Using a welder for electroplating



Yes, plus the bar itself would be properly called a copper strip.
In any case, as I said, we became sidetracked.
I know that my welder is a constant current welder. I am not sure why we are arguing over this.
The original question was, is a constant current welder such as my 100% duty cycle cybertig, capable and usable for slectrolytic rust removal.
The answer from Don Foreman indicates, sensibly, that the answer is yes. I also suspected so and am now convinced that it is the case.
And yes, my welder is able to supply as little voltage as necessary, when faced with low resistance.
i
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On Thu, 17 Nov 2005 21:22:55 +0000, Andy Dingley

Sorry, this is annoying me.
You just can't get 2V across any short length of copper, of any thickness - it would melt almost instantly.
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Andy Dingley wrote:

If he shorts his welder, the voltage AT THE SHORT is zero. You are correct, Andy. However, the connections and the welding lead have nonzero resistances, and so some current is still flowing through those resistances and generating a small voltage back at the welder. So he is also correct.
It's easy to think of a piece of welding lead as having zero resistance, because in many ways that's an excellent approximation. However, *nothing* has zero resistance except maybe a piece of superconductor.
His welder is a fancy one, too, one of those TIG stick machines.
GWE
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You are 100% right. There is enough resistance to make the voltage at terminals about 2 volts. Even the copper strip that I used warmed up considerably.
i
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replying to Andy Dingley, S G wrote: for a car body like a mustang use 6v 1000amp for 2 hours , start the solution off hot, ue carbon/graphite blocks to avoid making sludge
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Andy Dingley wrote:

Good TIG welders will happily run down to 5A output. The open circuit voltage may be ~30V, but it will be lower when under load.
Pete C.
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On Wed, 16 Nov 2005 01:44:26 GMT, Ignoramus15120

I have used my AC oil cooled welder with an external rectifier to de-rust rebar at 50 Amps. I was using an 8'x1'x8" pond made up from plastic sheet supported by wooden railway sleepers and using a washing soda electrolyte with lead anodes. The only thing limiting me to 50 A was the bridge rectifier that I had ( I now have some 100A rectifiers). I did have to top up the water in the electrolyte several times over the 5 days that I was using the rig. I measured 15V across the electrodes and this gives 750W dissipation... the water was hand hot and steamed gently where it was not frothing over the rebar.
Mark Rand RTFM
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That's interesting (lead anodes). Why did you decide to use lead? Is the resulting solution environmentally safe?

Wow, that's very impressive. I did not realize that even at 50A, derusting takes so long (and so much energy). I thought that at that sort of amperage, I could expect rust to disappear within minutes.
i
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Like I say, the amount of current you need is proportional to the surface area. In this case he had a lot of surface area so he used a lot of current.
I like the idea of lead anodes. Lead is quite insoluble and of course it is used in about a billion car batteries as an electrode ..
GWE
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The reason why I am so interested is that I have some lead sheet leftovers that I could use as anodes.
i
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wrote:

I didn't mention that I was actually processing two batches of rebar a day. The current density was much higher than ideal, but I was in a hurry.
Next time I'll buy new rebar and scrap the rusty stuff :-)
Mark Rand RTFM
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On Thu, 17 Nov 2005 15:59:23 GMT, Ignoramus24428

Lead isn't a good anode, but lead peroxide is. It's like a car battery, you really need to form the coating rather than just dump in bare metal and hope. A convenient way is just to lift plates out of an old battery.
The resultant solution will be full of lead (and maybe other more-noxious heavy metals). Dispose of it as for battery waste. In theory it's possible to avoid the lead going into solution, but not in practice.
There's a similar problem if you use stainless steel anodes (as I do). However this is _not_ hexavalent chrome (the really nasty stuff) and you can avoid much of the problem by not storing the anodes in the tank when not in use.

This is usually a symptom of excess voltage, or excess currrent density in some areas. This puts too much energy into electrolytic gas generation, rather than de-rusting. It's just wasted energy.
For big pieces, where I can really control the current density by using big plate anodes, then I'd rather run at about 8V or even 5V (5V is a bit low, but big PSUs are conveniently available) Anything over 12V is definitely too high - if you have to use these voltages to get a decent current, then look at providing better anode area or checking the electrolyte concentration.

I don't think this process can be used to remove "significant" rust in under a day. If you try to push it too hard, the chemistry gets bored and goes off into other areas instead.
All "electro-plating" processes are extremely sensitive to current density and to local variations from the ideal current density. Read a commercial handbook for advice here - industrial plating goes to a lot of trouble with magic additives to allow higher currents (and thus higher deposition rates) For the rest of us it's easier just to keep the current down and be more patient.
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replying to Andy Dingley, S G wrote: for a car body like a mustang use 6v 1000amp for 2 hours , start the solution off hot, ue carbon/graphite blocks to avoid making sludge
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Load vs temperature rise testing of the variable power supply I built from a 50A, 20% duty cycle arc welder transformer and a variac showed that the output current should be limited to around 20A continuous or 25A for 1/2 hour. This corresponds to a derating to 20% of the squares of the currents, ideally 22.36A.
P = I^2 * R, where R is the copper winding resistance.
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Arc welding transformers have a fairly high internal impedance to cheaply provide high no-load voltage to start the arc and current limiting when welding, especially when the electrode sticks. The tradeoff is low efficiency. This means that the voltage drops significantly as load current increases, and conversely the voltage rises if the load's current demand decreases, as when a battery nears full charge.
This voltage and current regulator is a good match to the 50A transformer: https://lygte-info.dk/review/Power%20DPS5020-USB%20UK.html
The rectified transformer output is almost useless and high enough to be considered dangerous without either the variac or DPS5020 to control the voltage. https://www.osha.gov/laws-regs/standardinterpretations/2015-09-04 "OSHA considers all voltages of 50 volts or above to be hazardous."
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