Duplex systems combine the superior protection of galvanized steel with the additional benefits of another corrosion protection system, such as powder coating or paint, to extend the corrosion protection of the piece even further.
The synergistic effect of using two coatings extends the service life for unparalleled corrosion protection, while the metallurgically bonded galvanized coating serves as an ideal primer to provide an impervious barrier for the base steel. When paint and galvanized steel are used together, the corrosion protection is superior to either protection system used alone and can last from 1.5 to 2.5 times the combined lifetimes of both systems. In other words, if you had a paint system that lasts 10 years and a hot-dip galvanized system expected to last 70 years, the duplex system would last 120 – 200 years – if you just let the paint and galvanizing wear away naturally.
The probability of specifying a duplex system and letting it weather away without maintenance is slim, so in a more practical sense, the added benefit of the system is that it extends the paint/powder coating maintenance cycle 1.5 to 2 times what it would be on bare steel. This extended maintenance cycle results in significant cost savings over the life of the project. To compare the initial and life-cycle cost of a duplex system to paint on bare steel, visit www.galvanizingcost.com.
In addition to providing unparalleled corrosion protection, a duplex system allows a great deal of design freedom. For example, projects requiring a particular color scheme can use a duplex system to take advantage of the corrosion-resistant protection of galvanizing while incorporating a range of colors.
Painting galvanized steel requires careful preparation, however, and a good understanding of both painting/powder coating and galvanizing. Past experience provides excellent historical data for how best to achieve good adhesion. The specifications for preparing hot-dip galvanized steel for painting and powder coating are ASTM D6386 and D7803, respectively. The Surface Preparation page contains all the information you need to successfully paint hot-dip galvanized steel.
For more detailed information about duplex systems, see our guide Duplex Systems: Painting over HDG. This brochure provides an overview of procedures for successful adhesion of paint to HDG, explaining why hot-dip galvanized steel is painted/powder coated/, the benefits of duplex systems, and a few case studies. For information on pre-treating, surface preparation procedures and recommendations for painting new or weathered surfaces—in the shop or field—view the surface preparation page or download the paint guide.