Soak the metal in vinegar. It’s smelly but works. I’d mix 1 part vinegar to 1 part water (more concentrated works faster, but then you have to buy a lot more vinegar!). The acid (acetic acid) will react with the rust and float to the surface of the liquid. If you don’t have a container large enough for soaking, vinegar-dipped rags will work, although not as well.
You can also rub a cut lemon on the metal (citric acid), but this will leave a slightly sticky film that you’ll need to wipe off later. Good luck!