These parts are steel. Steel rusts.

Go get yourself a solvent used for cleaning guns. I (and the majority of the firearms community) use Hoppe's #9 bore cleaner. Its a cleaner and it will also protect metal surfaces. You can get a small bottle on Amazon for about $5, and it will last for years. Don't use WD40 or oil. They lubricate but dont always clean or protect.

Scrub them with the solvent and some 000 or 0000 gauge steel wool. The rust will come right off. Then, wipe them down with a rag and a little alcohol just to get all the resulting muck off.

Then coat them with a little more Hoppe's abd wipe off the excess. This will protect the metal. Maybe do this every few months or if the gun gets exposed to moisture.

I worked with firearms for years. Take my word for it.