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Editor's note: This article originally appeared, in slightly different form, in Products Finishing magazine (November 1999). It is used here with permission.
The Right Finish for Architectural Aluminum
What is Anodizing?
Aluminum oxide is a hard, durable, weather resistant coating that protects the base metal. The coating may be clear or colored using various methods. The coating itself grows from the base aluminum metal by way of the electrochemical process, so the coating is integral to the metal and cannot peel or flake. The coating's structure consists of many small pores that can be used to color the aluminum. Once color is added, these pores are sealed to provide durability. This durability has been recognized by the aerospace industry, which selected anodizing as the finish of choice for the Space Station.
Advantages of Anodizing
Anodizing: the Renewable Finish.
When a paint film fails, the usual options are to recoat the surface with another paint or replace the metal. Scrubbing can damage a painted finish. When an anodized coating appears to have failed, cleaning often results in a renewed appearance. Anodized surfaces, like other building components, must be protected from chemical attack after installation.
Anodizing and the Environment
Chemical wastes from anodizing are used by many municipal wastewater treatment facilities to balance pH levels of treated water. The aluminum hydroxide from the effluent of anodizing plants improves the separation of solids in wastewater treatment plants. If local wastewater treatment plants cannot handle the solids generated in the anodizing process, equipment is available that enables anodizers to remove aluminum hydroxide solids from the effluent.
Care must be taken when applying touch-up paints to anodized finishes because a perfect match is impossible between the factory-applied finish and a finish applied in the field. For this reason, touch-up paints are a problem for both painted and anodized coatings.
Customers often present questions about color variation within the context of color range. A "range" implies a two-dimensional axis, for example, a range from light to dark. Research has shown that lightness is only one of at least three dimensions of appearance. Most people are familiar with the controls on a television set that affect brightness, color, and hue. When these same three components of appearance are combined with gloss and texture, we have at least four variables that can influence appearance besides light and dark.
The word range is discouraged when describing color. It is better to focus on scientific color measurement systems that include color, lightness, and gloss. With scientific color measurement techniques, a production run can be compared objectively to an approved standard. If a component is significantly different from an approved standard, the finisher should not ship it. It is also the customer's responsibility to avoid using metal that is not acceptable. When large anodized panels or extrusions are used in close proximity to each other, it may be possible to see color variations. In application, it is common to sort parts to obtain the desired effect.
The industry has visual comparisons as the criteria for color matching and for the most part color reproducibility has not been a problem. Color evaluation using color instruments is helpful but not the solution to producing a consistent color match. To avoid problems, the customer should agree on color standards with the anodizer.
Low-cost anodized sheets are often sold with a coating thickness of 0.15 mil. A finish of this thickness also might be called a 200, A21, A22 or A24. While this coating thickness is suitable for many applications, the integrity of this finish will not last more than a few years in exterior architectural applications and the expected lifetime is much less in coastal environments. Coating thickness makes an even more important difference in the durability of organically dyed finishes. Dyes fade more quickly with thinner coatings since they contain less color substance.
There are many options for finishing aluminum, and this is one reason aluminum is such a popular material. It is not always easy to decide which finish to apply. Communicate your needs with your finisher or your finisher's supplier. Consider not only appearance, but also the environment, maintenance requirements, and life cycle costs.
The Aluminum Anodizers Council