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Protection of Stainless Steel Tank for Nickel Bath

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What affects the amperage to go high in a Nickel Plating Stainless steel tank? We use a rectifier with 0.75 volts and a maximum of 4 amps. How can we prevent the amperage from going up?

Hagos Bahta
Printed Circuit Board - Sterling, Virginia, USA


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Sorry Hagos, but I'm a little lost. Are you talking about an anodic system for a stainless steel tank that keeps electroless nickel from plating out on it? If so, what is the size of the tank?

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


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Hagos, before anyone can help you have to let us know if this tank is used for Electroplating Nickel or Electroless Nickel deposition. If it is an electroplating bath, what type of nickel bath are you using, what type of anodes, and as Ted asked, the tank dimensions. Please advise.

Mark Baker
process engineer - Malone, New York


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The tank is used for electroless nickel deposition. It is equipped with an anodic protection system for electroless nickel deposition system (to prevent the nickel bath from plating out on the tank). The anodic protection system we are using is called Protektostat SST. The tank and cathode are made of type 316 stainless steel. The tank is 20" x 51" x 30". or about 120 gallons.
I have to drain the nickel bath at about 5 Metal Turn Overs or whenever the current reading on the Protektostat SST reaches 3 amps. My problem is sometimes, for reason that I don't know, the current goes high so early in the life of the bath. I would appreciate your advice on how to prolong the life of the bath by maintaining a low current reading until at least 5 metal turn overs.

Hagos Bahta
- Sterling, Virginia, USA


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Hello Hagos,
Thank you for responding with the added info. I had a similar problem a few years ago. We determined the tank had a few hot spots that led to plate out, even though our tank had anodic protection. We also determined that our load per gallon (sq ft/gal) was shy of what it should have been. We improved our solution agitation system, increased our load capacity accordingly. We were able to get an additional 2 MTO's from the bath. I forget what the current cut off point was on our system before dumping the bath. Our tank was 160 gallons and our voltage was about 1.1. We had to passivate our ss heaters of course. There could be other causes to your problems such as the activity of the solution itself, or magnetic properties in or around the tank that you may be unaware of. I hope this will be of some help to you.

Mark Baker
Process Engineer - Syracuse, New York


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Mark:
Thank you for the information. It helps a great deal. I would appreciate some more input on the following issues, if you may.
1. What is the principle behind the anodic protection system? How does it protect the tank?
2. Why does the ampere reading go high as the bath ages?
3. The situation we had is that, while chemically the bath is fine (it's not active and the orthophosphite level is not high), the current reading on the rectifier tends to go high. Why does this happen?
4. I noticed that the cathode rods in the tank get plated significantly. Are they supposed to be plated?

We changed the rods and cleaned/changed some of the wiring on the tank. Now the current is, fortunately, minimum with the bath age at about 2MTOs and we are continuing to observe how the system behaves.

Thank you for your help.

Hagos Bahta
Printed Circuit Board - Sterling, Virginia, USA


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Hello again Hagos,
The reason for anodic tank protection is to help avoid plate out of the EN on the stainless steel tank (which you already knew). In a sense it makes the stainless surface passive. The reason the current increases is that the rectifier is compensating for a greater amount of conductive area in the tank. In your case you mentioned plating out on the cathode rods, are they a series 300 stainless as well? If so, they should be passivated in a solution of 20% by volume nitric acid at 130 degrees F for thirty minutes. This will eliminate excessive plate-out on the rods. I am not a EN expert, I can only share what has worked for me. It's good to hear the amperage is remaining low on your system. Good Luck!

Mark Baker
Process Engineer - Syracuse, New York


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Dear Sir
1- You can have more information regarding anodic protection if refer to books concerning corrosion and protection.
2- Anodic protection work in a limited range called passivation, so at the beginning your current should go up and then down ,after that if you increase the voltage you'll get into a pre-passivation stage where your current will shoot up and up by increasing the voltage.
3- I believe you should ,electrically insulate your tank , you have to calculate the surface in contact with the solution, and your current density stay within passivation required limits.
4- While filling the tank keep the current within passivation limit, by slowly increasing it.
This for the best of my knowledge and it may help and god luck.


Cair Shishani
aircraft maintenance - Al Ain, United Arab Emirates


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This is not the answer you wanted but have you considered using disposable specially made plastic liners for the tank? If that eliminates the need for nitric acid so much the better.

Nick Clatworthy
- Whitstable, Kent, UK

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