Applying a hot dip galvanising process to steelwork provides fabrications with a robust, durable and corrosion protective finish that under normal conditions will last for many years without maintenance of any sort. The process itself has several stages that are required to achieve the final finish. These are all by immersion and they are summarised as follows (rinse stages omitted):
Degreasing – This may be carried out using either acid or alkaline-based proprietary products and may be heated or used at ambient temperatures. The target is to produce a surface which is not contaminated with oil or grease-based products.
Pickling – This is carried out in dilute hydrochloric acid, which dissolves rust and scale and produces a ‘chemically clean’ surface which will react with the molten zinc.
Fluxing – A mixture of zinc chloride and ammonium chloride in solution is the standard fluxing agent of choice. This is usually used between 50°C and 70°C, which helps the steelwork to dry after it is withdrawn. Drying is essential as it helps prevent zinc splash, and a separate drying stage is sometimes employed.
Zinc Immersion – This ‘final’ stage utilises a unique bath holding molten zinc at 450°C. The clean steel is immersed in the zinc, and while it is submerged, it alloys with the iron in the steel to form zinc/iron alloy layers. These layers form the basis of the coating, which is then overlain with free zinc as it is withdrawn from the galvanising bath. The result is a coating that not only has excellent abrasion resistance but one that, according to the most recent exposure tests*, lasts between 34 and 170 years before the base steel is exposed. With S02 levels in the atmosphere continuing to decline, this should further increase the life of galvanised coatings in the future.
Small items such as nuts, bolts, chains, and screws need to be as rust resistant as the steel structures they support, however they are too small to hot dip galvanise in a hot dip galvanising bath. With spin galvanising small items can be placed in a perforated basket and immersed into molten zinc. Once the zinc/alloy layers form, the basket is centrifuged at high speed so that the spinning action removes the excess zinc and creates a clean profile.
Powder coating is a type of paint that is mainly applied to metals to provide a harder finish than a normal paint. The paint is applied as a powder electrostatically from a spray gun and is cured in an oven under a high heat to form a tough, durable skin. Powder coating paints come in many colours and can be used to create different finishes such as textured or metallic.
Before metal is hot dip galvanised, it needs to be cleaned to give it a smooth surface. One of the most effective ways to create this surface is to shot blast it. Shot blasting fires a high-pressure spray of abrasive steel materials onto a rough surface to create a smooth surface.
Fabrications should ideally be symmetrical, suitable for single-dipping and incorporate sections of as near equal thickness as possible at the joints, perfectly fitted components to avoid the use of force or restraint during joining, continuously welded joints using balanced welding techniques to reduce uneven thermal stresses and the largest possible radii on all curved members.
To achieve the best quality zinc coating, it is important to ensure that the molten zinc can flow freely over all surfaces, externally and internally. So, where there are sealed hollow sections or cavities, vent and drain holes are essential. A copy of the Galvanizers Association ‘Engineers and Architects Guide to Hot Dip Galvanising’ may be downloaded here. Please also download Venting Tips for Hot Dip Galvanising.
To ensure a good hot dip galvanising finish, a design should ideally incorporate easily handled components or fabrications that can be single dipped or assembled by bolting with galvanised fastenings or welding. Weld areas must then be protected with zinc-rich paint, by zinc spraying or with low melting-point alloy repair rods. Double dipping is a possible alternative. Seek advice before proceeding.
Suspension holes or lifting lugs may be needed if there are no suitable points for locating hooks or wires. Once again, lifting points should be positioned to maximise venting and drainage.
Hot dip galvanising is a total immersion process, and to facilitate this, all hollow section material requires a free flow of zinc through the whole structure and a means for air to escape. With the necessary holes, it is possible to galvanise hollow sections successfully. A mitred corner, gusset plate or recess that could pool liquid or trap air will also require drilling for the same reasons. If pre-treatment chemicals are trapped inside a fabrication, there is a significant risk of explosion when the item is immersed in molten zinc.
For guidance on drilling requirements, please refer to our Design for Galvanising fact sheet and booklet or call one of our offices for advice.
The bulk of the galvanised coating consists of alloy layers that are dull grey in colour. When work is withdrawn from the galvanising bath, a pure zinc layer usually cools on the surface to give a shiny silver appearance. However, variations in the surface chemistry of the steel significantly influence its reactivity. With more reactive steels, the surface zinc layer continues to react after withdrawal and converts the zinc to alloy, thus giving a duller appearance. This can occur in isolated areas or across the whole surface. Nevertheless, the variations in surface colour are not detrimental to the corrosion protection offered by the coating.
This depends on the corrosion rate of the work’s environment and the thickness of the galvanised coating. Since atmospheric corrosion rates in the UK have fallen significantly over the last few decades, the general level of protection offered by a galvanised coating has, in many cases, more than doubled. With typical corrosion rates averaging 1-2 microns per year, a coating life of more than 30 years would not be uncommon.
The standard for hot dip galvanised coatings is BS EN ISO 1461. Which all our galvanising conforms to.
BS EN ISO 14713-2:2020 gives additional guidance for protecting against corrosion of iron and steel in structures.
This depends on the material thickness and the type of material used. The following is a summary of mean coating thickness for the most common materials:
Steel ≥ 6mm thick – 85µm
Steel ≥ 3mm to < 6mm – 70µm
Steel ≥ 1.5mm to < 3mm – 55µm
Tables 2 and 3 within EN ISO 1461:2009 give the full details for both conventionally dipped and centrifuged work respectively.
To achieve a thicker coating, the steel would usually require blasting to SA2.5 with a G24 chilled iron grit. For regular products or larger contracts, it may be possible to specify controlled silicon steels that are more reactive and give a controlled level of reactivity above standard. Please contact us at one of our offices for further information.
Hot dip galvanised coatings have excellent abrasion resistance. In fact, the outer zinc layer is soft enough to absorb impact and abrasion while the alloy layers that are metallurgically bonded to the steel, are harder than the steel substrate. Minor abrasion will have little impact on the life of the coating.
The most common method for repairing galvanised coatings is to use a suitable zinc rich paint. This needs to be applied in accordance with the manufacturer’s instructions and to a thickness equal to or 30µm more than the surrounding coating depending on whether the item is to be over coated. An alternative method is to have the area zinc thermal sprayed to the required thickness.