I honestly don't know how the magnum wheel is made. Most of it is basic steel and painted to prevent rust while other smaller areas are a steel gray color that doesn't rust. No chrome on the set that I have although others are chromed and don't require trim rings.
I sand blasted a set of rallyes once years ago and didn't clean all the sand out of the little crack or nip where the center meets the wheel. That is where the new rust started.