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Synergistic Effect

Synergistic effect of Hot-dip galvanizing and duplex systems

As previously stated, duplex systems are often specified for aesthetic reasons. However, the synergy between the two coatings provides a much more practical benefit - extended protection. Used independently, both paint/powder coatings and galvanizing provide corrosion protection to steel, however, when utilized together in a duplex system a more sophisticated corrosion resistance is achieved, known as the synergistic effect.

The exterior layer of paint or powder coating slows down the rate at which the zinc is consumed, greatly extending the life of the galvanized steel. In return, once the exterior layer has been weathered down or damaged, the zinc beneath is still available to provide cathodic and barrier protection. Furthermore, the hot-dip galvanizing "primer" ensures no underfilm corrosion will form as the paint/powder coating begins to deteriorate. 

As a result, the substrate steel is afforded corrosion protection for 1.5 to 2.3 times the sum of the expected life of each system alone. For example, if a galvanized coating alone on black steel would provide 50 years of maintenance-free protection and a paint coating would not require any maintenance for 10 years, the combination duplex system would provide maintenance-free protection for 90 to 138 years in the same environment. A periodic maintenance schedule can extend this synergistic lifetime even longer.

In mathematical form, the synergistic effect would be defined:

Mduplex = Duplex Multiplier x (Mgalvanizing + Mpaint)
M = time to first maintenance

This equation assumes no maintenance will be performed on the paint or powder coating, and as it naturally wears away the underlying galvanizing would  provide the majority of the corrosion protection. This is only likely to be applied in theory, as in practice, the project owner will maintain the paint system to keep the aesthetic appearance intact. Therefore, in practical terms, the synergistic effect is realized in the delayed maintenance cycle (touch-up, maintenance repaint, and full repaint) of the paint/powder coating of the duplex system. This delay of approximately 1.5 to 2.3 times for each of the three elements of the maintenance cycle means holding capital set aside for maintenance longer and fewer maintenance cycles over the project life. 

For predicting duplex coating performance, you will need to know:

Synergistic effect of Hot-dip galvanizing and duplex systems
  1. Estimated coating life of the paint (in the expected environment) independent of hot dip galvanizing, provided by the paint manufacturer (typically ~15-20 years).
  2. Estimated coating life of the hot dip galvanized steel (in the expected environment) which can be estimated using the Time to First Maintenance chartsprovided on the AGA website, and adjusted as necessary should the presence of corrosive elements be known (exposure to regular flooding, road salting, etc.)
  3. The 1.5 – 2.3 multiplication factor when discussing the synergistic effect is called the Duplex Multiplier, which should be selected based on the corrosivity of the environment.
    • Duplex Mutliplier (1.5 to 2.3x) based on the exposed environment:
      • 1.4-1.6 Extreme Marine/pollution
      • 1.5-1.6 Sea Water (immersion)
      • 1.8-2.0 Industrial Marine
      • 2.0-2.3 Non-aggressive environment

Keep in mind if the HDG coating is never exposed to the environment, (i.e. routine maintenance is performed on the paint or powder coating system), the overall coating system can last indefinitely.

Duplex Systems Case Study

Center For Great Apes: Wauchula, FL, 2010

Center for Great Apes, Wauchula, FL

With a winding maze of chutes and walkways woven throughout the treetops, the Center for Great Apes is a charitable organization created to provide sanctuary for chimpanzees and orangutans that have retired from the entertainment industry, completed research, or formerly served as pets.

After working in Borneo tracking and observing orangutans in 1984, Patti Ragan developed a passion for great apes. She had the vision and desire to provide long-term care and sanctuary for the animals, who can never be returned to the wild, and worked to develop an enjoyable, functional, and durable facility to achieve this goal. Ragan bought a tract of land in south central Florida and created a facility that would provide the animals with a habitat as close to natural as possible.

It was also necessary for the structure to be hardy and durable, as the apes will scratch, chew, and abuse any building material. The humid coastal environment, corrosive ape wastes, and continuous habitat cleanings also would provide extreme challenges for corrosion protection. A duplex system of paint over hot-dip galvanized steel was the ideal solution to meet all of these needs.