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Corrosion-Resistant Finishes for High-Traffic Projects

Friday, October 12, 2018

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By Tammy Schroeder, LEED Green Associate, Linetec Senior Marketing Specialist

 

When choosing finishes for a building envelope’s architectural aluminum products, the risk of corrosion should be a key consideration. Regardless of the project’s proximity to the ocean’s coastline, transportation facilities, transit-oriented developments and other high-traffic city centers can present significant challenges in protecting exterior-facing architectural aluminum.

Without proper precautions and finishes, corrosion to finished aluminum components ultimately can damage the structural integrity of the building envelope and can lead to systemic failure. Such failures can lead to dangerous and costly results when they affect the connecting points and places that compose our urban landscapes and skylines.

Courtesy of Extech Inc.

When choosing finishes for a building envelope’s architectural aluminum products, the risk of corrosion should be a key consideration. Regardless of the project’s proximity to the ocean’s coastline, transportation facilities, transit-oriented developments and other high-traffic city centers can present significant challenges in protecting exterior-facing architectural aluminum.

Windows, storefronts, entrances, curtain walls, sun shades, canopies, skylights, column covers, rain screens and exterior panels all commonly are manufactured from aluminum and integrally connect to a building’s facade. In almost any city, these architectural aluminum products are continually exposed to not only weathering, but to pollution, chemicals and the constant bumps and scuffs of pedestrians. While salt spray performance considerations usually are reserved for coastal conditions, it’s important to remember that salt mixture often is used to de-ice roads making it an equal concern in colder climates.

As a prominent part of the building’s exterior, the finished aluminum adds color and design to the project; this coating also protects the building from unsympathetic surroundings. The American Architectural Manufacturers Association continues to set the highest standard for architectural finishes, especially in highly corrosive environments. When selecting a coating that will be required to withstand such conditions, select either the highest-performing organic paint coating that meets the AAMA 2605 specification, or a Class I anodize that meets AAMA 611.

 

High-Performance Painted Coatings

High-performance, 70 percent polyvinylidene fluoride (PVDF) resin-based coatings on architectural aluminum products allow architects, specifiers and building owners the capability to select nearly any conceivable color or combination of colors while shielding the building against weathering, pollution and aging.

PVDF is known for its exceptional chemical stability and excellent resistance to ultraviolet radiation. It is used in architectural applications as a coating on aluminum where it provides exceptional resistance to environmental exposure.

The carbon-fluorine bond, used in the 70 percent PVDF, including Kynar 500 resin-based architectural coatings, is one of the strongest bonds known. These paint coatings can withstand enduring and intense UV radiation. Such attributes support long-term color and gloss retention, and chalk resistance.

These highest-performing 70 percent PVDF coatings meet the most stringent exterior architectural specification, AAMA 2605, “Voluntary Specification for High-Performance Organic Coatings on Architectural Extrusions and Panels.” 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.

 

Orlando Airport’s Painted Skylights Stand Up to Sun, Salt

The new Intermodal Terminal Facility at the Orlando International Airport’s South Airport Complex offers a seamless travel experience for the 44.3 million annual passengers. Designed by HKS Architects Inc., the ITF features an 8,000-square-foot, heavy-duty commercial skylight. This daylighting system helps connect those arriving and working within the space to the welcoming surroundings and Florida sunshine outside.

“Although the aesthetics are certainly important, the skylight specifications are more performance-driven than aesthetic,” acknowledged HKS Inc.’s associate principal, project manager, David Thomas, AIA. “Firstly, we didn’t want it to leak and the skylight system needed to meet Florida product approval. The finish also needed to be able to withstand the harsh Florida climate and local airport conditions.”

Spanning 40.5 feet wide by 197.5 feet long, Acurlite Structural Skylights Inc. manufactured and installed the low-rise, segmented barrel vault skylight. Hundreds of aluminum-framed segments compose the total skylight system. Each aluminum extrusion was manufactured with recycled content and finished in a Bright Silver color using Fluropon Classic II 70 percent PVDF Architectural Extrusion Coating Systems.

Courtesy of Bess Adler, Thornton Tomasetti

Designed by HKS Architects Inc., the ITF features an 8,000-square-foot, heavy-duty commercial skylight. This daylighting system helps connect those arriving and working within the space to the welcoming surroundings and Florida sunshine outside.

