Brakes are grinding on rear of new car - In poor condition and pitted (Car is 12 months old and done 6000 miles). Could the work on the rear pads be done under the warranty.
If there is some defect in material or workmanship then the issue MAY be covered under warranty. You need to have some reason why the discs are defective but IMHO that's going to be tough. At 12 months and 6000 miles you are into the territory where environment and normal wear are likely. While 6000 miles is a bit low for rotors to be failing, it's not so low that it's clear they were bad from the factory.
Your best bet is to have a heart-to-heart talk with the service adviser at your dealer and see if they can find any evidence that the rotors were defective. Otherwise you'll need to pay for their replacement.