I've been reading about using electrolysis to remove rust and scale from metal. I have several old tools I'd like to return to use that are completely bound up with rust and scale as they've been dug up out of the garden and retrieved from other similar situations. According to the information I gleaned from the internet, all you need is a battery charger, a plastic container, water and Sodium Carbonate, aka washing soda, aka household lye. Arm & Hammer Super Washing Soda is commonly available in the laundry section of larger supermarkets.
You mix the soda at a rate of 1 heaping tablespoon per gallon of water. Manually clean a spot on the tool to be "derusted" and attach it to the negative clamp on the battery charger. Find a piece of iron to use as an electrode and attach it to the positive clamp. Suspend both the item to be cleaned and the electrode in a plastic container with enough soda-water to completely immerse the object to be cleaned. Fire up the charger. If you have good connections to the electrode and the tool, you should immediately see fine bubbles rising from the tool.