I've used this quite a few times with some success. If you google it or search youtube there are plenty of how-tos, most of them recommend using washing soda (sodium carbonate) which you might find in a large supermarket, DIY shop, or hardware place amongst the cleaning products.
It's not this however that removes the rust, it's only added to the water to make it more conductive, and you don't need much, I think about a table spoon per gallon(5 litres) is recommended. The rust is removed by the electricity actually reversing the rusting process, it splits the iron oxide back into iron and oxygen, the oxygen bubbles to the surface and the iron is left on the piece as grey/black deposits, these need wire brushing off afterwards but they come off very easily. The piece of sacrificial steel will be eroded away, so be very careful to get the wires connected the right way around, or the piece you intended to de-rust will come out with holes in it.
Any DC power source is suitable, but the higher it's out-put the quicker the process, a basic battery charger is perfect, even better if it has a built in ammeter, as then you know it's working.
Some of the guides warn about the mixture of hydrogen and oxygen given off being a fire risk. Being the curious type I tried putting a lighter to the bubbles on the surface of the water, I got a couple of small 'phut' and 'pop's but nothing interesting. Perhaps if you collected the mixture in a bin bag it might be more dramatic, but overall I wouldn't be too concerned.
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