Naval Jelly, at least the stuff labeled and sold by Locktite is phosphoric acic primarily with a tiny bit of sulphuric acid in at as well (1% or less).
Probably some other proprietary chemicals as well.

I used it quite a lot in removing bluing from guns, Bluing is a form of rust/iron oxide no matter the method it is applied.
It quickly wipes hot salt bluing from the surfaces. It takes a 5 or 10 minutes and some agitation to get thru most rust blue.
I use a very weak soln of muriatic acid now to do the same. Cheaper and easier to handle.
That will remove heavy rust, heat scale, ect as well if you can soak the part(s) in it for a while.
I use a couple tbs /per gallon water.