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
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.
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.
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
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.
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 ..
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 :-)
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
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
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
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.
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.
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
This voltage and current regulator is a good match to the 50A
The rectified transformer output is almost useless and high enough to
be considered dangerous without either the variac or DPS5020 to
control the voltage.
"OSHA considers all voltages of 50 volts or above to be hazardous."
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