Had rust stains on our drive from the old tt chains and jack pads. Tried everything to remove them and someone suggested "The Works" toilet bowl cleaner. Got a bottle from Dollar General and tried it. Hey, it really works and it's cheap. It removed the stains immediately. You have to use caution as it is very caustic and you have to wash it off immediately.
I'm a monument maker and we use Manganese7. It will remove rust, won't harm concrete and we leave it on polished granite with no ill effects. We get it from Granite City Tool in St. Cloud, MN. Granitecitytool.com