This has me curious...because the stock rims are aluminum.

Chroming is usually done with copper and nickel...no iron.

http://www.advancedplating.com/process/process.htm

Iron is what rusts...aluminum typically corrodes with that white pitting...typically called "white rust".

You might want to check the wheels to make sure they are really Ronal wheels. I'm wondering if they are some sort of cheap knock-off?