Re: Electric Rust Removal Questions


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Posted by Sherman in Idaho [108.162.246.23] on Monday, May 23, 2016 at 02:39:53 :

In Reply to: Re: Electric Rust Removal Questions posted by Joe Lorenzino [108.162.242.206] on Monday, May 23, 2016 at 01:18:19 :

Titanium's worth a try if you can get it for free. Let me know how it works. I suspect it may develop a passivating oxide layer that makes it non-conductive, but that's just a guess. I don't think the pH is very important. The main thing is to make the solution conductive with something that will cause a reducing reaction at the cathode and not produce noxious fumes or be hazardous to get on your skin. The grungy rusty gunk in the wash bucket is really no worse that dirty wash water out of a washing machine. You don't want it alkaline enough to make your skin slippery. I generally put maybe a couple ounces of washing soda in 5 gallons of water. As for current density, you can't over-do it. This isn't an acid pickle so it won't dissolve metal. What it does is reduce red iron oxide to black iron oxide which then either falls to the bottom or reduces back to iron. But once it's iron, it stays that way forever at the cathode. There is no loss of detail, or pitting, from leaving it in too long or pushing it with too much current. Parts with a lot of detail need to be left in a long time because the projections closest to the anode draw most of the current. In a really tough case like a radiator elbow, I'll put a SS anode rod down inside the part.

As for current density, I'm lucky to have a variable power supply (actually a battery charger with a variac in it) that is continually adjustable between 0 and 75V. It will push 10 amps continually at up to 35V and 5 amps above that. I tend to add enough soda that I can run it near 10 amps at 10-20 volts for most parts. Heat in the water is proportional to volts times amps, so reducing the voltage for the same amount of amps reduces the heat. Large parts need less voltage and small parts need more, because their area is less. A plain 12 volt battery charger is probably fine in most cases -- it just takes longer than when you can crank the voltage up. Also, when the solution is fresh and cold, it is much less conductive than when it's hot, so I'll usually crank the voltage up to get it working and then when it gets hot, turn it down to keep it below 10 amps.

The only danger to the workpiece with this method is hydrogen embrittlement. The reducing action at the cathode is due to the generation of atomic hydrogen on the surface of the workpiece. When hydrogen gets down into microscopic cracks in the metal it makes it more brittle. This is only an issue with springs, because they're highly stressed in operation. Electro-platers deal with the same problem. I've cleaned several rusty old guns this way (it leaves a nice almost-blued finish) but I never put the springs in the bucket. A long bake-out in an oven is supposed to drive the hydrogen out, but I'd rather not bother with it.

One of the great things about the electrolytic method is that neither the chemical composition of the solution nor the current density is very important. It's not like electro-plating where everything is a variable and the various chemicals in the "secret sauce" are vital to good results. Here, you're just removing or reducing oxides and once they're removed, nothing further happens.



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