What is Anodizing?

What is Anodizing?

Anodizing is an electrochemical process that converts the metal surface into a decorative, durable, corrosion-resistant, anodic oxide finish. Aluminum is ideally suited to anodizing, although other nonferrous metals, such as magnesium and titanium, also can be anodized.

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 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.

Anodized finishes have made aluminum one of the most respected and widely used materials today in the production of thousands of consumer, commercial and industrial products.

Anodizing's Benefits

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 aluminum for total bonding and unmatched adhesion.

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 andusage 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 variations. Unlike other finishes, anodizing allows the aluminum 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 degrees 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.

Anodizing and the Environment

Aluminum Life Cycle Enhancement with Anodizing
The environmental advantages of aluminum are widely acknowledged. Aluminum is one of the most durable and versatile of metals, offering improved mileage in automobiles by virtue of its lightweight and tremendous recyclability. According to the Aluminum Association, about one-third of all aluminum produced in the U.S. today is from recycled sources, saving some 95 percent of the energy required to produce aluminum from raw materials.

In light of environmental impacts, anodized aluminum is the "naturally green" material. Anodizing accentuates the environmental benefits of aluminum without compromising them, and the process is relatively benign when compared to other materials and processes

In light of environmental impacts, anodized aluminum is the "naturally green" material. Anodizing accentuates the environmental benefits of aluminum without compromising them, and the process is relatively benign when compared to other materials and processes

Anodizing enhances aluminum and its environmental virtues. 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.

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.

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.

Aluminum Saves Energy and Materials
Aluminum metal is a good conductor of electricity; the anodic coating is an insulator. Combinations of the
two properties can be incorporated into systems that save energy and materials. The metal can serve both a structural and conductive purpose, while the anodic coating insulates the circuit and preserves the structure. This simplifies physical design for electric circuits and saves space and wiring.

All of the aforementioned properties of anodizing contribute substantially to a product's life cycle and reduce energy demands.

Environmental Aspects of the Anodizing Process
Anodizing is a water-based process and uses no 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 halogenated hydrocarbons or similar toxic organics are used in anodizing.

Similar neutralization reduces most anodizing chemicals to common dissolved minerals. Most anodizing is performed without generation of hazardous waste, and in many cases aluminum-rich anodizing wastes are environmentally valuable in removing pollutants and settling solids in domestic sewage treatment processes.

Anodizing is not metal plating.The two are sometimes confused, but in fact, are completely different processes. The anodic coating is generated from the base metal and, thus, has essentially the same constituents as the aluminum. The surface builds

from the metals as an ultra-thin, nontoxic aluminum oxide. Added materials constitute a minute amount of mass to a product; Material Safety Data Sheets for anodized aluminum are identical to those for the metal.

Under EPA rules, conventional anodizing generates no hazardous waste; it does not use VOCs or EPA-listed toxic organics. The involvement of heavy metals is dramatically lower than exterior-use paint pigments or plating.

Recyclability is unaltered by anodizing and no intermediate processing is
needed for anodized metal to reeenter the recycle chain, unlike thicker organic or plated metallic coatings.

Anodized aluminum is the environmentally sound choice for various applications.

Anodizing, Definitions and Method

Anodizing successfully combines science with nature to create one of the world's best metal finishes.

It is an electrochemical process that thickens and toughens the naturally occurring protective oxide. The resulting finish, depending on the process, is the second hardest substance known to man, second only to the diamond. The anodic coating is part of the metal, but has a porous structure which allows secondary infusions, (i.e. organic and inorganic coloring, lubricity aids, etc.)

Anodizing Definitions and Methods

While the chemical anodizing process remains the same for all applications, the mechanical methods vary according to the two physical types and shapes of metals used:

  • Batch Anodizing - Involves racking parts and immersing them in a series of treatment tanks. Extrusions, sheets or bent metal parts, castings, cookware, cosmetic cases, flashlight bodies, and machined aluminum parts are just a few of the items that are batch anodized. 

  • Continuous Coil Anodizing - Involves continuous unwinding of pre-rolled coils through a series of anodizing, etching and cleaning tanks, and then rewinding for shipment and fabrication. This method is used for high volume sheet, foil and less severely formed products such as lighting fixtures, reflectors, louvers, spacer bars for insulated glass, and continuous roofing systems. 

