5 Things You Need To Know About Anodizing Aluminum

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Anodized Aluminum Parts

What Is Anodizing?

Anodizing is a process,which is used to increase the thickness of the natural oxide layer on the surface of metal parts. Anodizing does not need to use heavy metals nor does it produce toxic waste, so it is an environmentally friendly process, also it meets the environmental and safety directives of the RoHS.

Some purposes of anodizing are to obtain good wear resistance, corrosion resistance, surface lubricity, adhesion, and aesthetics.

Aluminum is ideally suited to anodizing, although magnesium and titanium can be anodized too. But steel or stainless steel cannot be anodized.

Anodized aluminum parts are commonly found in aircraft and architectural components, as well as home appliances, sporting goods, motor vehicle components, food preparation equipment, furniture, and electronics, etc.
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Normal Anodizing Process of Aluminium.

The first step is to pre-treat the aluminum part through degreasing and pickling and providing a visibly smooth, clean surface.

The second step is that the aluminum part acts as the anode, then it is submerged in an electrolytic solution bath along with a cathode. When a current is passed through the electrolytic solution, hydrogen is released from the cathode(the negative electrode), and oxygen at the surface of the aluminum anode(the positive electrode), creating a build-up of aluminum oxide. This results in an anodized aluminum layer growing on the surface of the part. The layer thickness can be more than 100 times as thick as an oxide layer, which would naturally exist on an aluminum part that is only exposed to oxygen.

The third step is color dyeing(if needed). The most common anodizing processes, for example, sulfuric acid on aluminum, produce a porous surface. After immersing the anodized aluminum in an inorganic metal salt bath. An electric current is applied to this bath as the metal salts oxidize in the aluminum’s pores. Depending on the chemical conditions of the bath and the length of time immersed, the aluminum color will vary. The most common colors are yellow, green, blue, black, orange, purple and red. Dyed anodizing is usually sealed to reduce or eliminate dye bleed out.

The last step is sealing. Acidic anodizing solutions produce pores in the anodized coating. These pores can absorb dyes and retain lubricants but are also an avenue for corrosion. When lubrication properties are not critical, they are usually sealed after dyeing to increase corrosion resistance and dye retention. Sealing the anodized aluminum parts can be done in three ways: a cold method, a hot method, or a combination of the two.
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Types of Anodizing.

The most common types of anodizing found in MIL-A-8625 are listed below:

Type I-Chromic Acid Anodize, it results in the thinnest anodic coat of the principal three types. It is good for tight tolerance parts or serves as a paint/prime base. It can be dyed in black, and not practical for other colors.

Type II Sulfuric Acid Anodize, it is the most common method for anodizing, and less expensive than other types of anodize concerning to chemicals used, heating, power consumption, and length of time to obtain the required thickness. Normally, the film thickness range from 1.8 μm to 25 μm, the overall thickness of the coating formed is 67 percent penetration in the substrate and 33 percent growth over the original dimension of the part. It is particularly suited for applications where hardness and resistance to abrasion are required. Also it is easy to be dyed with a variety of colors.

Type III Sulfuric Acid Anodize, known as Type III, hard coat, hard anodizing, or engineered anodizing, the coatings are thicker than 25 μm. Hardcoat is specified for aluminum components subject to extreme wear applications where superior abrasion resistance is needed, or corrosive environments where a thicker, harder, more durable coating is necessary. It is suited for applications of valves, sliding parts, gears, swivel joints, and so on. It can be black dyed, but less decorative for other colors. 
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Benefits of Anodizing Aluminum.

Anodized aluminum parts are commonly found in a wide range of applications, such as home appliances, sporting goods, electronics, architectural and aircraft components.

The benefits of anodizing include:

Durability. Anodizing is a reacted finish that is integrated with the underlying aluminum. Most anodized products have an extremely long life span, result in offering significant economic advantages through maintenance and operating savings.

Aesthetics and Color Stability. Anodizing offers a large number of color alternatives and eliminates color variations. It allows the aluminum to maintain its metallic appearance. Also, exterior anodic coatings provide good stability to ultraviolet rays.

Ease of Maintenance. During the process of fabrication, handling, installation, surface dirt on the anodized parts may be generated. Normally, rinsing or mild soap and water cleaning will restore an anodized surface to its original appearance. Mild abrasive cleaners can be used for more difficult deposits.

Cost-Effective.  The anodizing part is with a much harder surface than painting and powder coating, but less expensive.

Health and Safety. Anodizing is one of the more environmentally friendly metal finishing processes. 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. The most common anodizing effluents, aluminum hydroxide, and aluminum sulfate, are recycled.
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Some Products Using Anodized Aluminum:

  • Building exteriors, such as storefronts, curtain walls and roofing systems.
  • Appliances such as refrigerators, dryers, coffee brewers, ranges, televisions, microwave equipment.
  • Vents, awnings, dust covers, light fixtures, storm doors, window frames, mailboxes, bathroom accessories, patio covers, and wall switch plates for buildings.
  • Display cases, pans, coolers, and grills for the food industry.
  • Tables, beds, files and storage chests for homes and offices.
  • Golf carts, boats, and camping/fishing equipment for the leisure industry.
  • Hundreds of components for motor vehicles of all kinds such as trim parts, wheel covers, control panels, and name plates.
  • Exterior panels for aerospace vehicles, clocks and electronic products, fire extinguishers, photo equipment, solar panels, telephones, picture frames, and bathroom accessories.
  • Interior decoration and trim.

Is Anodizing Right For You?

If you have any more questions about anodizing, please feel free to contact us, we can offer advice to you not only on anodizing but also on many different kinds of finishing, which might be used on the cnc machining parts or prototypes of your projects. Also we will help you to find the solution that meets your target price and delivery date.
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