Anodizing Aluminum: Everything You Should Be Aware Of

The process of anodizing, also known as anodization, transforms the surface of aluminum into a thicker, more uniform oxide layer through an electrolytic process.

Electrochemical anodization of aluminum yields an aluminum oxide surface that possesses:

  • Resistance to corrosion
  • A visually appealing appearance
  • Resistance to wear
  • Enhanced bonding for paint or adhesives
  • Improved lubricity
  • Enhanced adhesion for plating

This article will primarily concentrate on the anodization of aluminum, although other metals such as magnesium, titanium, and aluminum alloy can undergo anodization. Various types of anodized aluminum finishes exist, including clear anodized aluminum, black anodized aluminum, and hard anodized aluminum. There are also commonly referenced anodizing types: Type I, Type II, and Type III.

Many industries typically adhere to military specification MIL-PRF-8625 (which supersedes MIL-A-8625) to define the anodizing process, including anodizing line design and patterns. We will reference it several times in this article. However, it is advisable to verify your anodizing requirements before specifying your design.

Advantages of the Anodizing Process:

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The anodizing process replaces the natural aluminum oxide layer with a new layer, enhancing part durability, paint adhesion, component appearance, and corrosion resistance. It also creates a chemically resistant coating against harsh cleaning agents. The image below showcases several parts that have been anodized and dyed in different colors.

The anodizing process utilizes an acid bath and an electrical current to form this anodic layer on the base metal. In essence, it creates a controlled and durable aluminum oxide layer on the component instead of relying on the thin oxide layer naturally occurring on bare aluminum surfaces. This process is akin to bluing, Parkerizing, passivating, and other conversion coating surface treatments used for corrosion resistance and surface hardening.

Typical Applications for Anodized Aluminum Parts:

Anodic oxide coatings find applications in various fields, with aesthetics being one of the most common uses, allowing components to be dyed in different colors.

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For example, thumb drives can be anodized and dyed for aesthetic purposes. The top thumb drive is class 1 (undyed), displaying a clean anodic coating with a color almost matching the base alloy. The bottom thumb drive has been dyed a vibrant blue color. Another example is the thread adapter below, anodized and dyed black.

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Anodized heat sinks, as seen below, can enhance heat dissipation by increasing surface emissivity over bare aluminum.

Additionally, anodized aluminum sheets are excellent for outdoor signage due to their lightweight nature, corrosion resistance, and greater durability compared to paint. The anodized surface allows easy screen printing or application of reflective materials.

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While we have covered a diverse range of applications, there are countless other potential uses for anodized aluminum. For more information and examples, we recommend exploring the Aluminum Anodizers Council’s website.

Available Anodizing Processes for Aluminum

The primary anodizing processes specified by the military standard MIL-PRF-8625 are Type II acid sulfur anodizing and Type III hard coat anodizing. Commonly referred to as regular anodizing, Type II, and tough or hard anodizing, Type III, these processes are widely utilized. Let’s delve into the details of Type I in a moment.

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The three anodizing types according to MIL-PRF-8625 are outlined below:

Type I Type I and IB – Chromic Acid Anodizing Type IC – Non-Chromic Acid Anodizing, serving as an alternative to Type I and IB Type II Type II – Conventional coating from a sulfuric acid bath Type IIB – Non-chromate alternative for Type I and IB coatings Type III Type III – Hard Anodize Each anodization type imparts distinct functional and aesthetic characteristics:

Types I, IB, and II are employed for corrosion resistance, abrasion resistance, and paint preparation. For applications with fatigue-critical requirements, Types I and IB, known for their thinner and more flexible coatings, are applied. For instance, these types are frequently used on highly fatigued structural components in aircraft.

Types IC and IIB are preferred when a non-chromate alternative is necessary, often due to environmental regulations. In the defense industry, new contracts may mandate environmentally friendly options (Types IC and IIB) due to the hexavalent chromium content in Types I and IB.

Type III primarily enhances abrasion and wear resistance, featuring a much thicker coating that outperforms other types in these aspects. While this coating may reduce fatigue life, it finds frequent application on firearm components, gears, valves, and other parts that experience relative sliding.

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Anodizing, in general, improves the adhesion of paints and other bonding agents compared to bare aluminum. Beyond the anodizing process, some parts may undergo dyeing, sealing, or treatment with additional materials, such as dry film lubricant. Dyed parts are categorized as Class II, while undyed parts are considered Class I according to MIL-PRF-8625 specifications.

