Robin Clarke | Executive Director | Hot Dip Galvanizers Association of Southern Africa (HDGASA) | mail me |
Duplex coating, which combines hot dip galvanising with painting, is an extremely effective method of corrosion control. Paint on hot dip galvanising is an excellent system. However, it does require that everyone in the value chain understands the requirements for best practice.
Several award-winning projects demonstrate its efficacy. These include the Kirstenbosch Gardens walkway, the steel work on the Cape Town soccer stadium and the Standard Bank façade in Maputo. All required high levels of corrosion control, as well as architectural synergy with their surroundings. These examples underscore the importance of corrosion control pillars, which bring together engineering practice and design excellence.
Grappling with technicalities
We frequently assist designers, architects, fabricators, galvanisers and paint professionals regarding the best approach to duplex coating. Our website also includes useful documentation and methodologies to guide them.
For example, a structure may require a paint overcoating to prolong service life in harsh marine and agricultural locations. It may also be necessary in exceptionally high pollution areas or corrosive micro-environments within processing plants.
Both aesthetic and legislative requirements can also play a role. These might relate to a specific aesthetic to complement the surroundings. They might also involve a legislative requirement, such as colour coding of pipelines or meeting Civil Aviation Authority (CAA) regulations for high-rise structures near airports or flight paths.
Tried and tested corrosion control
I describe hot dip galvanising as a well-established, tried and tested, preferred corrosion control method. Extensive research between the 1940s and 1960s on hot houses in the Netherlands produced very repeatable outcomes for service life.
The same applied to certain paint systems. These were applied onto the same galvanising base as a third leg of the experiment. The outcome showed that the lifetime of the coating exceeded the sum of the two individual parts. In aggressive environments, it lasted at least 1.5 times longer, while in less corrosive situations it extended up to 2.8 times longer.
Laboratory tests also determined that when paint became porous, the zinc from galvanising plugged and sealed the micro-porosities. This process gave new life to the coating.
Another benefit is avoiding under-creep on a paint system. If paint is penetrated, the steel can rust beneath it. That does not occur when paint is applied onto a galvanised or zinc-based coating. The zinc sacrifices itself to restore the coating. If it cannot restore it, the zinc still sacrifices itself to protect the carbon steel substrate. This layered defence illustrates one of the most practical corrosion control pillars: using complementary systems that reinforce each other for extended protection.
Collaboration and co-ordination to mitigate corrosion
Once duplex technology has been specified, important decisions around steel selection and preparation must follow.
We frequently advise on the best corrosion control approach in extreme environments. This often involves specifying silicon-controlled steel in terms of the SANS 1471 (parts 1 and 2) guidelines.
The best manufacturing practice starts with selecting the most appropriate steel, which is silicon-controlled steel. It also requires ensuring the consistency of steel supply throughout the project. If a substantial project requires various steel suppliers, it becomes very difficult to deliver the same paint aesthetic.
In terms of SANS 14713 for best manufacturing practices, the design must ensure an even flow of cleaning materials across the steel articles. We also emphasise best practices in welding, control of de-fluxing and anti-spatter. Safety is equally critical in the case of enclosed containers, where venting and filling holes must be placed appropriately.
Communicating to achieve the perfect outcome
We recommend commencing with a gentle sweep blast of air to remove any products of oxidation, remnants or contaminants on the surface.
We also strongly advocate that the paint manufacturer make a recommendation in accordance with the ISO 12944 standards. These standards relate to a variety of painting systems that will be effective depending on the coating durability required for the project.
I emphasise that the paint coatings for a single project should come from the same manufacturer to avoid incompatibility. The engineer, the architect or designer, and the fabricator must inform the galvaniser that the material is to be over-coated or painted before the galvanising process starts. At that point, the galvaniser and the painter also need to liaise regarding the requirements for post-galvanising surface preparation.
The painter will have reviewed the our codes of practice and prepared the surface accordingly. They will then follow the manufacturer’s recommendations to ensure the paint is correctly mixed, applied and cured. Therefore, adherence to collaboration, co-operation and communication ensures that the client ultimately receives a hot dip galvanised and painted article.
Instead of lasting under ten years, the service life of such an article will be doubled. These three practices stand as corrosion control pillars for any project that aims to deliver both durability and safety.
