Duplex coating, a method that combines hot dip galvanizing with painting, is widely regarded as one of the most effective strategies for corrosion control. According to Robin Clarke, Executive Director of the Hot Dip Galvanizers Association of Southern Africa (HDGASA), the key to achieving success lies in the collaboration, co-ordination, and communication among all stakeholders in the steel value chain.

‘Paint on hot dip galvanizing is an excellent system, but it requires that everyone in the value chain understands and follows best practices,’ Clarke notes. This methodology has already yielded remarkable outcomes in notable projects such as the Kirstenbosch Gardens walkway, the Cape Town stadium, and the Standard Bank facade in Maputo – all examples of corrosion control and seamless architectural design integration.

Understanding the technicalities of corrosion control

The HDGASA plays a guiding role in helping designers, architects, fabricators, galvanizers and paint professionals implement best practices during the duplex coating process. Clarke explains that certain conditions – such as harsh marine environments, industrial zones, or corrosive micro-environments – necessitate paint over hot-dip galvanizing to prolong the service life of steel structures.

‘Aesthetic and legislative considerations may also dictate the choice of duplex systems, such as ensuring specific colour coding for pipelines or meeting compliance standards of the Civil Aviation Authority (CAA) for structures near airports,’ Clarke elaborates.

The HDGASA website offers extensive resources, documentation, and methodologies that simplify the complexities involved, helping professionals achieve optimal results in corrosion control.

The Dependable duo: paint and galvanizing

Clarke emphasises that hot dip galvanizing has been a well-established method for corrosion protection for decades, supported by extensive research. Studies conducted in the Netherlands from the 1940s to the 1960s demonstrated that the lifespan of a duplex coating system exceeded the sum of its two parts by at least 1.5 times in aggressive conditions and up to 2.8 times in less severe environments.

Additionally, laboratory tests proved that zinc from the galvanizing process seals micro-porosities in paint that forms over time. ‘Unlike standalone paint systems that risk under-creep rusting when penetrated, hot dip galvanizing sacrifices itself to protect the steel and restore the coating,’ Clarke explains.

Building on collaboration and co-ordination

Successful corrosion control begins with selecting the right materials, such as silicon-controlled steel per SANS 1471 (parts 1 and 2) guidelines. Clarke stresses the importance of consistency in steel supply and advises project teams on best practices, especially for large-scale initiatives involving multiple suppliers.

To ensure optimal outcomes, HDGASA advocates proper preparation techniques like sweeping the surface of steel to remove residuals, addressing welding issues, and managing venting and anti-spatter for steel articles.

Robust communication among stakeholders is critical for duplex coating projects. According to Clarke, aligning all parties – from engineers and architects to painters and galvanizers – before starting the galvanizing process leads to better results.

Key steps include:

• Removing oxidation and contaminants from steel surfaces using gentle sweep blasting
• Following ISO 12944 standards when selecting compatible painting systems
• Using paint products exclusively from a single manufacturer to prevent compatibility issues

‘When methodologies of collaboration, coordination, and communication are embraced, a galvanized and painted structure can achieve a lifespan well beyond ten years—often double that,’ Clarke concludes.