Electrolysis works very well though slowly, and will not of course damage the heat treating/tempering of the steel.
Takes only water, a power supply such as a battery charger, and washing soda. The part must be free of all oil and grease, and that can be tricky if very rusty parts have been soaked in oil as it penetrates the scale and probably can't be got out even by a hot caustic bath. A tablespoon of soda to a gallon of water (it's not necessary to be precise), some rebar or expanded metal and that's it. Connect the item to be cleaned to the negative lead and the expanded metal or rebar to the positive lead and let it "cook". The expanded metal or rebar must be cleaned off every day with a brush and water to keep the process working "full speed".
Some machinery collectors in Australia have had good success using a molasses based mixture, which sets up some kind of reaction that removes rust.
http://www.antiqueautoranch.com/mont...rian/rust.html