Hi, I'm trying to replace a broken piece of what I belive is cast iron pipe with new pvc on a bathrrom sink. Not positive its cast iron, but the piece that broke was about a 5 inch long connector piece threaded on both ends. One end to the main pipe and one end going into the plastic under the sink. The threads from the connector piece came apart and it was very corroded. So now I need to install new pvc but I'm not sure how to attach it to the metal pipe. Any thoughts?