If the bar has rusted it was likely a lower grade of metal with a chrome finish. Steel wool is about all that might improve it's appearance followed with a dab of neatsfoot oil. I use neatsfoot in case it gets on the leather, no harm done.
I always though rust was caused by water/oxygen.... We left saddles/harnesses in the cold for a long time and never had anything rust...unless it got wet and wasn't dried off right away.
You can try the steel wool as already mentioned, or you can buy new stirrups for either an english saddle or a western saddle. They can be fairly cheap, and you could possibly find a used pair at an auction or used tack store.
Well, stirrup bars were mentioned and that is what I responded to. The metal on tack/harness should be nickel coated brass and is often marked with a small anchor. On cheaper equipment the metal is a cheap alloy that may or may not have a chrome finish. Chrome has a high shine and looks nice at first but it will begin to pit and rust and get worse over time.