Galvanized Black Pipe 3" Coupling. Galvanized Black Pipe, due to its black oxide scale surface finish, is ideal for the use with natural and propane gas lines applications. For example it will be used to run a line from the meter to inside the house, and inside the house to the appliances. This form of pipe is what's typically used for those applications. Also This Pipe is generally used in commercial grade fire sprinkler systems (Water Application). This versatile pipe is heat resistance, and water damage resistant. Like the pipe, the high quality fittings are designed to connect, adapt, and function in the same applications as the pipe. This Galvanized Black Pipe 3" Coupling is the quickest and most secure way to connect two lines. The smaller (2"-4") fittings are generally used on gas and water applications.