Galvanised steel coatings are celebrated for their longevity, durability, and cost-efficiency in preventing corrosion. The choice between pre-galvanised and hot-dip galvanised steel depends on your specific project requirements and intended application. RHS Steel Supplies offers a diverse range of steel products to meet your unique needs.
What is Galvanising?
Before exploring the differences between pre-galvanising and hot-dip galvanising, it's essential to understand the galvanising process itself. Galvanising involves applying a zinc coating to metals like steel or iron to protect against rust and corrosion. This zinc layer acts as a sacrificial barrier, enhancing the metal's durability and lifespan. While galvanised coatings cannot prevent corrosion indefinitely, they significantly extend the life of steel compared to untreated metal.
The Pre-Galvanising Process
Pre-galvanising, also known as ‘in-line,’ ‘continuous,’ or ‘mill’ galvanising, involves galvanising steel products using an automated process before they are cut to size and processed. Brands such as Duragal, Allgal, and Supagal employ this method. In pre-galvanising, steel components are immersed in a molten zinc bath, and excess zinc is removed through mechanical wiping, air knives, or steam, resulting in a relatively thin zinc coating.
The Hot-Dip Galvanising (HDG) Process
Hot-dip galvanising (HDG) involves immersing the fully fabricated steel product into a molten zinc bath. The steel is lowered at an angle, allowing air to escape and ensuring the zinc flows over and through the entire piece. During this process, the iron in the steel reacts with the zinc, forming zinc-iron layers topped with pure zinc. This reaction creates a robust bond between the steel and zinc, resulting in a thicker and more durable coating than pre-galvanising.
Key Differences Between Pre-Galvanised and Hot-Dip Galvanised Steel
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Coating Thickness: Pre-galvanised steel typically has a coating thickness of about 20μm for sheets and 20-30μm for tubes and wires. HDG coatings are significantly thicker, with minimum average requirements ranging from 45-85μm.
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Coating Continuity: HDG provides a continuous coating over the entire steel structure, while pre-galvanised steel may have uncoated areas at the cut edges.
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Coating Bond: Both HDG and pre-galvanised coatings form strong metallurgical bonds with the base steel.
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Coating Formability: The thinner coating on pre-galvanised steel can typically withstand forming without damage, whereas HDG should not be formed post-galvanising due to the risk of damaging the thicker coating.
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Coating Appearance: Pre-galvanised coatings are generally uniform and bright, while HDG coatings can vary in appearance.
Understanding these differences is crucial for selecting the appropriate type of galvanised steel for your project. At RHS Steel Supplies, we are dedicated to providing high-quality steel products tailored to your specific requirements.
