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.

As a galvanized steel pipe supplier, Mejor Steel supplies hot dip galvanized pipe from China, galvanized to the base pipe specification the order names, with the coating thickness, the end finish and the threaded-and-coupled option set out on this page.

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.

Galvanized pipe: the coating class sets the service life

Galvanizing is a corrosion barrier whose life is proportional to how much zinc is on the surface, so the useful question is not whether the pipe is galvanized but to what coating weight or thickness and to which standard. Hot dip galvanized and pre-galvanized are different products and are not interchangeable.

Coating weight is the number to specify, not just thickness. For schedule-wall pipe (steel 6 mm and over), ISO 1461 requires a minimum average of 85 µm (≈610 g/m²) and a local minimum of 70 µm (≈500 g/m²); ASTM A123 sets the equivalent by steel thickness. State the minimum coating weight per unit area and the standard it is applied to, and confirm the figure on the test certificate.

Grades supplied and minimum properties

The grade fixes the minimum strength and the chemistry. The table below gives the requirements that separate the ASTM A53 grades ordered most often; the full composition and mechanical table for each grade is on its own page.

The complete chemistry and mechanical table for each grade, with the notes that qualify it, is on the individual grade page – for example ASTM A53 Grade B pipe. Galvanizing is applied to the finished pipe and does not change these requirements; the class (light, medium or heavy) is what sets the wall.

Source: specified minimum yield and tensile strength for each grade, per ASTM A53 / ASME SA53, Table 2. The standard is the governing document.

• Grade A
• Grade B
• S235JR
• St 37

Download Hot Dip Galvanized Steel Pipe data sheet (PDF) One page: supply range, grades and key sizes, what to state on the enquiry, and the inspection documents that come with the order. The standard is the governing document.