Iggy is right diluted muratic is fast and cheap, wants to re-rust. I stop it
with a squirt of purple engine cleaner (strong caustic) after a water rinse.
Still need to oil or paint.

For many parts I like phosphoric acid better. I buy it already diluted as
"Lime Away" cleaner at my local Fleet Farm. This works differently, it
converts rust to a black iron compound that won't re-rust. If you'd rather
pay more, you can get the same chemistry in Naval Jelly or rusty metal prep.

The electrolytic method others have mentioned is best for a small amount of
rust on a delicate part.

Karl