You can use electrolysis to remove rust from iron or steel. The process uses a small electrical charge to break the bond between the iron and oxygen atoms that comprise rust.
What you need:
~ an automotive battery charger, 2 amps or greater, preferably with an ammeter
~ a non-conductive container
~ sodium carbonate, commonly found as “washing soda” by Arm & Hammer. I got some online at SoapsGoneBuy.com but you can also use baking soda.
~ an anode: a piece of unpainted iron or steel to provide the electrical connection from the battery charger to the electrolyte fluid
Remove any oil / grease / dirt from the rusty part. Use a tablespoon of washing soda per gallon of water, and mix up enough to immerse the rusted surfaces of your part in your container. Clip (or electrically attach) the black/negative lead of the battery charger to the rusty part; clip the red/positive lead to the anode. The rusty part (the cathode) and the anode should not touch, but instead should be separated by 2 inches or so of electrolyte fluid.
The process works best when the anode’s surface is parallel to the rusted part. You can get creative by using an anode that bends to match the contour of your rusty part. Surface area and line of sight are key.
Turn on the battery charger: the ammeter should indicate a flow of electricity and bubbles should appear on both the anode and cathode. Time required in electrolysis depends on how bad the rust is. You may need to rotate the part to give it line of sight exposure to the anode.
The rust will turn from orange / red to grey or black; you can remove this with a scotch brite pad. Paint your part or take steps to avoid future rust.
Notes and warnings:
~ Do this in a well ventilated area: the bubbles on the cathode are hydrogen gas and generated in minute quantities, but why risk a hydrogen accumulation (and the associated explosion and fire)
~ You can reuse the electrolyte fluid. Some people discard it by pouring it on the lawn, saying it is beneficial because it is rich in iron.
~ Keep copper connections on the battery charger out of the electrolyte, or else they themselves will erode and cause additional rust on your part.
~ Web sources say do NOT use stainless steel as the anode, because it creates toxics in the electrolyte that are illegal to dump and can cause skin problems / cancers.
Why would you do this?
~ on small or intricate parts, or when typical methods are impractical
~ to remove as little of the original finish as possible, or where presence of scuff marks from sandpaper, cup brush, wire wheel, etc. is not acceptable
~ in a restoration or art project, you might leave the grey / black residue on as a patina to enhance the visuals (conditions permitting: the black / grey is often porous iron, and rusts quickly)
~ annoy your spouse / stupefy your neighbors (people around here are easily impressed, as proved by the availability of organized adult kickball leagues)
~ The setup: my battery charger, connected to a length of angle iron as anode and rusty bolt cutters as cathode
~ “Before”: my rusty bolt cutters
~ “After”: clean bolt cutters, after an hour or so in the process. This was probably too long, could have been 30 minutes or so.