get your panties out of that bunch! NO MANUFACTURER will cover corrosion unless it is through the metal. Sometimes a dealer will cover it for you on their own and worry about getting paid from the manufacturer later, just to satisfy the customer. yes, there may be a spot under the hood. is it actually growing in size? are you 100% sure it is corrosion under the paint and not some excess sealer that is painted over?
if you are so worried about it, take it to an independent shop and have them repair the surface imperfection and spot paint it for far less than you were quoted. trust me, trying to get a manufacturer to cover something like this is not worth the headache, Ford is not the only one. if you think foreign cars do not have similar problems, you have not been exposed to them enough.