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Science + Nature= Durable: Getting the best anodized finish

Anodizing successfully combines science with nature to create one of the world's most durable metal fi nishes. Anodize is an electrochemical process that thickens and toughens the naturally occurring protective oxide. The resulting finish is the second hardest substance known to man, second only to the diamond. This characteristic makes anodize an excellent choice for use in high-traffic areas, such as doors and railings, where hardness and abrasion-resistance properties are vital.

The purpose of anodizing is to form a layer of aluminum oxide that will protect the aluminum beneath it. The aluminum oxide layer has a much higher corrosion and abrasion resistance than aluminum. A typical anodize finish used in the architectural industry is called "two-step electrolytic." The actual anodizing and coloring of the aluminum occur in separate steps of the process. The anodizing step takes place in a tank that contains a solution of sulfuric acid and water.

Specifying Anodized Aluminum

To ensure a long-lasting anodize finish on building products, the Schaumburg, Ill.-based American Architectural Manufacturers Association's AAMA 611-98 specification should be referenced at the time of the order. This specification addresses finish mil thickness, color ranges and performance of architectural anodize (see chart).

If requested, the finisher should submit, for approval, representative samples of the allowable color range. A set of color range samples consists of a light sample and a dark sample, which provide customers with a visual reference to represent the extremes of appearance to be expected on the finished parts. The range samples should not differ by more than 5 Delta E.

Keep in mind that range samples aren't always from the same alloy, temper and shape that will be used on a project; therefore, they are meant as an illustration of the degree of possible color variation. These may not be an exact representation of the color achieved for a customer's specific project.

Minimizing Color Variation

"All Aluminum can be anodized." This is a valid statement but only to the extent it is understood that only the aluminum gets anodized, while the other metal constituents (zinc, magnesium, silicon) present in the alloy do not respond in the same manner. The variance of metal constituents in an alloy is the major reason for color variation-especially when using secondary, recycled aluminum billet.


Other variables affecting color variation in the anodize process include temper, anodize tank chemistry, shape geometry and material load size. To assure the most consistent anodize color:

  • Maintain metal consistency. The easiest way to ensure metal consistency is to work with one metal source/extruder per project and request that all metal come from one "lot" of material.
  • Do not mix aluminum alloys. Mixed alloys, or even mixed tempers, will not produce uniform results. For best results use 6063 alloys for extrusions and 5005 for flat sheet stock and fabricated parts. (6061 and 5052 can be used if structural alloy is required but will not give similarly acceptable results.)
  • Perform as much bending and forming as possible prior to finishing. Anodic films are very hard, and as a result, most post-production bending causes the film to "craze," which produces a series of small cracks in the finish, giving it a spider-weblike appearance.
  • Be aware of anodizing's effect on welds. The heat developed from the welding process will change the metallurgy on nearby metal or heat-affected zone, or HAZ, and cause localized discoloration, often referred to as a "halo" effect, after anodizing. Use the proper 5356 alloy welding wire and the lowest heat possible.
  • Select an anodizer who utilizes automation to reduce inconsistencies in the anodize process.

Selecting Green Anodize

Unlike other finishes, anodizing highlights aluminum's metallic appearance. Because it is an integral part of the substrate, the anodic coating results in a hard, durable substance providing excellent wear and abrasion resistance with minimal maintenance. Anodized aluminum is 100 percent recyclable and can be stripped back to raw aluminum through a traditional etch process and then can be painted or re-anodized.

Etching is an important step during the anodize process. Etching is designed to dissolve a thin layer on the surface of the aluminum to develop a smooth uniform finish. Linetec, a finishing provider in Wausau, Wis., recently made a change to its etch chemistry from conventional caustic etch to a more eco-friendly acid etch process.

This new etch process creates an aesthetically appealing, "frostier" matte finish that improves the ability to hide small defects, such as extrusion die lines and mild scratches, that may occur on the aluminum surface of architectural products. Although neither conventional caustic nor acid etch removes all irregularities in the aluminum, acid etch does a better job of concealing them. This gives the material a more architecturally desirable, aesthetic finish on both primary and secondary (recycled) billet.

This eco-friendly, acid etch process also can provide a copper-colored anodize that gives the look of rich, real copper, as well as resists stains from salt runoff, galvanic corrosion and the formation of patina. It involves using actual copper to color the aluminum while isolating the copper in the coating. This process makes the copper color very stable and consistent. The rich copper color that is observed is actually copper that will not patina and will remain stable to provide many years of service.

Other attributes of eco-friendly anodize include:

  • Thanks to the finish's ability to hide small surface defects, secondary billet can be successfully anodized, contributing to recycled material content recognized by many green building rating systems.
  • The low gloss level of eco-friendly anodize reduces glare in bright sunlight.
  • Aluminum removal is reduced during the etching process, allowing for a more consistent extrusion tolerance between paint and anodize.
  • The process byproducts from the acid etch bath are recyclable. The acid etch anodize process greatly reduces landfill waste used with traditional etch processes.
  • Eco-friendly acid etch has the viscosity of water and will not collect in the small recesses of aluminum extrusions or narrow aluminum tubes, enhancing the durability and lifecycle of the finished product.

Tammy Schroeder is the national marketing specialist for Linetec in Wausau, Wis. With a decade of experience in paint and anodize finishing, Schroeder develops and maintains Linetec's AIA/CES and other educational presentations. Learn more at