Soak the rusty chrome pieces like that front derailleur in white vinegar for a few hours, maybe 4 to 6 but don't leave them in the vinegar too long as it can cause the plating to peel more and will attack any aluminum pieces. A short soak won't effect the aluminum though. Most of the rust will wipe right off after about a 4 hour soak. To get what doesn't come off the first time you can then try soaking them a little longer or hit the remaining rust with #0000 steel wool soaked with WD-40, I prefer the steel wool and WD. Any areas where the plating is gone or heavily pitted can be coated with clear finger nail polish to protect them from moisture.