It is covered with a zinc material to make the steel pipe more resistant to corrosion. The primary use of galvanized pipe is to carry water to homes and commercial buildings. The zinc also prevents the buildup of mineral deposits that can clog the water line. 
This kind tube commonly used as fencing, scaffolding frame, heating, ventilation and air conditioning (HVAC) on both commercial and residential areas; it can also be used in bridgework and business to make handrail and guardrail, because of its resistance to corrosion, and the strength and durability.