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Silver anode now turns black during Silver Plating

adv.    

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We are doing Silver plating process on Nuts and bolts to make them anti-seize. Our Tank is Old and solution is checked and additions are made periodically to keep the contents within limits of Standards that we follow. The Tank was working O.K. in the past But recently we are facing a problem that the ANODE which is Pure 99% Silver starts getting a black film during plating. The black layer on Anode slows the deposit rate and we have to take out the Anode from the bath and clean it using an abrasive pad, rinse and again put it back. We have to do this 3 to 4 times in a 15 minutes plating operation. We have checked the bath for KCN and AgCN and their contents are within limit.
Please suggest any troubleshooting/remedial action?.

Muhammad Najeeb
Plasma Shop Employee - Karachi, Pakistan


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Hi Mohammed,
silver anodes can be available up to 99.97% of purity. Get that.Change the supplier if he cannot.50% of your problem will be solved.
Now take care in the following areas:
1.Low free cyanide
2.Low pH
3.More current.
Mohemmad why don't you mention the bath composition instead of mentioning "within limit"-what is the limit!
There is a ratio of metal and cyanide. But first pl. let us know the "limit" you maintain!Then may be we will help u.
Try to post maximum details rather than leaving things for guessing.
Regards,

T.K. Mohan
    plating process supplier 
Mumbai, India



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Dear Mohan
Thank you for your suggestions. The Bath Make Composition is Four Chemicals are added in De-mineralized water. These are Potassium Silver Cyanide = 13 Oz.wt./gal, Potassium Cyanide = 10 Oz.wt./gal, Potassium Carbonate = 4 Oz.wt./gal, Potassium Hydroxide = 1 Oz. wt./gal.

Gallon is US gallon = 3.785 Liters.

PH value is 12.5 ,

The Solution Control Limits are Silver Metal = 6.0 - 7.5 Oz/gal (Present value 6.1), Potassium Cyanide = 6.0 - 9.0 Oz/gal (Present value 6.5), Potassium Carbonate = 4.0 - 16.0 (not checked recently), Potassium Hydroxide = 0.5 - 2.0 (not checked recently).

We will look for a 99.97 Silver Anode in our local market. Meanwhile, based on the bath details, if you have any other suggestion please let us know.

Regards
Najeeb

Muhammad Najeeb
Plating Shop - Karachi, Pakistan


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Najeeb,
Can you convert these conc. into g/l!
I am leaving for a holiday. will be back on 15th
Regards,

T.K. Mohan
    plating process supplier 
Mumbai, India


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Muhammad,
you would be care full in following areas if your chemical parameters are in range..
1.filtration of solution
2.quality of cyanide
3.current density
4.quality of water
5.pH of solution

regards,

Uroj Shahnawaz
- Karachi, Pakistan


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If these anodes were operating OK in this solution until now, then the problem is most likely in the solution. As you indicate that the current is also dropping it sounds like the anodes are polarizing so as your Free cyanide is at the lower end of your range I would try adding a couple of ozs/gal of Potassium Cyanide and see what happens.


Geoff Whitelaw
- Port Melbourne, Victoria, Australia


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Hi,

Your Anodes are impure, also not annealed and pickled after rolling. It is probably grease that comes out every 15 mins or so since it can be cleaned by an abrasive. please also check the operating voltage at the bus bars.

deepak whorra Deepak Whorra
     silver craftwork
New Delhi, India

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