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Coastal areas pose the greatest challenges in protecting exterior-facing architectural aluminum products from corrosion. Without proper precautions and finishes, corrosion to these components ultimately can damage the structural integrity of the building envelope and can lead to systemic failure.
According to the Aluminum Association, unless exposed to some substance or condition that destroys the protective oxide coating, the aluminum remains resistant to corrosion. Aluminum is highly resistant to weathering, including many industrial and harsh atmospheres, which often corrode other metals.
In addition to obvious coastal zones near seawater, buildings located inland in cold climates also may need to consider protecting their architectural aluminum products if salt is used as a de-icing agent. These de-icing chemicals will attack aluminum and its coating if allowed to dwell on the surface.
Durable Finishes
Painted coatings and anodized finishes are among the most durable finishes available for exterior-facing architectural aluminum products.
Architectural anodize provides excellent wear and abrasion resistance with minimal maintenance in most environments. It resists the ravages of time, temperature, corrosion, humidity and warping. Anodize is created from the substrate itself, integral to the part on which it forms. It should meet the Class I specifications of the American Architectural Manufacturers Association (AAMA) 611-12 including a minimum oxide coating thickness of 18 microns or 0.7 mil.
Painted aluminum coatings also protect the building from unsympathetic surroundings. High-performance 70 percent polyvinylidene fluoride (PVDF) coatings can be selected in nearly any conceivable color or combination of colors, while shielding the building against weathering, pollution and aging.
The first and one of the most important defenses against a paint failure is proper pretreatment of the aluminum. Without proper pretreatment, premature failure of the finish system almost is guaranteed. Paint systems are designed to be applied over clean metal that has been properly pretreated. Pretreatment of the aluminum building components to be used in a severe corrosive or coastal environment is crucial.
The most time-tested, proven pretreatment system for architectural aluminum products is a chrome phosphate conversion coating. This process conforms to ASTM D 1730-3, Type B, Method 5 as required by AAMA 2605-11, “Voluntary Specification, Performance Requirements and Test Procedures for Superior Performing Organic Coatings on Aluminum Extrusions and Panels,” published by AAMA.
Offering the longest life cycle and true sustainability, chrome phosphate conversion coatings continue to be recognized by the world-class coating manufacturers-PPG, Pittsburgh; Valspar, Minneapolis; and AkzoNobel, Nashville, Tenn.-as the most effective, robust pretreatment for aluminum. As a result, products installed along the seacoast and in other harsh industrial environments may not be warranted, or the warranty length and coverage may be compromised, if a chrome pretreatment system is not utilized.
After the aluminum has been pretreated, a primer coat is applied prior to the paint coating application. The paint coating is then sprayed to meet the AAMA 2605 specification of a 30-microns or 1.2 mil total film thickness. When the formula of the paint coating dictates, a clear coat is applied over the paint topcoat. This protective layer shields the flake in metallic coatings and increases protection against ultra violet (UV) degradation of highly chromatic and exotic colors.
These highest-performing 70 percent PVDF coatings meet the most stringent, exterior, architectural specification AAMA 2605-11. This specification requires paint coatings to meet rigorous testing performance standards including more than 4,000 hours of salt spray, and heat- and humidity-resistance.
Special Consideration
The shape and machining of the architectural aluminum products also may facilitate or deter corrosion. As examples:
- Machined holes and cut ends of factory-finished aluminum components are protected by thin, naturally forming aluminum oxide. This oxide, while tenacious in its bond to the underlying aluminum substrate, may be susceptible to attack from strong cleaners or heavy salt deposits.
- Hems and seams on aluminum components may be formed in a way that will collect sand. With movement, over time, this sand can erode away the painted coating or anodized finish.
- Components may be shaped with areas that are left holding pooling or ponding water. This often becomes a major issue for corrosion.
- Specific to curtainwall and window systems’ aluminum framing, ensure the weeps are large enough to avoid becoming plugged by salt deposits.
Cleaning and Maintenance
Studies have shown that increasing levels of atmospheric pollution can have a negative effect on finish longevity in the absence of periodic maintenance. Run-off from adjacent materials on-site must be considered in a corrosion prevention plan.
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 paint finishes can be managed by rough attempts to remove such build-up.
AAMA 609 and 610-02 “Cleaning and Maintenance Guide for Architecturally Finished Aluminum,” and CW -10-04 “Care and Handling of Architectural Aluminum from Shop to Site” are provided as general guides for these precautions and cleaning activities.
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Tammy Schroeder, LEED Green Associate, is the senior national marketing specialist for Linetec, Wausau, Wis. With a decade of experience in paint and anodize finishing, Schroeder develops and maintains the company’s American Institute of Architects/Continuing Education System (AIA/CES) and other educational presentations. To learn more, visit www.linetec.com.