chemical level with no chemicals or fumes or anything nasty.
Before (these are sub frame bolts exposed to years of the elements under the front):

Let soak 24h:

Rinse off and voila:

It works really well on our old rusty bolts that are hard to replace. Treat these
with a rust inhibitor/seal afterwards for many years of rust free hardware.
It costs about 10 bucks/liter in Canada, prob way less in the states.