cleanout pipe threads corroded!

[tumbled]

My sink line was clogged so I unscrewed the cleanout cap and snaked out the pipe. Unfortunately the pipe end is so corroded/rusted that I can't put the cleanout cap back on without leaks. The pipe is 2 inches and I'm using a new plastic cap (with teflon tape). The problem isn't subtle--there's so much corrosion/rust that you can't really screw the cap on. What can I do? Would it help to put a rust remover jelly on the threads? Is there a product/coupler piece that I could solder onto the end of the cleanout pipe that would have new threads? Thanks very much for any help!