Can't see any pictures but if it's external corrosion then that is specific to the paint finish, not the frame. The dealer you bought the bike from is your first point of contact.
Seeing as it's a mountain bike you may have difficulty convincing them it's a paint fault, the finish could have easily been damaged riding off-road in muddy conditions or by stone chips and the like.
Worth pointing out that if you are not the original owner of the bike neither the shop nor the manufacturer has any obligation to honour any warranty and that the warranty period is specifically from the original date of purchase. After that it's down to good will from the manufacturer.