I have a turbo exhaust manifold that has been setting outside for a while and I know there is some big rust flakes inside. do you think this would work for this process? will it remover or loosen the bigger chunks
Yes, this works great on exhaust manifolds. An exhaust manifold was my second test part when I was in the 'this is too good to be true' phase of experimentation. Remember, though that the sacrificial cathode has to be in line of sight from the parts you want the rust to disappear from. For big pieces, this usually means you turn it over/turn it around in the bucket a few times to get all the sides and corners.
Your manifold will be gray and rasp-like when it is done. That's bare cast iron. You'll need to hit it with some rust preventative pronto or it will turn brown. I use phosphoric acid to pickle it, like OsPho, Prep Sol, etc. For exhaust manifolds, you hit it with the phosphoric, let it dry off, then spray it with header paint. I blow a heat gun through one port with all but one other blocked off to get the manifold hot and cure the paint. If you keep fingerprints off of it, the header paint will stick very well to the derusted, phosphate-coated iron.
I've generally used scrap black pipe from plumbing jobs as the sacrificial electrodes. I also had an exhaust manifold with broken ears that worked well. For something irregular shaped, I'm guessing the expanded metal mesh would probably be the ticket. Nearly anything ferrous metal will work, as long as part of it projects out of the bath to be used as a wire attachment point.
I've generally found the hardest part is maintaining good contact between the sacrificial electrode (I usually have several in a daisy chain) and the wires coming from the power source. The electrodes rust away, and so you're usually going after the out-of-water end with a bastard file, wire brush, or something to find some clean metal to get a connection to. It's not trivial, because running voltages are usually in the 2-5 volt range and it doesn't take much resistance to stop that entirely.
Power supplies do indeed have to be rated for the amount of current you're pulling. The good news is that you can adjust that current by using a lower concentration of washing soda or lye.
The easiest way, frankly, is to use an ammeter. Stay at about 50% or less of rated current on the supply and all should be well. If you overshoot, don't panic - just dip out some electrolyte and replace with plain water.
Also be warned that, like most chemistry, it's heat sensitive. Current flow goes up when the water gets hot. If you are putting lots of current into the water, you get it hotter, which increases current flow and next thing you know you've fried the power supply. Yes, I've done this. That's why I say do it at 50% of rated current.
As for current demand, that basically figures out as how fast you get your part cleaned. Current over unit area is a constant, so more is necessary for a bigger part to get the same rate of removal . If the voltage across the electrodes is high enough, you WILL get removal. It might, however, be very slow... If the inter-electrode voltage drops below the minimum EMF required to convert the rust into black magnetite then absolutely nothing will happen, except that your part will rust in the water bath...
Put the two pieces about 3 inchs a part.
