July 10, 2007
Anodizing combines with nature to create one of the world's best metal finishes. It is the process of electrochemically controlling, accelerating, and enhancing oxidation of an aluminum tube, creating a durable, scratch-resistant coating on the surface comparable to a sapphire. Architectural anodize finishes are limited to certain colors, and it is impossible to produce a perfect color match. Perform as much bending and forming as possible before finishing.
Because of its strength and durability, anodized aluminum is used in a number of applications. Many structures and buildings have anodized aluminum in places where the metal framework is exposed to the elements, such as handrails, metal coping, column covers, HVAC ducts, and decorative metalwork.
Anodizing combines with nature to create one of the world's best metal finishes. It is the process of electrochemically controlling, accelerating, and enhancing oxidation of an aluminum tube, creating a durable, scratch-resistant coating on the surface (see lead image).
Preparation. The anodizing process typically begins with the aluminum tube being cleaned in a nonetching alkaline chemical cleaner to remove all shop dirt, water, and soluble oils that may have accumulated on the tube's surface during handling and manufacturing. After it is cleaned, the tube is ready for caustic etching.
The caustic etch process produces a matte finish and also minimizes minor surface imperfections, such as light die lines and travel marks. Caustic etching does not eliminate all surface imperfections. A good rule is if the imperfection can be felt with a fingernail before the tube is anodized, it is likely that it will not be removed by caustic etching.
The tube is then desmutted and rinsed to remove residues left from the caustic etch. This is the final preparation stage prior to anodizing.
The Process. The sulfuric acid (Type II) anodizing process produces a protective and decorative oxide finish on aluminum tube. The aluminum oxide layer is made thicker by passing a direct current through a sulfuric acid solution, with the tube serving as the anode, the negative electrode. The current releases hydrogen at the cathode, the positive electrode, and oxygen at the surface of the aluminum anode, creating a buildup of aluminum oxide.
Coloring. If the tube requires coloring, it is moved to a two-step electrolytic coloring tank. Tin metal is electrochemically introduced into the anodic pores to produce bronze tones ranging from light champagne to black (see Figure 1). After it is anodized and colored, the material is sealed in a midtemperature hydrothermal seal and is then given a final hot water rinse. This final, important step ensures that the high-quality anodized finish maintains its color for many years.
Advantages. An anodize finish satisfies the requirements for aluminum tube:
Disadvantages. A few challenges associated with an anodize finish are:
Architectural Class I and Class II anodic coatings are designations created by the American Architectural Manufacturers Association (AAMA) for the purpose of codifying the specification of anodized aluminum (see Figure 2).
|Characteristic||Class I||Class II|
|Color & Gloss Retention||Excellent||N/A|
|Gloss Options||40 - 80||40 - 80|
|Salt Spray Resistance||Fair||Very Poor|
|Effect of Poor Substrate Quality||Significant||Significant|
|Initial Cost||Low||Very Low|
A Class I coating is a high-performance anodic finish used primarily for exterior building products and other products that must withstand continuous outdoor exposure.
A Class II coating is a commercial anodic finish recommended for interior applications or light exterior applications receiving regularly scheduled cleaning and maintenance, such as storefronts.
The AAMA 611 specification for anodized architectural aluminum is the most commonly used, although other specifications may be followed as required.
Preparing the aluminum tube correctly for anodizing will reduce waste and produce the optimal finish:
Initiate cleaning procedures for aluminum as soon as it is practical to do so after installation is completed to remove construction soils and accumulated environmental soils and discolorations.
The simplest way to remove light soils is to flush the surface with water using moderate pressure. After the surface is air-dried, if soil is still present, scrub it with a brush or sponge and try concurrent spraying with water. If soils still adhere, use a mild detergent cleaner with brushing or sponging. Use uniform pressure to wash it—first horizontally then vertically. Thoroughly rinse the surface with clean water.
You must take certain precautions when cleaning anodized aluminum surfaces. First, identify the finish to select the appropriate cleaning method. Never use aggressive alkaline or acid cleaners. Avoid cleaning hot, sun-heated surfaces because possible chemical reactions will be highly accelerated, resulting in nonuniform cleaning.
Strong organic solvents, while not harmful to anodized aluminum, may extract stain-producing chemicals from sealants and affect the function of the sealants. Strong cleaners should not be used on window glass and other components on which it is possible for the cleaner to come in contact with the aluminum. Avoid excessive abrasive rubbing because it could damage the finish.
For added protection, apply wipe-on surface protectants, which are estimated to provide protection for 12 to 24 months in the harshest environments and make subsequent maintenance easier.
One of the best steps a tube fabricator can take to ensure a quality tube finish is to form a good working relationship with the finisher. Ongoing, open communication will be among the most valuable factors in minimizing waste and maximizing results.
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