I just got a keg in the mail and was going through an initial cleaning of it. After taking off the posts, I noticed a lot of corrosion or something around the threads the posts connect to (I'm not sure what the official name for these are). I proceeded to clean it with PBW but it did not affect the build up. Is there a way to remove this? Is it worth the effort? Am I going to just ruin the seal by trying to clean it up?