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Posted by David Sherman on Sunday, February 15, 2009 at 13:49:07 :
In Reply to: Re: There is a lot of info in the Forum Archives posted by Nick on Saturday, February 14, 2009 at 17:51:33 :
Battery chargers are fine. Some people use old computer power supplies because they're free. I see Paul (NY) has a fancy HP lab type power supply. Whatever you have, so long as it puts out DC will work. The key is the amps, which depends on the surface area of the part and the conductivity of the solution. I just dump the soda in by eye -- maybe a tablespoon or so in 5 gallons, and turn up the voltage until I get 5-10 amps going through. I use an adjustable battery charger that can be dialed from zero to 75 volts, which I find is very handy. I've done small parts that needed the whole 75 volts to get 5 amps through them, and large parts that would draw 10 amps at 15 volts. If you just have a 12 volt battery charger, you probably won't get more than a few amps on most parts, but that doesn't really hurt anything. All it means is you have to leave the part in longer, like half a day versus an hour or so. Obviously, if you crank the voltage up past 30 volts or so, you want to keep your hands out of the solution and off the wires while the juice is on.
I've never found any damage from pushing it hard with lots of volts if I want to do a lot of parts in a short time. Another reason to push it is if the shape of the part means that some parts are recessed or are a lot further from the anode than other parts. You'll be bubbling a lot of gas off of the more active parts before you get all the rust off of the recessed parts.
I always use stainless as the anode (either an old sink for big parts, or various cylinders and tubing for smaller parts). I know some people say I'm going to poison everyone and everything, but I just don't see it since the stainless is never the slightest bit pitted or corroded, which means no significant amount of chromium is going into solution. Heck, "Chem-Dip" carburetor cleaner has potassium dichromate right in it, and nobody seems too terrified of that, though I always wear rubber gloves when dealing with that stuff.
I usually dump out the solution after each project. I don't see any reason to save a bucket of gunky, rusty washing soda water, any more than I'd try to save the wash water after doing my laundry. The soda is cheap and benign.
Also, the gas that bubbles out is not, as somebody thought, toxic. It's just hydrogen and oxygen, formed by breaking down the water at the anode and on the places where the rust has been removed. One clue to the process being complete is when hydrogen is being evolved evenly all over the workpiece. If you watch it, you'll notice that initially only the clean parts produce hydrogen, whereas at the rusty parts, one way to look at it is that the hydrogen is reducing the iron oxide back to iron. The fun thing about the bubbles is that once the solution gets sufficiently dirty for the bubbles to start accumulating a good head of foam on top of the solution, you can ignite them. There's a good mixture of hydrogen and oxygen in the bubbles, and one spark will explode the whole mess, blowing dirty rusty foam all over the place.