You are looking for FIT's caustic paste or you could also use store bought oven cleaner which is a similar composition but more expensive while being much more handy. Lye or sodium hydroxide solutions will eat aluminum, but leave steel alone. For decades this was the solution used to 'boil out' an engine block. Heat helps a lot with the speed, they really did boil them.
Muriatic acid, is the same thing as hydrochloric acid and will eat steel crankshafts along with aluminum and your KFC dinner (it's in your stomach at a much diluted level) - I wouldn't use that one, so bad advice? It will eat rust before steel and back when, it was used to 'clean' cooling systems of excess rust, but no more than a half hour exposure was allowed before being drained, flushed, and neutralized.