Seaside and coastal environments are beautiful locations for window views but also present some of the most challenging conditions for metal coatings. High-performance finishes offer optimal protection for exterior-facing aluminum products while enhancing their appearance.
Finishes are key to delivering the desired performance and aesthetic for large, exposed surface areas of metal wall panels and the many sizes and shapes of aluminum framing members that compose a window, curtain wall, storefront, or entrance system. Without proper precautions and finishes, corrosion of these aluminum components can damage the building envelope’s structural integrity, leading to systemic failure.
Corrosive conditions
Corrosion rates vary from place to place and during different times at the same location. The duration of wetness, temperature, and frequency of temperature cycles are relatively higher in many coastal applications, including Florida. This is why a finishes testing site was established at the Kennedy Space Center, Cape Canaveral, in the 1960s. Throughout the years, thousands of coated test panels have been evaluated in this seacoast environment with high levels of salt, humidity, and ultraviolet rays.
The primary variables affecting corrosion rates near the coast are the salt content in the air, the time of wetness of the metal surface, the temperature, and the level of other atmospheric pollutants. Several environmental factors control these variables, including distance to the ocean, elevation, wind direction, wave action, rainfall, humidity, the degree of shelter, and the level of industrial air pollution.
A corrosive environment can consist of many different elements, including the most common—salt. Salts are ionic compounds, and sodium chloride is the primary salt in seawater. The water could also come from rain, condensation of humidity, or fog.
These types of environments rarely contain only one type of corrosive. The more severe the atmosphere, the more critical a proper protective coating becomes.
High-performance finishes
When selecting a finish to help architectural aluminum cladding, framing, and fenestration withstand harsh coastal and corrosive environments, these two specifications from the Fenestration and Glazing Industry Alliance (FGIA) offer the highest performance:
- AAMA 611, Voluntary Specification for Anodized Architectural Aluminum
- AAMA 2605, Voluntary Specification, Performance Requirements and Test Procedures for Superior Performing Organic Coatings on Aluminum Extrusions and Panels
Both recently updated specifications require 2,000 hours of cyclic corrosion testing. For real-world validation, these specifications also require high-performance anodized and painted finishes with a minimum of 10 years of south Florida exposure on-fence testing.
Anodized aluminum finishes
These were first used on an industrial scale in 1923 to protect the parts of seaplanes from corrosion. The anodizing process was quickly adopted for aluminum in architectural applications. Anodized finishes are integral to the aluminum substrate and highlight the natural metallic tones. The anodic coating is uniform and hard and protects the aluminum substrate from deterioration, providing excellent wear and abrasion resistance in most environments.
Class I anodic coatings are high-performance finishes used for exterior building products that withstand continuous outdoor exposure. To meet AAMA 611, Class I finishes must have a minimum thickness of 18 microns (0.7 mil). Class I anodized finishes offer resistance to corrosion in coastal environments and durability in high-traffic areas.
Painted metal coatings
Coatings that meet AAMA 2506 include 70 percent fluoropolymer resin-based coatings. These colorful painted finishes have been used for 60 years on aluminum products for architectural projects.
Painted finishes meeting AAMA 2605 are tested for superior performance, including film integrity, color retention, chalk resistance, gloss retention, and erosion resistance. In addition, these high-performance coatings must undergo more than 4,000 hours of heat and humidity testing, similar to coastal climate conditions.
AAMA 2605 includes one of the most important defenses against paint failure—proper pretreatment of the aluminum. Without proper pretreatment, premature failure of the finish is almost guaranteed. Paint systems are designed to be applied over clean, appropriately pretreated metal. Pretreatment of aluminum building components to be used in a severely corrosive or coastal environment is crucial.
After the aluminum has been properly pretreated, a primer coat is applied before the paint coating application. The paint coating is then sprayed to a minimum total film thickness of 23 microns (0.9 mil). Two-coat systems consist of a primer and a color coat. Three-coat and four-coat systems are used for metallic and exotic colors, which include a clear topcoat and sometimes a barrier coat.
The clear topcoat is essential to three- and four-coat systems because it protects against ultraviolet, humidity, and chemical attack. It is also required for bright and exotic colors and all colors containing metallic flake.
Preventative care and maintenance
Without periodic maintenance, increased salt and other atmospheric pollutants can impact the performance and longevity of metal coatings and finishes.
Runoff from adjacent site materials must be considered in a corrosion prevention plan. For example, mortar, cement, and even gypsum dust can accumulate as alkaline deposits on aluminum surfaces and must be promptly rinsed. This is especially true of mill finish or anodized surfaces. While somewhat more resistant to alkaline attack than anodized surfaces, high-performance painted finishes can be managed with freshwater rinses to remove such buildup.
AAMA 609/610, Cleaning and Maintenance Guide for Architecturally Finished Aluminum, and AAMA CW-10, Care and Handling of Architectural Aluminum from Shop to Site, offer general guidelines, precautions, and cleaning recommendations.
With preventive measures, finished architectural aluminum retains its intended look and long life while providing the desired performance in the harshest environments, including highly corrosive seacoasts. These qualities reduce the need to replace materials and components, conserve resources, optimize labor, and save money.
With more than 25 years of experience in the finishing and fenestration industries, Tammy Schroeder is the director of marketing for Apogee Enterprises’ Architectural Framing Systems segment and its Alumicor, Linetec, EFCO, and Tubelite brands. She can be reached at tammy.schroeder@apog.com.