Appearance options and quality are improved through the use of dyes and special pretreatment procedures. This makes the aluminum look like pewter, stainless steel, copper, brushed bronze or polished brass and can also be colored with brilliant blues, greens, reds, and many varieties of metallic gold and silver. 

The unique dielectric properties of an anodized finish offer many opportunities for electrical applications.

The surface of the aluminum itself is toughened and hardened to a degree unmatched by any other process or material. The coating is 30 percent thicker than the metal it replaces, since the volume of oxide produced is greater than that of the metal replaced.

The resulting anodic coating is porous, allowing relatively easy coloring and sealing.

Hard Anodizing is a term used to describe the production of anodic coatings with film hardness or abrasion as their primary characteristic. They are usually thick by normal anodizing standards (greater than 25 microns) and they are produced using special anodizing conditions (very low temperature, high current density, special electrolytes). They find application in the engineering industry for components which require a very wear resistant surface such as piston, cylinders and hydraulic gear. They are often left unsealed, but may be impregnated with materials such as waxes or silicone fluids to give particular surface properties. 

Batch and Coil Anodizing

Batch and coil anodizing are accomplished in five carefully controlled, calibrated, quality-tested stages:

  1. Cleaning. Alkaline and/or acid cleaners remove grease, and surface dirt.

  2. Pre-Treatment
    ◦ Etching. An appealing matte surface finish is created with hot solutions of sodium hydroxide to remove minor surface imperfections. A thin layer of aluminum is removed to create a matte or dull finish.

    ◦ Brightening. A near mirror finish is created with a concentrated mixture of phosphoric and nitric acids which chemically smooths the aluminum's surface.

  3. Anodizing. The anodic film is built and combined with the metal by passing an electrical current through an acid electrolyte bath in which the aluminum is immersed. The coating thickness and surface characteristics are tightly controlled to meet end product specifications.

  4. Coloring. Coloring is achieved in one of four ways: 

    • Electrolytic Coloring (The two-step method) - After anodizing, the metal is immersed in a bath 
      Anodizing Line containing an inorganic metal salt. Current is applied which deposits the metal salt in the base of the pores. The resulting color is dependent on the metal used and the processing conditions (the range of colors can be expanded by overdyeing the organic dyes). Electrolytic colors can be specified from any AAC member. Commonly used metals include tin, cobalt, nickel, and copper. This process offers color versatility and the most technically advanced coloring quality.

    • Integral Coloring - This so-called one-step process combines anodizing and coloring to simultaneously form and color the oxide cell wall in bronze and black shades while more abrasive resistant than conventional anodizing. It is the most expensive process since it requires significantly more electrical power.

    • Organic Dyeing - The organic dyeing process produces a wide variety of colors. These dyes offer vibrant colors with intensities that cannot be matched by any other paint system in the market. They can also provide excellent weather-fastness and light-fastness. Many structures built with these finishes have lasted more than 20 years. The color range can be broadened by over-dyeing the electrolytic colors with the organic dyes for a wider variety of colors and shades. This method is relatively inexpensive and involves the least amount of initial capital of any other coloring process.

    • Interference Coloring - An additional coloring procedure, recently in production, involves modification of the pore structure produced in sulfuric acid. Pore enlargement occurs at the base of the pore. Metal deposition at this location produces light-fast colors ranging from blue, green and yellow to red. The colors are caused by optical-interference effects, rather than by light scattering as with the basic electrolytic coloring process. Further development will produce a greater variety of colors.

  5. Sealing. This process closes the pores in the anodic film, giving a surface resistant to staining, abrasion, crazing and color degradation.

    Quality control. Throughout the entire anodizing process, AAC members monitor the process and quality of the product. The application of electrical power and color is preprogrammed and verified on all batches and coils. 

    This quality control ensures uniformity to end product specifications for film thickness, density, abrasion resistance, corrosion resistance, color uniformity, fade resistance, reflectivity, image clarity, insulative properties, adhesion and sealing.

    In many cases, AAC members use Statistical Process Control (SPC) methods to meet rigorous quality assurance standards.

For more information or to order samples, please contact Dylan at Design Strategies today.


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