Prior to finishing, the extrusions were stretch formed to the skylight’s required radius. Maintaining close and consistent tolerances, the stretch-forming process yields a smooth and even curved surface. To obtain the best finish quality and to keep parts fully warranted, it is necessary to thermally improve and finish the aluminum framing members after they have been stretch formed. Installing the thermal barrier in the metal after it has been curved helps minimize stress on the thermal barrier and ensures performance as specified.

 

Corrosion Resistance Requirements

Section 8.8 of AAMA 2605 refers to the corrosion resistance requirements of all coatings that must pass these stringent guidelines:

  • Humidity—The sample is exposed in a controlled heat and humidity cabinet for more than 4,000 hours at 100 degrees Fahrenheit and 100 percent relative humidity. No formation of blisters to the extent greater than “few” blisters, as defined by ASTM D714.
  • Cyclic corrosion testing (previously referred to as salt spray resistance)—Score the film deep enough to expose the base metal. Expose the sample for 2,000 hours according to ASTM G85, Annex A5, dilute electrolyte cyclic fog/dry test. The sample must score a minimum rating of 7 on scribe or cut edges, and a minimum blister rating of 8 within the test specimen field, as defined in ASTM 1654.
  • South Florida exposure—The coating shall maintain its film integrity, color retention, chalk resistance, gloss retention and erosion-resistance properties for a minimum of 10 years on the South Florida on-fence testing site.
  • Color retention—Maximum of 5ΔE Units (Hunter) of color change after the minimum 10-year exposure test. A ΔE unit is the variance or color difference measured on a vector scale from a specific point in the color space.
  • Chalk resistance—Chalking shall be no more than that represented by a No. 8 rating for colors and No. 6 for whites after 10 years of test-fence exposure. Per ASTM D4214, chalking is measured on a numerical scale with higher numbers representing better chalk resistance.
  • Gloss retention—Gloss retention shall be a minimum of 50 percent after the 10-year exposure testing, as described by ASTM D 523.
  • Resistance to erosion—Less than 10 percent film loss after the 10-year exposure testing. AAMA notes that in high-humidity environments such as, but not limited to, seacoast or industrial environments, performance of corrosion resistance may be diminished.

 

High-Performance Anodize Finishes

Class I anodize finishes that meet or exceed all requirements of AAMA 611, “Voluntary Specification for Anodized Architectural Aluminum,” also resist the ravages of time, temperature, corrosion, humidity and warping. Anodized aluminum withstands extreme temperature changes and weather conditions, constant exposure to vehicle exhaust, and daily use by passengers. Over-sprayed salt de-icing can be managed with a simple rinsing as needed. With basic maintenance, architectural aluminum products enjoy a long life cycle.

The anodizing process, because it is an integral part of the substrate, produces an oxide film that is uniform, hard and protects the rest of the aluminum substrate from deterioration providing excellent wear and abrasion resistance. The coating produced is extremely durable, and the hardness of the surface is comparable to a sapphire—the second hardest substance on earth. This characteristic makes anodizing an ideal choice for use in high-traffic areas where resistance properties are important.

Images: © Eduard Hueber, Arch Photo

The Forge, a 272-unit luxury residential tower in New York, sits solidly within its urban environment displaying a facade of copper-colored anodized aluminum cladding.

In the most challenging applications, anodized aluminum will perform as specified and will not reduce the service life of the aluminum, but may affect the natural beauty of the surface finish. Avoid any conditions that quickly can corrode an anodized finish such as mortar, cement and other alkaline materials.

  • To meet AAMA 611 requirements, Class I anodize must have a dry-film minimum thickness of 0.7 mils to pass these stringent guidelines:
  • Abrasion resistance (Michael Clarke Test)—A go/no-go abrasion test using abrasive papers coated with silicon carbide, garnet and glass, respectively. This is used to discriminate between films of the correct hardness and those that may give poor service because they are too soft. The specification grade of abrasive paper is Abrasive Type: Glass, Grade: “Flour,” Mohs’ Hardness Scale Number: 4.5-5.5.
  • Corrosion resistance testing (previous referred to as salt spray resistance)—Expose the sample for 3,000 hours according to ASTM B117 using 5 percent salt solution. Test samples shall show no more than a total of 15 isolated spots or pits none larger than 1 mm in diameter, in a total of 381 cm of test area grouped from five or more test pieces.
  • South Florida exposure—The coating shall maintain its color retention, gloss retention and erosion resistance properties for a minimum of 10 years on the south Florida on-fence testing site.
  • Color retention—Maximum of 5ΔE Units (Hunter) of color change after the minimum 10-year exposure test.
  • Gloss retention—Gloss uniformity shall be within established gloss range.

