Malcom Wright, director at B.E Wedge, Wedge Group Galvanizing’s dedicated spin galvanising division and recognised European Centre of Excellence for centrifuge galvanising, looks at the process of galvanising and explains why it’s one of the most reliable processes for treating fastenings and threaded components to ensure their structural reliability. He also identifies some of the issues that may occur if the wrong finish is used, which could potentially pose a risk to the overall structural reliability of a project.
Galvanising is the finish of choice for many industries, not only for the long-term protection it provides against various types of corrosion including weathering and rust, but for the ease of application and fast turnaround time it offers.
Galvanising can be used for a wide range of projects from large-scale structures right through to sculptures and artwork, however one of the most vital – yet often overlooked aspects – is ensuring the structural reliability of the project. Nuts, bolts and other fastenings are vital for reinforcing and securing steel projects, so it’s essential they are treated appropriately.
For the vast majority of steel related projects, hot-dip galvanising is simply the most effective method to provide a long-lasting protective coating against rust and corrosion.Because it forms a series of alloy layers with the steel, it is much more robust than other coatings that only bond chemically or mechanically, and it also has the added advantage of fully coating the steel – both inside and out. Galvanising is long-lasting and can protect an item for up to 60 maintenance free years (or longer) depending on where it is located. It is environmentally friendly with low whole life costs. It’s these qualities plus its flexibility across a whole range of projects that makes it a popular choice throughout the construction process.
Although larger steel components significantly benefit from galvanising, perhaps one of the most vital aspects to consider during construction is protecting the fasteners such as bolts, nuts, and screws that hold the core structural components together. If fasteners fail, the integrity of the entire structure is at risk, and corrosion can essentially cause this.
Threaded components form a key element in construction projects such as bridges, stadiums, and tunnels which wholly rely on the resilience and durability of the fixtures to reinforce them and maintain their stability.
Centrifuge galvanising differs from the conventional hot-dip galvanising, which is used to treat larger steel components. Leading facilities follow the BS ISO 10684 industry standard, which is directed at hot-dip galvanising fasteners and specifies alternative material, process and some performance requirements for centrifuged hot-dip galvanised. It’s not recommended for hot-dip galvanising of threaded fasteners smaller than 8mm in diameter, and states 10.9 fasteners may be hot-dip galvanised providing a certificate of compliance is issued by the galvaniser stating that the galvanising has been carried out in terms of the recognised standard.
Galvanising threaded parts also requires a slightly alternative process compared to larger components, primarily due to the size of the fasteners and their end requirements. Prior to galvanising, the surface of the steel must be clean from light grease, scale and dirt. However, the technique of acid pickling used to clean larger steel components can cause embrittlement in smaller parts, which can damage the mechanical properties and compromise its performance. Hydrogen embrittlement may occur when atomic hydrogen is absorbed by the steel during the acid pickling process. This embrittlement can potentially lead to the loss or partial loss of ductility in the steel and consequently result in the premature failure of the fastener in the field.
Small components that are 10.9 or less should be shotblasted, which ensures the steel product isn’t damaged prior to treatment. At B.E.Wedge the Galblast 450 process is used, which is designed to meet the requirements of the fastener industry for reliable, long-life, high strength nuts and bolts, typically grade 8.8 to 10.9.
Spin galvanising is used for products that cannot be conventionally treated, primarily small parts, as it ensures any excess zinc is removed ensuring a smooth surface and preventing items sticking together. The process sees the threaded components and other small parts immersed in molten zinc in a perforated basket which, after the coating has formed, is spun at high-speed – it’s this spinning action that removes any surplus zinc and ensures a clean finish. After centrifuging, the contents of the basket are immersed in a quench tank where the steel is allowed to cool.
Similarly to standard galvanising, smaller components can be hot-dipped at conventional temperatures that provide a robust outer layer that can cushion the component against direct blows and help resist abrasion.
In recent years, high temperature galvanising (HTV) has been used across Asia as an alternative to conventional galvanising and sees steel treated at temperatures between 560ºC and 633ºC. Although some believe the process is more suitable for smaller components due to its thinner outer layer, in reality this decreases the products strength and doesn’t provide the protection of conventional galvanising, as it doesn’t provide an additional protective outer zinc layer. So, although the process provides a thinner coating, there are a number of risks that could essentially compromise the strength of the fastening.
It is commonly assumed once fasteners have been galvanised they will no longer fit the previous size. This can be overcome through the process of over-tapping, a procedure that sees the fastener manufacturer make the nut threads slightly larger to accommodate the slight dimensional increase in the bold thread that can result from galvanising. Following galvanising treatment if the thread is still not large enough, it can be re-tapped. Re-tapping will not necessitate re-galvanising because of the cathodic properties and the zinc coating will continue to protect re-tapped fasteners, an ideal process for bolts kept in stock.
Wedge Group Galvanising has 14 plants strategically located across the UK offering a national galvanising service ranging from centrifuge galvanising of small components, such as bolts and washers, right the way up to 29 metre steel beams.
Having spent a decade in the fastener industry experiencing every facet – from steel mills, fastener manufacturers, wholesalers, distributors, as well as machinery builders and plating + coating companies, Claire has developed an in-depth knowledge of all things fasteners.
Alongside visiting numerous companies, exhibitions and conferences around the world, Claire has also interviewed high profile figures – focusing on key topics impacting the sector and making sure readers stay up to date with the latest developments within the industry.
