The wall pipe under the kitchen sink has a hole. I already disconnected the end that attaches to the trap, but the side that connects to pipe in the wall is badly corroded and the nut won't come loose. I already tried banging the pliers with a hammer and WD40. I can't get it to budge at all. Any suggestions would be greatly appreciated. I am worried about using a hack saw, because if I go to far I could damage the pipe in the wall...