by Tom Minnon, LEED® AP, CDT, Eastern Region Sales Manager for Tubelite Inc.
What is Anodizing?
Anodizing is an electrochemical process that converts aluminum’s metal surface into a decorative, durable, corrosion-resistant, anodic oxide finish. It is readily available for storefront, curtainwall, entrances and daylight control systems.
Exposed to the earth’s atmosphere, aluminum combines with oxygen to form a protective surface film, which inhibits further oxidation of the aluminum. This natural oxide is extremely thin, loosely adhered to the aluminum surface and easily removed by handling. Anodizing is a process, which thickens the natural oxide film resulting in a heavy aluminum oxide film of controlled thickness having the hardness similar to that of a ruby or sapphire. Anodizing is, therefore, a matter of highly controlled oxidation—the enhancement of a naturally occurring phenomenon.
Anodizing uses the base metal – the aluminum alloy – to create a thin, extremely strong and corrosion-resistant finish. The anodized surface is very hard and thus preserves and extends the life of the aluminum product. Architectural aluminum anodic coatings provide good stability to ultraviolet (UV) rays and does not chip or peel.
The anodic oxide structure originates from the aluminum substrate and is composed entirely of aluminum oxide. This aluminum oxide is not applied to the surface like paint or plating, but is fully integrated with the underlying aluminum substrate, so it cannot chip or peel. It has a highly ordered, porous structure that allows for secondary processes such as coloring and sealing.
Anodizing offers a range of colors in earth tones, such as champagne, bronze-tones and black. Unlike other finishes, anodizing allows the aluminum to maintain its metallic appearance. Clear anodizing does not incorporate any pigments.
When aluminum is anodized, an electrical current is passed through a bath of sulfuric acid (the electrolyte), while the aluminum being treated serves as the anode. This produces a clear film of aluminum oxide on the aluminum’s surface. This layer is mostly porous with a very thin barrier layer at the base. This structure lends itself very well to electrolytic coloring.
Anodizing is a water-based process and uses no volatile organic compounds (VOCs). There are no vehicle solvents, no carrier resins, and any pigmentation used in anodizing is created by extremely small amounts of metals or dye securely locked within the hard surface. No toxic organics are used in anodizing. Recyclability of aluminum is unaltered by anodizing and no intermediate processing is needed for anodized metal to re-enter the recycle chain.
Anodizing is a safe process that is not harmful to human health. An anodized finish is chemically stable, will not decompose and is non-toxic. The anodizing process is non-hazardous and produces no harmful or dangerous by-products. Under U.S. Environmental Protection Agency (EPA) rules, conventional anodizing generates no hazardous waste; it does not use VOCs or EPA-listed toxic organics.
Anodized products have an extremely long life span and offer significant economic advantages through maintenance and operating savings. Anodizing is a reacted finish that is integrated with the underlying aluminum for total bonding and unmatched adhesion.
The Anodizing Process
* Clean – The anodize process begins with the material being cleaned in a non-etching alkaline chemical cleaner to remove all shop dirt, water, soluble oils and other unwanted surface contaminates. which may have accumulated on the material during handling and/or manufacturing.
* Rinse – After cleaning, the material is rinsed and is then ready for etching.
* Etch – Etching is an important step during the anodize process. It is designed to dissolve a thin layer on the surface of the aluminum to develop a smooth uniform finish. Most anodizers have changed their etch chemistry from conventional caustic etch to a more eco-friendly acid etch technology.
The eco-friendly acid etch creates an aesthetically appealing, “frostier” appearance that helps hide small defects, such as die lines, flow lines, minor corrosion and scratches, that may occur on the aluminum surface. Although neither conventional or acid etch removes irregularities in the aluminum, acid etch does a better job of concealing them. This gives the material a better aesthetic finish on both primary and recycled aluminum extrusions.
* Desmut – Material is then moved to deoxidzing and desmutting process, which further prepares the aluminum surface for subsequent finishing. This step removes surface oxides. It also removes smut, which is a combination of intermetallics, metal and metal oxides remaining on the surface after cleaning and etching. And, it actives the surface for the electrochemical anodizing.
* Anodize – In the anodizing tank, the electrochemical oxidation of an aluminum surface takes place to produce a stable film of oxide. In this process, a porous, insulative layer composed of aluminum and oxygen is produced by passing electricity through the aluminum in a conductive medium. The basic structure of an anodic coating is based on a series of hexagonal columns of oxide, each with a central pore and a thin barrier layer separating the electrolyte in the base of the pores from the underlying metal.
* Electrolytic Color – The coloring of an anodic film is designed to enhance the appearance of the material and broaden the opportunity for anodized aluminum applications. In electrolytic coloring, or “two-step” coloring, anodizing is followed by the electro-deposition of a metal. AC power is used to deposit tin metal. Deposition takes place at the bottom of the pore. The intensity of the color is dependent on the amount of tin deposited and the packing density.
* Seal – After anodizing and coloring, the material is sealed in a mid-temperature hydrothermal seal and then given a final hot water rinse. This last, important step ensures that the high-quality anodized finishes will maintain their beauty and durability for many years.
Watch for part 2 on anodizing in March and in “Architect’s Guide to Glass and Metal.”
Tom Minnon, LEED® AP, CDT, is the eastern region sales manager for Tubelite Inc., serving clients from Maine to Georgia. With nearly four decades of industry experience and many professional accreditations, he regularly provides educational and consultative support to architects, buildings owners and glazing contractors regarding storefront, curtainwall, entrances and daylight control systems.