 

Forged in New York, Anodized in Copper

The Forge, a 272-unit luxury residential tower in New York, sits solidly within its urban environment displaying a facade of copper-colored anodized aluminum cladding. Developed as a “village in a building” by Brause Realty Inc. and Gotham Organization, the wedge-shaped site and stringent zoning requirements led The Forge to take a slender form with stepped massing in a neo-modernist style. While it is a single 38-story, 266,000-square-foot structure, the property appears as if it could be three separate wafer-like buildings set side by side with a fourth, far shorter, one nestled at the foot of the other three.

It is on The Forge’s south side that the copper-colored aluminum wall system clads the entire height of the building, accentuating its vertical presence. To create the industrial metallic look specified by FXCollaborative, Linetec finished more than 92,000 square feet of Dri-Design’s aluminum wall panel system in a proprietary copper color anodize and painted an additional 4,800 square feet in a Black Iron color.

The building’s copper-colored anodize resembles real copper’s lasting luster, but without such shortcomings as salt run-off stains, galvanic corrosion, patina or intensive maintenance. It does not require a clear coat or ongoing treatment to maintain color stability. The finish is available for extrusion, stretch-formed and flat sheet aluminum, before or after assembly. Anodized aluminum also weighs less than half of real copper, for easier transportation and installation.

It is on The Forge’s south side that the copper-colored aluminum wall system clads the entire height of the building, accentuating its vertical presence.

The Forge’s other elevations also showcase the copper-anodized cladding punctuated by julienne strips of windows to maximize daylight and views toward the East River and Manhattan, all while preserving outdoor zones. The project is pursuing LEED Silver certification.

 

Durability and Sustainability

Anodized aluminum is an inert, non-combustible material that is 100 percent recyclable and poses no health risks. At the beginning of the new millennium, Linetec and other environmentally responsible finishers, changed from the traditional caustic etching process to a more eco-friendly etch system allowing customers to use secondary (recycled) billet in the anodizing process.

Small surface defects, such as those common in recycled material, are hidden by the eco-friendly anodize finish’s “frosty” matte appearance. Architects prefer the aesthetic of this matte finish. The resulting surface also reduces glare in bright sunlight. Gloss level reading is typically reduced from 15-25 with conventional anodize, down to a gloss level of 3-12 for eco-friendly color anodize.

The eco-friendly anodize process reduces landfill waste used with conventional etch processes by 75-80 percent. Landfill waste directly relates to the production of greenhouse gas. It also has the viscosity of water and will not collect in the small recesses of aluminum extrusions or narrow aluminum tubes, which enhances the durability and lifecycle of the finished architectural aluminum product.

For painted architectural aluminum products, the leading paint manufacturers and applicators recommend the use of a PVDF-based paint system on aluminum material for all corrosive environments. Offering the longest lifecycle, a 70 percent PVDF resin-based coating system, pretreated with chrome phosphate, along with an inhibitive chrome-rich primer, should be used. This coating type meets or exceeds all the requirements of AAMA 2605.

Beyond ensuring the highest quality application, industry-leading environmentally responsible finishers also can manage liquid paints’ volatile organic compounds in their facilities before the coated material returns to the product manufacturer or arrives at a job site. Using a 100 percent air capture system, they can safely destroy the VOCs with a regenerative thermal oxidizer. The more innovative finishing applicators then re-use the heat energy byproduct to improve process energy efficiency. This process of re-use is completed before the material exits the paint line.

Choosing the right finish for aluminum components on transportation facilities, transit-oriented developments and other high-traffic city centers not only supports the environmental and structural integrity of the building envelope, it creates a lasting impression of quality and safety for all to see.

 

About the Author

Tammy Schroeder, LEED Green Associate, is a senior marketing specialist at Linetec, the nation’s largest independent architectural metals finishing company. With 18 years of experience in the finishing industry, she serves as an industry educator on high-quality, high-performance architectural coatings and services. These include liquid paint coatings, powder coat, anodize, thermal improvement and stretch forming for aluminum. She enjoys sharing her knowledge with architects, specifiers and architectural product manufacturers work in commercial and residential building markets. Schroeder can be reached via email at tammy.schroeder@linetec.com.

   

Tagged categories: Aluminum; Anodization; Corrosion protection; Linetec

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