Hot Dip Galvanized Steel Pipe

How Hot Dip Galvanizing Protects Steel

In hot dip galvanizing the cleaned steel pipe is immersed in molten zinc at approximately 450 degrees C, so the zinc reacts with the iron to form a series of zinc-iron intermetallic layers that are metallurgically bonded to the steel. This bond is far stronger than a paint film, so the coating does not flake off under impact. The protection works in two ways. First, as a barrier that seals the steel from the environment. Second, and more importantly, by sacrificial action: zinc is more electrochemically active than iron, so at a scratch or cut edge the zinc corrodes preferentially and protects the exposed steel. This is why galvanized pipe continues to perform well at cut ends and damaged areas, unlike painted pipe. A typical hot dip coating of 500 g/m2 provides a service life measured in decades in rural or mild industrial atmospheres.

Light, Medium and Heavy Class Pipe

BS 1387 and EN 10255 classify galvanized pipe into light, medium and heavy classes for each nominal size, where the outside diameter stays constant and only the wall thickness changes. Medium class corresponds roughly to SCH 40 and is by far the most common, used for general water and gas service. Heavy class has a thicker wall for higher pressure and mechanical strength, and is often used for structural columns and load-bearing frames. Light class is thin walled and is restricted to low pressure or purely decorative applications. When ordering it is essential to state the class, because a light class pipe and a heavy class pipe of the same nominal size are visually almost identical but differ significantly in weight, pressure rating and price.