Bob Duxbury, technical director at Wedge Group Galvanizing, one of the UK’s largest hot-dip galvanizing organisations, explains just how the galvanizing process works, the technology behind it, and the financial and sustainable benefits of incorporating the finish into architectural design.
Structural steel has been the material of choice within the architectural industry for decades, but whilst steel is both strong and versatile, it’s also particularly prone to rust and corrosion. Which is why it’s paramount that a finish is selected which will not only protect the steel from the elements, but is long-lasting too. For this reason, galvanizing has long been recognised as one of the most durable and cost-effective finishes on the market. It’s flexibility can be demonstrated through the sheer range of projects it is used on, from affordable housing projects and airport expansions, through to RNLI Lifeboat stations and commercial buildings.
The initial stage of the galvanizing process involves rigorous cleaning of the steel with chemicals to help ensure that the steel is in the perfect condition to react with molten zinc, which includes removing all grease, scale and dirt. Commonly, the steel product is dipped into an alkaline or acidic degreaser, or even shot blasted, and is then rinsed thoroughly in cold water before being dipped into hydrochloric acid at room temperature.
During the next stage of the process, the product is dipped in a flux solution, typically made up of zinc ammonium chloride, and held in a temperature range between 65°C and 80oC. The final stage of this process removes the last traces of oxide from the surface and coats the product with a thin film of flux. Once the steel has dried fully, it’s dipped in molten zinc at around 450°C, which is when the galvanizing reaction takes place.
The thickness of the coating is created when the metallurgical reaction slows down and it’s this cooling down process which results in the bright, shiny appearance generally associated with galvanized products. It protects steel by forming a surface film which is insoluble in rainwater, and prevents moisture and oxygen reaching the steel itself. This makes it more robust than other coatings, which only bond chemically or mechanically, and it has the added advantage of fully coating the steel, inside and out.
Hot-dip galvanizing is often perceived as a more expensive method compared with paint or powder coating because of its high coating performance, but in fact it can prove a much more cost-effective option compared with other methods. Because painting a structure is labour-intensive, costs can work out to be higher when taking into account the need for regular maintenance visits, and repainting, to ensure that it remains at the required standard. The hot-dip galvanizing treatment, however, lasts up to 60 years or longer in the right conditions, and is undertaken in a controlled, highly-regulated factory, which means that labour costs are significantly reduced.
The process has not only proved popular in architecture because of its ease of application, cost effectiveness and long-term durability, but also because of its own inherent sustainable qualities. Treated steel needs no maintenance which means that the whole life costs of products protected by hot-dip galvanizing can be further reduced because there is no need for the expense, down-time, and sheer inconvenience of repeated applications.
The overall cost of protecting steel throughout its lifetime can depend on the cost and durability of the initial coating, including the cost of maintenance. Take two popular forms of steel protection as an example. The first is a method which consists of cleaning, with an undercoat plus two coats of paint, and the second, a method which consists of blast cleaning and using three coats of paint.
Although they may last years with some maintenance, both are expected to need repainting twice over a 25 year period. And even the cost of a ‘cheaper’ brand of paint is almost 70 per cent more expensive than the whole lifetime cost of galvanizing, a much different perception from the outlay.
Galvanizing also has the ability to protect steel in various types of environments – both in exterior or interior spaces – including immersion in cold and hot water, corrosive sea water, and contact with other metals. This means that steel can be used in even the harshest conditions, including internal and external parts of the most complex architectural projects.
An estimated 4 per cent of the world’s GDP is lost every year to corrosion, and with architecture being one of the top industries committed to enhancing and improving its sustainability efforts, it’s no surprise that galvanizing is becoming the finish and protection of choice for the industry.