The use of multiple coating processes in tandem can significantly heighten the durability of steel-based infrastructure items such as bridges, making them far more resistant to corrosion-related degradation and extending their projected lifespans.
Two of the most popular coating processes for steel bridges when it comes to enhancing long-term corrosion-resistance are hot-dip galvanising and zinc spray metallisation. Engineers are increasingly choosing to apply the two methods in conjunction in order to compensate for their respective limitations or shortcomings, and enhance their ability to protect ferrous infrastructure.
Hot-dip galvanisation involves the complete coating of an iron or steel component by means of full immersion in a bath, also referred to as a kettle, of super-heated molten zinc.
The bath of molten metal consists of 98 per cent zinc heated to a temperature of approximately 443 degrees Celsius, which reacts metallurgically with the steel to form the hot-dip galvanised coating.
While the surface of the coating consists of pure zinc, the underlying layers are comprised of inter-metallic zinc-iron alloys that bond tightly enough with the steel to essentially form an intrinsic part of it.
This underlying zinc-iron layer greatly enhances the toughness of the original steel or iron material because of its heightened abrasion resistance compared to the uncoated base.
Hot dip galvanisation has the added advantage of creating a full, even coating across the complete exterior of steel components, even around edges and corners or along the interior surfaces of hollows.
This is because the process involves the complete immersion of steel components in the bath, so the molten zinc comes into contact with all parts of them equally, while the chemical reaction with the zinc produces inter metallic layers at perpendicular angles to the coated surfaces.
Hot dip galvanisation also produces thicker, denser zinc coatings compared to other processes, which translates into greater durability and resistance over a long time period.
The ASTM A123/A123M Standard Specification for Zinc [Hot-dip Galvanized] Coatings in Iron and Steel Products, for example, mandates a minimum coating thickness of 0.1 millimetres for structural steel with a thickness of 15 millimetres, which provides a time to first maintenance in an industrial environment of 72 years.
A major shortcoming of hot-dip galvanisation, however, particularly when applied to large-scale structures such as bridges, is the need for individual parts and components to be completely immersed in molten zinc in order for the process to be fully effective.
This means the process can be unwieldy when steel components are too large for either traditional or progressive dipping, or when the infrastructure item is already located at a site which is difficult to access.
In stark contrast to hot dip galvanisation, zinc spray metallisation involves the use of a heated gun powered by combustible gases or compressed air to spray melted zinc onto steel components. The guns are usually operated by a trained human technician, although automated processes for spray metallisation do exist.
While the coating produced by means of spray metallisation consists of 100 per cent zinc, the process does not result in a bond that is strong as that produced by hot-dip galvanisation. The bond strength of spray metallisation is approximately 10,300 kPa, as compared to 24,281 kPa for hot-dip galvanising.
Another issue associated with spray metallisation is the creation of a coarse coating surface that can be slightly porous in character. While the spraying process can be applied until a set thickness requirement is satisfied, coatings that are excessively thick can also suffer from problems with adhesion.
Spray metallisation is also significantly more expensive than hot dip galvanising, with an estimated cost of $125.88 per square metre for the former as compared to $24.98 for the latter. Metallisation costs are further increased when performed with smaller pieces or on site, while full life cycle costs are also greater compared to hot-dip galvanising as the later does not require maintenance.
Given the respective shortcomings and advantages of these two processes, it makes perfect sense for engineers to apply them in tandem to certain infrastructure projects when their components parts are of variable dimensions, and thus lend themselves better to one or other of the two methods.
This is particularly the case given the high level of compatibility of the finished coatings produced by the two process, which possess the same electrical potential, thus preventing the emergence of corrosion cells, as well as a similar appearance after weathering that lends greater visual unity to structures for which they’re jointly applied.
The general rule should be to galvanise steel parts whenever possible given the cost and protection advantages of this process, and use metallisation for the remainder of cases where galvanisation proves inconvenient.
An outstanding example of the use of these two processes in tandem is Indiana’s Castleton bridge, which was originally subject to hot-dip galvanising on its northbound side and painted on its southbound side back in 1970.
While the hot-dip galvanised coating on the northbound side held up to standard requirements after four decades of operation, maintenance of the painted southbound side ended up costing more than the full bridge construction over the same period.
The bridge owners eventually opted to use spray metallisation to cover the southbound side of the bridge in 2002, resulting in a structure with an extended life span courtesy to a full-body zinc coating that had been applied by means of two disparate methods.