Although aesthetics may be the primary factor, there are a number of other reasons why utilizing a duplex system is advantageous. As more specifiers understand the importance of corrosion protection, galvanized steel used in conjunction with paint/powder coatings has risen.
In addition to extended life (synergistic effect), here are some other benefits of specifying a duplex system:
- Color Coding/Safety
- Extended Corrosion Resistance
- Ease of Repainting
Hot-dip galvanized steel provides an attractive, metallic-gray finish many specifiers have come to appreciate. However, when the industrial finish of galvanizing does not suit a particular project, painting or powder coating can offer an aesthetic alternative. Whether the architect prefers a vibrant color, or the owner prefers the project blend with its surrounding environment, painting or powder coating over galvanized steel provides superior corrosion protection along with the color preference. Duplex systems are becoming a common trend in university and professional stadiums as the top coat provides team branding, and the underlying galvanized steel provides the corrosion protection and service life required.
A duplex system also may be used to conform to safety regulations. Duplexing increases safety in many environments by enabling color-coding of gas, steam, or chemical pipes, identifying hazardous work areas and walkways, and marking high-voltage electrical lines and equipment. For example, the Federal Aviation Administration (FAA) requires structures over 200 feet tall to be painted in the alternating pattern of white and international orange. Duplexing these types of projects ensures the proper color coding is in place without sacrificing corrosion protection on often difficult to maintain structures.
Extended Corrosion Resistance
Because of the synergy between paint/powder coatings and galvanizing provide, an obvious and important reason for specifying duplex systems is the added corrosion protection it provides. In harsh environments where galvanizing or paint/powder coatings alone have difficulty protecting steel, utilizing a duplex system can provide the necessary corrosion resistance.
Ease of Repainting
As outlined in the synergistic effect section, an important advantage of painting over galvanizing is the extension of the maintenance cycle. In addition to an extended maintenance-cycle, painting on a galvanized surface also facilitates the maintenance repainting. As the paint film weathers, the zinc in the galvanized coating is present to provide both cathodic and barrier protection until the structure is repainted. The exposed zinc surface then can be repainted with minimal surface preparation.
Ease of Powder Coating/Touch-Up
While the substrate galvanized steel will provide both barrier and cathodic protection for decades, similar to paint, the exterior powder coating may, in time, need to have small nicks or scratches touched up. This can be achieved in the field by using automotive touch-up paint, which is both relatively inexpensive and will provide a durable seal. It can, however, be somewhat difficult to match colors exactly using this method.
To apply touch-up paint, the affected area must be prepared by removing a small amount of powder coating surrounding the breach, roughening the metal and adjacent powder coating with a polymer abrasive pad, then wiping away any sanding by-products with an isopropyl alcohol wipe. Then the touch-up material may be applied with a brush application.
Duplex Systems Case Study
101 Freeway Overcrossing: Los Angeles, CA, 2015
The overcrossing is more than just functional, it is also a visual addition to the freeway. The 123-foot long arc-canopy structure is impressive at 22 feet tall and 17 feet wide, utilizing multiple sizes of hot-dip galvanized radius pipes. The tapered arc-canopy also has an artistic finish - a detailed stenciling pattern painted over the galvanized coating. Finally, the overcrossing has a series of etched aluminum panels with colored LCD lights fastened to the main structure by an interconnecting network of netted stainless steel cables, which form a mesh-like awning. The awning offers pedestrians a large degree of shade during the day and a colorful lit path to guide them on a moonlit night.
The artistic, duplex (paint over hot-dip galvanizing) finish was a solution to enhance the overall appearance of the galvanized pieces. The galvanized arc-canopy design required varying sizes of pipes to accommodate the tapering aspect, and all the pipes had slightly different steel chemistries. As steel chemistry has the most profound effect on the final zinc coating appearance, this led to some differences in the look of the coating. After the pieces were hot-dip galvanized and constructed, they were set for a final inspection. Unfortunately, the city inspectors had envisioned a consistent, uniform galvanized finish, similar to galvanized sheet metal, but, due to the variances in steel chemistry, the final product showed visual differences between the connecting radius pipes.