Muriatic Acid is the good stuff. Its a pool chemical that eats rust, and if not properly neutralized, good metal. Available everywhere. Its pretty bad stuff. Avoid it possible.
You'll probably want phosphoric acid. Much tamer, wont hurt good metal but will convert rust to a harmless oxide. You want at least a 30-40% phosphoric acid product. just wash it out with water. No need to neutralize it.
