Black pipe is plain steel pipe without any protective coatings. Black pipe is used for a variety of applications around the home. It is very common to see black pipe used for your natural gas line and sprinkler system lines. Since black pipe has no protective coating, it may rust easily in wet or humid environments. To stop the pipe from rusting or corroding on the outside, you should provide a layer of protection on the outside of the pipe. The easiest method is painting it.