The coil and extrusion coatings industry is at a crossroads. Certification and testing bodies such as the American Architectural Manufacturers Association (AAMA) and the American Society for Testing and Materials (ASTM) have introduced more stringent corrosion testing to better reflect real-world difficult industrial and seacoast environments.
Coil coating pretreatments can protect metal and world health
This is corrosion on a powder-coated aluminum extrusion.
At the same time, there is growing demand to eliminate proven corrosion inhibitors such as chromium from architectural metal coatings. Although these priorities may seem to be in conflict, innovations in metal coatings formulation are well on the way toward achieving both.
Corrosion-Resistant Pretreatment Technologies
For the past 50 years, the building industry has relied on zinc-coated steel and/or chromium pretreatments in combination with chromium primers to protect steel and aluminum building components, especially from corrosion, in industrial and seacoast environments. Chrome also enables coatings to adhere more strongly to the metal surface. The three pretreatment options detailed below are those most commonly used for aluminum substrates.
Hexavalent Chrome. Hexavalent Chromium is the most robust solution for corrosion protection, yet it also is the least safe for the environment and human health. Because it is toxic and carcinogenic, Hexavalent Chromium has been placed on the Red List of products to avoid by administrators of the Living Building Challenge, one of the world’s most rigorous green building certification systems.
Trivalent Chrome. Trivalent Chrome, Cr3 or chrome-three, features a lower concentration of chrome than Hexavalent Chrome and has proven to be an effective corrosion prevention solution.
However, it still does not facilitate the goal of fully eliminating heavy metals from the waste stream. Although it is not currently considered carcinogenic, trivalent chrome is a heavy metal and likely to be placed on the Red List in the future. It does not have the same level of corrosion protection or self-healing properties as Hexavalent Chrome but can be a very effective corrosion deterrent if applied within the correct parameters.
Chrome, Not Intentionally Added (NIA). Pretreatments specifically formulated without intentionally added Hexavalent or Trivalent Chrome have been developed and commercialized in recent years. They offer excellent corrosion resistance but are not as robust as legacy chrome alternatives.
In the applicator’s process, chrome (NIA)-containing pretreatment products have much tighter operating tolerances. When these materials are run, they demand much shorter test intervals to ensure the chemicals are being applied at optimum performance levels. Operating outside of these rigorous tolerances can significantly diminish the corrosion protection performance of the finished product.
Corrosion-Resistant Systems Options
The need to protect metal from corrosion is not restricted to the substrate and pretreatment layer. This section summarizes the advantages and disadvantages of three corrosion protection layers and/or systems and components for building products, including two alternative pretreatment technologies that are gaining traction overseas:
Pretreatments. One-layer chrome pretreatment or primer systems are acceptable as an alternative but will not perform as well as coatings systems with two layers of chrome.
In Europe, manufacturers are pursuing two alternative pretreatment technologies to address stricter environmental regulations. The first is improved etching with acid cleaners to remove more oxide from aluminum surfaces. The second alternative is flash anodizing, which imparts a fine layer of protection to the surface of the metal to prevent the formation of filiform corrosion.
Primers. Primers are as important to protecting metal substrates as pretreatment systems. They help coatings adhere to metal surfaces and provide a barrier against moisture penetration. As with pretreatment systems, one-coat products do not provide corrosion protection equal to two-coat systems.
Clearcoats. These products aid directly in the prevention of corrosion by functioning as an extra protective barrier against salt penetration. They also tend to rinse and clean more easily than color coats, which can help remove external contaminants from their surfaces.
Summary
Legacy coating systems have demonstrated their ability to last more than 50 years on landmark building in harsh industrial and seacoast environments. As they move into the future, it will be important for pretreatment chemical and coatings companies to innovate and improve corrosion resistance with alternative materials. In the years ahead, it is likely that new technologies will emerge, and that they will set new standards of performance for the commercial building market.
Scott Moffatt is market manager, building products at PPG Industries Inc., Pittsburgh. To learn more, visit www.ppg.com.
Building and metal structures along the Gulf Coast are most susceptible to corrosion failure due to the preponderance of heat, humidity, salt and wind. Salt concentration drops significantly one mile inland; however, salt air from the sea can carry well beyond those distances.
