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May 3 - May 7, 2021

Are there any advantages in applying a protective coating over a hot-dipped galvanized surface?

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From Alana Fossa of American Galvanizers Association on May 11, 2021:
Known as a “duplex system” paint or powder coatings can be specified on projects where corrosion protection is critical, but aesthetics or color selection is also a high priority. However, aesthetic freedom is not the biggest benefit to specifying a duplex system. When hot-dip galvanized steel is painted or powder coated, the duplex system provides a more sophisticated manner of corrosion protection known as the synergistic effect. Used independently, both paint/powder coatings and hot-dip galvanizing provide corrosion protection to steel; however, when utilized together, the two coatings work in synergy. The exterior layer of paint or powder coating slows down the rate at which the zinc is consumed, extending the life of the galvanized steel. In return, once the exterior layer is weathered or damaged, the galvanizing beneath continues to provide cathodic and barrier protection. As a result, the steel is afforded corrosion protection for 1.5 to 2.3 times the sum of the expected life of each system alone, based on the corrosivity of the environment. 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, a duplex system using these same coatings would provide maintenance-free protection for 90 to 138 years in the same environment. This formula assumes no maintenance of the paint/powder coating of the duplex system. In reality, it is often not practical to allow the coating to deteriorate and weather away naturally. It is more likely the owner will maintain the coating system to avoid the unpleasant appearance of a peeling and deteriorating paint system. Further, the synergistic effect allows paint or powder coating over hot-dip galvanizing to last an additional 1.5 to 2.0 times longer than over bare steel. This extended time to first maintenance results in a delayed maintenance cycle for the paint/powder coating system, leading to significant economic advantages over the life of a project. To compare the initial and life-cycle costs of a duplex system to various other corrosion protection systems, visit the AGA’s Life-Cycle Cost Calculator at Additionally, where maintenance is performed for the paint/powder coating of the duplex system, the hot-dip galvanized coating is never exposed to the environment and the overall duplex coating system will last indefinitely. For more resources from the AGA including case studies, cost data, and technical guidance:

From Tom Schwerdt of TxDOT on May 7, 2021:
I suggest using the search term "duplex coatings"

From Marco Fabio Ramenzoni of ZINGA METALL BRASIL on May 6, 2021:
Yes! HDG is the finding of Humanity to protect steel. Usually, in most atmospheric environments, the active protection provided by ZINC towards ferrous metals is highly effective and efficient. However, in some environments, the pure ZINC exposed on the surface may react faster than expected. Applying a protective coating over HDG may extend its lifetime significantly (sometimes to over 2 times longer), as experimented several times by Professor Fernando Fragata, ex head of the Corrosion Lab of the Electricity R&D Center (CEPEL) in Brazil. The setbacks that comes with coating over HDG is the attention needed with proper surface preparation and the request for the HDG supplier not to finish the job with any passivation process.

From Janet Mazeau of CT DOT on May 5, 2021:

From Nourhan Demirdjian of Waterproofing Technologies on May 4, 2021:
In coastal areas & aggressive chemical environments yes.

From rON Lewis of Corrosion Management Ltd on May 3, 2021:
Certainly there is.....Consider a HDG structural piece being submerged in the sea, near a waste water outlet. Wouldn't we believe the 'topcoat' might extend the life of the HDG piece as being "extended" with the addition of a high quality epoxy coating topcoat.....(?) "Sure we do - especially if we consider that we are corrosion control saavy".

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Tagged categories: Asia Pacific; Coating Application; EMEA (Europe, Middle East and Africa); Hot-dip galvanizing; Latin America; North America

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