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Steel Finishing FAQ'S
Read the topics below to find answers to your questions about hot dip galvanizing, shot blasting, powder coating, and duplex coatings. We’ve provided some definitions and FAQs.
Hot dip galvanizing
Using a hot dip galvanizing process on steelwork creates a strong, durable, and corrosion-resistant finish, usually lasting many years without maintenance. Several stages are involved in the process to achieve the final finish. These all involve immersion, and they are summarized as follows (rinse steps excluded):
Degreasing - Degreasing can use acidic or alkaline commercial products, with or without heat. The objective is to create a surface clear of oil and grease contamination.
Pickling – Using dilute hydrochloric acid dissolves rust and scale, resulting in a ‘chemically clean’ surface for the molten zinc to react with.
Fluxing – The preferred standard fluxing agent is a mixture of zinc and ammonium chloride in solution. It typically works best from 50°C to 70°C, aiding the drying of steel after removal. Drying matters since it prevents zinc splash; a separate drying step can be used.
Zinc Immersion – In this last step, a special bath filled with molten zinc at 450°C is used. The clean steel combines with the zinc while submerged, forming zinc/iron alloy layers. The coating’s foundation is made of these layers, and it’s covered with free zinc when pulled from the galvanizing bath. The coating is highly abrasion resistant and, according to recent exposure tests*, protects the steel for 34 -170 years. As atmospheric S02 levels continue to drop, this should prolong the lifespan of galvanized coatings.
Spin galvanizing
Tiny components like nuts, bolts, chains, and screws must resist rust like the steel they support, but are too small for hot-dip galvanizing. Spin galvanizing allows you to immerse small items in molten zinc using a perforated basket. Following the zinc/alloy layers’ creation, the basket spins to remove excess zinc, resulting in a clean profile.
What causes fluctuations in the colour of galvanised coatings?
Most of the galvanized coating is made of dull grey alloy layers. A layer of pure zinc normally cools on the surface after the work is removed, creating a shiny silver look. Yet, its reactivity is largely influenced by variations in the steel’s surface chemistry. For steels that are more reactive, the surface zinc layer will keep reacting after removal, transforming the zinc into an alloy, which causes a duller appearance. This could happen in specific spots or generally. Variations in surface colour don’t harm the coating’s corrosion defence.
What’s the lifespan of the galvanised coating?
This is determined by the environment’s corrosion rate and the galvanized coating’s thickness. The UK’s reduced atmospheric corrosion has more than doubled galvanized coating protection in many instances. A coating life exceeding 30 years wouldn’t be unusual, given average corrosion rates of 1-2 microns annually.
Which standard is my work’s hot tip galvanized to?
Our galvanizing process meets the requirements of BS EN ISO 1461, the standard for hot dip galvanized coatings.
BS EN ISO 14713-2 offers further guidance on how to prevent iron and steel structures from corrosion.
What is the coating’s thickness?
This is dictated by the material’s thickness and the type of material.
Here’s a summary of the average coating thickness for the most common materials:
Steel > 6mm thick – 85µm
Steel ≥ 3mm to ≤ 6mm – 70µm
Steel ≥ 1.5mm to ≤ 3mm – 55µm
EN ISO 1461 gives the full details for both conventionally dipped and centrifuged work respectively.
Can a thicker galvanized coating be applied beyond the standard?
To get a thicker coating, the steel typically needs SA2.5 blasting using G24 chilled iron grit. For regular products or larger contracts, it might be possible to specify controlled silicon steels for more reactivity. Contact our office if you require additional information.
Will it last long if the galvanized coating gets scratched?
Coatings that are hot dip galvanized are highly abrasion-resistant. The zinc outer layer is soft and absorbs impact, while the alloy layers are harder than the steel they are bonded to. A slight scratch won’t affect how long the coating lasts.
Can galvanizing be fixed if I need to do on-site repairs?
A suitable zinc rich paint is the most common method for repairing galvanized coatings. Apply as the manufacturer directs, with a thickness similar to or exceeding the surrounding coating by 30µm depending on if the item will be over coated. You could also use zinc thermal spray on the area to get the needed thickness.
