Black pipe is used for gas because it's durable and doesn't corrode when exposed to gasses like natural gas and propane.

Black pipe is used for gas because it is less likely to corrode and cause a gas leak. Metal pipes are usually made of copper, galvanized steel, or PVC. Copper and galvanized steel are both susceptible to corrosion, while PVC is not. That's why black pipe is often used for gas lines - it's less likely to cause a gas leak.