By Tammy Schroeder
Anodizing protects and enhances aluminum cladding in exterior and interior applications. Installed on a building exterior, anodized wall panels must withstand continuous exposure to direct sunlight’s ultraviolet (UV) radiation, precipitation including salt spray, and pollution and other climatic conditions of the project’s location. For interiors, anodize cladding often is specified for its consistent durability in holding up to daily wear, bumps, and scratches from people within the building.
The anodize process
Unlike architectural painted coatings that are applied on top of a material’s surface, anodizing is a finish produced by electrochemically controlling, accelerating, and enhancing oxidation. Anodize is an integral part of the aluminum substrate.
Architectural aluminum anodize typically involves a two-step electrolytic process where the aluminum material is submerged in a series of tanks. After the material is cleaned from grease, oil, dirt, and other contaminants, the first step is placing it into a tank containing an acid and water solution.
The acid dissolves a thin layer on the surface of the aluminum which allows it to develop a smooth uniform finish. Up until the early 2000s, anodize mainly was produced through a conventional, caustic etch process which relied on an aggressive alkaline chemical. Now, most modern finishing facilities rely on an acid etch fluoride-based ionic chemistry. The acid etch process takes approximately 15 minutes and removes only 0.01 kg (0.002 lb) per 0.09 m2 (1 sf) compared to the caustic etch, which took up to 80 percent longer and removed 90 percent more aluminum.
The anodize tank is charged with an electrical current which causes natural oxide to form on the aluminum’s surface. This heavy oxide film has a controlled, uniform thickness, and an extremely hard surface which protects the metal substrate from deterioration. In fact, the hardness of anodize is comparable to a sapphire, the second-hardest substance on Earth. This material property imparts the finish with excellent resistance to wear and abrasion, making anodize an optimal finish choice for aluminum cladding systems.
To minimize potential inconsistencies, one should specify 5005 aluminum alloy, in anodize quality, for flat sheet as well as consider sourcing all the metal from one lot of material from a single supplier. When anodizing flat sheet for exterior wall cladding panels, the aluminum needs to be a minimum thickness of 1 mm (0.04 in.).
Clear anodize results after the material has gone through the cleaning, etching, and anodizing process. The result highlights the aluminum’s natural silver color. If other metallic tones are specified, the aluminum continues an additional step, where the anodized aluminum is immersed in the electrolytic color tank(s).