Black pipe gets its name from the black oxide that appears on the surface of the steel after forging, which is then coated with protective oil to minimize rust. Galvanized pipe is steel pipe coated with zinc to reduce corrosion.
Black pipe may be used to route gas and air to homes and buildings, while galvanized pipes may be used for some types of non-residential water fixtures and outdoor construction uses such as fence posts or rails.
Because steel rusts and corrodes when exposed to water for long periods of time, neither black pipe nor galvanized pipe are recommended for residential waterlines. The zinc coating on galvanized pipes prevents it from being used for gas lines, as the gas can cause the zinc to flake off and cause obstructions.
Since both pipes are formed from steel, they can be joined together to complete plumbing jobs so long as the coating is removed at the join site.