Eletrolytic Rust Removal Question

I am trying to use a leftover AC/DC adapter(s) to power an electrolytic rust removal rig I built. It works fine on a small scale (1/2 gallon of
sodium carbonate solution) and removes rust. Scaling it up to work on a big piece of rusted steel (12 gallons of solution), I encounter problems. The AC/DC adapter dies after a short time. I've killed 2 adapters so far, without removing all the rust. What's going on and how do I fix the problem? Thanks.
--
John Redmond

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John Redmond wrote:

May I suggest a small car battery charger? The older models that don't have circuitry to sense voltage and autostop can be quite inexpensive. I found one for $10 using the free ads on craigslist. To find the CL closest to you visit http://www.craigslist.org/about/cities.html
I suspect you are drawing out more current than the small "wall wart" can handle.
GWE
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Second that. It also helps to have the ammeter on the charger, it gives you an idea of whether the setup is working or not.
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OK. But how do I know if my rig is or is not drawing more current than a small car battery charger can handle? Is it the amps that the charger supplies that determines if it can handle my rig? How do I figure how many amps I need?
TIA.
--
John Redmond


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John Redmond wrote:

Little car chargers are pretty rugged. I doubt you'd have much trouble. They also normally have ammeters on the front and a switch 6/12 volts. So you can start out at 6 volts, see if it's bubbling nicely and if not, kick it up to 12. The amps you need are really dependent on the size of your tank, your electrodes, the strength of your solution, the surface area of your workpiece, and the composition (e.g. steel or stainless steel or graphite or lead ..) of your electrodes. Lots of guys figure something in the 4-15 amp range is OK for small items. If you're doing a big truck bumper in a ditch eight feet long lined with visqueen, you might need 120 amps. I wish someone scientific would come out with some heuristics on the optimal current density per square inch workpiece area for EDR.
GWE
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One of my "wall warts" was 9 volt and 0.3 amps while the other was 13 volts and 0.8 amps. No wonder they died, although I did get good results with the 9 volt in my 1/2 gallon tank. I've got another wall wart that is 30 volts and 0.83 amps that I might try, but I've read that I need to keep the voltage at about 12 or less. Is that right or should I give it a try?
--
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Calculate a current limiting resistor - or put a 10 ohm or such in series - to limit the short current... without a limiting resistor the wart will have to limit - and that is typically to expensive for them to do.
It will save the wart. Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH, NRA Life NRA Second Amendment Task Force Charter Founder
John Redmond wrote:

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On Fri, 25 Nov 2005 19:21:33 GMT, John Redmond

Wall warts are too small. If you can find one of the 2A models made a few years ago to power early digital cameras, then it _might_ work.
For de-rusting woodworking hand tools (my main use) then I like to see 1A - 1.5A at 12V. The biggest thing I can fit in my default tank would use about 3A max. For working on sheet I've used up to 50A at 5V, but that's getting complicated.
You need an ammeter so you can see what's going on. You should have a circuit breaker because things do tend to fall over accidentally and finding a short in the morning is a lot better if it's a popped breaker or fuse, not a smoked transformer.
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Try an old computer power supply the output is 12V and 5V

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wrote:

Computer PSUs are switchmodes. They don't take kindly to this sort of zero-load / random short abuse in an ad hoc derusting tank. You're much better off with a simple battery charger: crude transformer and rectifier to deliver power, ammeter to see what's going on, and ideally a resettable circuit breaker in case of over-current.
Also make sure the soda concentration is adequate and that the anode plate is large enough (bigger than the workpiece).
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If you can get your hands of a Variac or Powerstat (two brands of variable transformer), plug the battery charger into that, and set the input voltage to produce a current that is, maybe 1/4 to 1/3 up scale on the current meter of the charger. Then just watch what happens. If it gets hot and makes lots of bubbles, turn it down. If it doesn't seem to do anything, turn it up.
Funny story: When I worked at Shell Development Co. one of the company executives brought in a badly corroded rare coin to the Materials Engineering and Corrosion Department, and asked them to clean it up. The department head turned it over to one of the PhD chemists, and he did essentially what you're doing, but with lab quality equipment: variable DC power supply, milliammeter in series, etc. He set it up with a very low current and went home for the night. His "friends," who were watching him, stayed late and played a trick. The removed the coin and put a conical pile of dark crud in its place. They turned the current up to max on the meter, and went home. In the morning, the first chemist came in, and nearly had a heart attack!
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1 tablespoon of Arm & Hammer Washing Soda per gal of water. Anodes are a couple of rebars. I guess I should add more of them, but I am not sure how I sould connect them. Will the charge equalize across the different rebars if I run a wire from one to the next?
--
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John Redmond wrote:

