, or you using any proprietary chemistry.


praveen kumar
Praveen Kumar
plating process supplier
Mumbai, India


October 17, 2008

I use SN-CO alloy bath but colour is not black, (grey) I want more black colour what do I do?
I use sulfate bath.

Fikret Aydin
- Istanbul, TURKIYE

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