Anodizing is a multi step process whereby aluminum is electrochemically processed to form a layer of aluminum oxide (anodic) coating which protects the aluminum beneath. The coating can also be cosmetically enhanced by dying it as a step in the anodizing process. Surface cleanliness and preparation is the key to good anodizing results.
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 cannot chip or peel. It has a highly ordered, porous structure that allows for secondary processes such as coloring and sealing.
Anodizing is accomplished by immersing the aluminum into an acid electrolyte bath and passing an electric current through the medium. A cathode is mounted to the inside of the anodizing tank; the aluminum acts as an anode, so that oxygen ions are released from the electrolyte to combine with the aluminum atoms at the surface of the part being anodized. Anodizing is, therefore, a matter of highly controlled oxidation—the enhancement of a naturally occurring phenomenon.
Benefits of Anodizing?
The unique anodized finish is the only one in the metals industry that satisfies each of the factors that must be considered when selecting a high performance aluminum finish:
Durability. Most 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 aluminium for total bonding and unmatched adhesion.
The corrosion resistance of anodized aluminum is well established for industrial applications. Transportation components, building elements, storage containers, and process equipment utilize anodizing to extend the life and expand the utility of aluminum structures. Anodized aluminum is safe for cookware and provides durable work surfaces for applications that require superior abrasion-resistance.
Anodizing also reduces friction and increases lubricity, an advantage with fitted components and for moving parts. Increased wear resistance means a longer life cycle. Hardcoat anodizing further improves wear resistance and general coating durability to physical forces.
Color Stability. Exterior anodic coatings provide good stability to ultraviolet rays, do not chip or peel, and are easily repeatable.
Ease of Maintenance. Scars and wear from fabrication, handling, installation, frequent surface dirt cleaning and usage are virtually non-existent. Rinsing or mild soap and water cleaning usually will restore an anodized surface to its original appearance. Mild abrasive cleaners can be used for more difficult deposits.
Aesthetics. Anodizing offers a large increasing number of gloss and color alternatives and minimizes or eliminates color varations. Unlike other finishes, anodizing allows the aluminium to maintain its metallic appearance.
Cost. A lower initial finishing cost combines with lower maintenance costs for greater long-term value.
Health and Safety. Anodizing is a safe process that is not harmful to human health. An anodized finish is chemically stable, will not decompose; is non-toxic; and is heat-resistant to the melting point of aluminum (1,221 degress F.)
Since the anodizing process is a reinforcement of a naturally occurring oxide process, it is non-hazardous and produces no harmful or dangerous by-products.
Environmental Advantages of Anodizing. In contrast to anodizing, coatings – paint for example – can dramatically reduce the ability to recycle the aluminum and can increase costs. Paints, plastics, and plating rely on problematic materials in their production that can compromise green objectives. Anodizing, on the other hand, is “recycle-neutral” with minimal use of such materials as volatile organic compounds (VOCs) and heavy metals.
Where is the Market?
• building products (curtain walls, roofing systems)
• commercial and residential products (vents, awnings, frames, fixtures)
• appliances (refrigerators, microwaves, coffee makers, barbecues)
• food preparation equipment (pans, coolers, grills)
• home and office furniture (tables, beds, cabinets)
• sporting goods (golf carts, boats, camping and fishing equipment)
• motor vehicle components (trim, hubcaps, panels, nameplates)
• electronics (television, photography equipment)
• aerospace (satellite panels)
What is the MIL Spec for Anodizing?
Anodizing is typically specified using MIL-A-8625F.
Altech Anodizing Ltd.’s standard processes for Type II and Type III sulphuric acid anodizing are generally in accordance with the process requirements of MIL-A-8625F.
One difference between MIL-A-8625F and Altech’s standard processes is the exception of paragraph 18.104.22.168 of MIL-A-8625F which requires that production parts or test specimens undergo a salt spray test to determine corrosion resistance. This test is not regularly performed by us for cost reasons. However, given our strict chemical and process controls, we have every reason to believe that all parts processed by us would pass the prescribed salt spray test. We can arrange salt spray testing of test specimens or your parts after processing if required.
What is Coating Thickness?
Anodizing converts aluminum into its oxide, aluminum oxide. The aluminum oxide coating is thicker than the aluminum consumed during the process and the dimensions of the anodized aluminum therefore change. Precise dimensional changes are a function of temperature, electrical current density applied during the anodizing process, chemical bath parameters and the alloy being anodized.
However, under typical Type II anodizing conditions, the dimensional growth of an anodized part will be 2/3 in and 1/3 out per surface. In other words, a 6/10 of a thou anodic coating will result in a dimensional growth of approximately 2/10 of a thou outwards.
Under Typical Type III anodizing procedures, the dimensional growth of an anodized part will be ½ in and ½ out. In other words, a 2 thou Type III anodic coating will result in dimensional growth of approximately 1 thou (.001 inches) per surface.
What Affects Final Product Dimensions?
During the anodizing process parts are chemically etched to prepare and clean the surface for anodizing. Etching results in the removal of some aluminum while ultimately providing a matte finish. The longer a part is etched, the more aluminum will be removed. Therefore, the dimensions of the part will be slightly reduced. We can vary the etching time depending on the tolerances needed or in some cases completely eliminate the step.
Does Anodizing Hide Imperfection?
No anodizing typically highlights scratches and other surface and material imperfections. Surface preparation is fundamental to achieving an excellent finished product.