If your anodes are connected with a wire, they will all be at the same potential. Think about it, if they weren't, then a large current would flow in the wire ..
I'm sure I use more electrolyte than 1 tbsp per gallon. Maybe much more.
GWE
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"Grant Erwin" wrote: If your anodes are connected with a wire, they will all be at the same potential. Think about it, if they weren't, then a large current would flow in the wire .. ^^^^^^^^^^^^^^^ If the anodes are not all of the same material, some of them will be called cathodes, and current WILL flow in the wire. ;-)
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OK. I'll up the Washing Soda and add rebars to the rig. I'll try the 30 volts/0.83 amps wall wart power supply and see what happens. Thanks for your help. I'll let you know what happens, but that won't be for a day or two.
--
John Redmond

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If the 12V wall wart died, the 30V one will probably go quicker.
Bob
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I had really good luck using a heavy duty on wheels " shop style " battery chager. sometimes it is on sale for $79, it is on wheels, has a timer, cooling fan, 6- 12 V setting and at least 4 amprage setting if I recall mine is 10 40 100 and 200 jump start. it would be hard to kill that bad boy.
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I agree with Grant - in general though - think of a fork. Input is the handle...
The connection wire should be able to carry the current easily or you have to make sure there is a sharing division. Due to the voltage drop on this wire.
I have a couple of gallons of vinegar in the shop to try - then this method. I think the foaming tank is the way to go. I wish I remembered more of my chemistry!
Martin
Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH, NRA Life NRA Second Amendment Task Force Charter Founder
Grant Erwin wrote:

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On Fri, 25 Nov 2005 23:11:15 GMT, John Redmond
I'd increase that. Try some experimentation and see what gives you a reasonable current.
My tank (maybe 4 gallons) has been set up for a year or two now and I have no idea what its concentration is. I top up evaporation losses and dredge the rust sludge out of the bottom. When I can't get adequate current, I just add another handful - excess doesn't hurt.

You need a "sizable proportion" of the workpiece area. Otherwise you're limiting the current and you may also get problems of local variations in current density.
For inter-linking multiple anodes, just make sure that the croc clips are high up and dry. Wet croc clips corrode in no time. All-plastic clothesline pegs are useful for arranging things under the water level, keeping anodes out of the sludge layer etc. They'll always run at slightly different voltages, because resistances are high with these sorts of crude connection and the curent flow will be affected by this enormous "saltwater resistor" you've built. However it's close enough for jazz.
I prefer stainless anodes, even though this does complicate the waste produced. Mine is a sheet of scrap (old washing machine drum) that fills one wall of the tank.
I wouldn't recommend a variac. You don't need anything over 12V and excess voltage just gets wasted in gas production, not in de-rusting. For many small pieces a lower voltage is fine, but 12V is easy to obtain.
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"Andy Dingley" wrote: (clip) I wouldn't recommend a variac. You don't need anything over 12V an excess voltage just gets wasted in gas production, not in de-rusting. For many small pieces a lower voltage is fine, but 12V is easy to obtain. ^^^^^^^^^^^^^^^^ My suggestion for a Variac was to plug a 12v battery charger into it, so you can get controllable voltages from 0-12v.
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