I found this:
Concrete
The removal of rust stains from your garage floor also will require a trip to the Hardware Store. One method is to use a tri-sodium phosphate (TSP) in a gallon of hot water. Wet down the concrete and apply the solution using a scrub brush. Scrub vigorously and then let the solution soak for 15 to 20 minutes. Rinse
Really bad rust stains may require the use of a pressure washer and muriatic acid. Apply the muriatic acid according to manufacturer�s directions, let soak for a few seconds, and then pressure wash at 2,500-3,000 psi. To neutralize the acid, sprinkle with baking soda and wash again. Warnings: These products can irritate your skin and burn your eyes so use caution. Follow the manufacturer's directions and precautions carefully. Muriatic acid is dangerous and should only be used as a last resort. It is important to not wash the Muriatic acid into storm drains.
http://www.theofficialscoop23.info/rust-stains-carpet-ho usehold-items.html