Considerations for Aluminum Anodizing Planning

Here are key factors to consider when designing aluminum parts intended for anodization:

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  1. Dimensions First and foremost, take into account the dimensional changes associated with anodizing aluminum components. Engineers or designers may specify post-processing dimensions on drawings to compensate for the anticipated changes. During anodization, surfaces may experience “growth,” where outer diameters increase, and holes become smaller due to the accumulation of the anodic layer forming aluminum oxide.The dimensional growth is roughly estimated to be around 50% of the total thickness of the anodic layer. Variations in thickness will depend on the specific alloy and process controls. Precision features may require masking to control growth, and for thick Type III coatings, additional processes like lapping or honing may be necessary at an extra cost.Additionally, consider radii for outside edges and inside corners, as anodic coatings may not form well on sharp corners, especially for Type III coatings.
  2. Wear Resistance Anodizing increases the hardness of the part’s surface. Although coating hardness isn’t typically specified, abrasion resistance is a key consideration. MIL-PRF-8625 specifies a test for abrasion resistance. For example, Type III anodize can have a hardness of 60-70 Rockwell C, contributing to wear resistance in applications like CNC hold-down clamps.
  3. Adding Color with Dyes Anodized coatings can be dyed for aesthetic, functional, or identification purposes. When dyeing anodized parts, discuss expectations with the anodizing shop. Consider different coloring methods, such as electrolytic coloring, dip coloring, integral coloring, and interference coloring. Achieving color consistency can be challenging, and factors like fading, dye responsiveness, and type of anodize must be considered.
  4. Conductivity Anodic layers are insulators despite the base metal’s conductivity. To ground to a chassis or component, a clear chemical conversion coating may be necessary. Testing surface conductivity with a digital multimeter can help determine if an aluminum part is anodized.
  5. Combined Coatings Anodizing is often done before other surface finishing processes due to its ability to enhance adhesion. Common combined coatings include paint, Teflon impregnation, and electroplating. Painted anodic coatings improve corrosion resistance and provide specific colors. Teflon impregnation reduces friction in Type III hard coats, while electroplating, such as chrome plating, offers decorative or functional benefits.Rare processes can further alter anodic coating properties, typically requiring specialized vendors. For specific needs, it’s recommended to consult with a coating specialist.


Considerations for Aluminum Anodizing

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Aluminum anodization offers numerous advantages, but it is essential to be aware of certain limitations. Here are key points to bear in mind:

  1. Thick anodic coatings might diminish the fatigue life of components, particularly those employing Type III processes.
  2. Geometric alterations should be taken into account for parts undergoing anodization. This is crucial for Type II and III processes but may be unnecessary for some Type I applications.
  3. Achieving color consistency can be challenging when dealing with multiple batches and extremely difficult when collaborating with different vendors.
  4. To ensure complete corrosion protection, it may be necessary to seal the pores of the anodic layer.
  5. The abrasion resistance of Type III hard coats is likely to decrease as the thickness approaches and surpasses 0.003 inches.
  6. Different alloys may react differently to the anodizing process. For instance, alloys with a copper content of 2% or more are generally less wear-resistant than other alloys during MIL Spec testing of Type III coatings. This implies that Type III hard coat on 2000 series aluminum and some 7000 series will not be as wear-resistant as 6061 with a hard coat. If you have concerns about the alloy you are using, we recommend consulting your vendor.

FAQs

  1. How does the aluminum anodizing process work? The anodizing process transforms the surface of aluminum into a thicker, more durable, and more uniform oxide layer through an electrochemical process. The resulting porous surface structure enhances paint and adhesive bonding, plating adhesion, heat emissivity, and allows secondary infusions of lubricants and colorants.
  2. What colors can you anodize aluminum? Most colors can be achieved by using dyes during the anodizing process or by post-anodizing painting (e.g., to achieve a pure white color).

Anodizing coloring methods include electrolytic coloring, dip coloring, integral coloring, and interference coloring (refer to the Coloring methods section). Possible anodizing colors comprise Black, Blue, Blue-Gray, Brown, Gold, Gray, Green, Olive Drab, Pink, Red, Violet, and Yellow. Some processes produce UV-resistant colors, while others generate colors through optical-interference effects or light scattering.

The anodized aluminum surface is also well-suited for paint application, screen printing, or the attachment of reflectives (e.g., reflective road sign materials) to achieve the desired color.

  1. What does clear anodized aluminum look like? Clear anodized aluminum exhibits a uniform, transparent film with enhanced corrosion and wear resistance. The film’s color varies from very light to dark gray as the coating thickness increases.
  2. Which type of anodizing should I choose? Most machined parts undergo Type II (or “regular”) sulfuric anodizing and Type III Hardcoat or “hard” sulfuric anodizing per Military specification MIL-PRF-8625. Type I anodizing uses environmentally unfriendly chromic acid and is rarely specified (e.g., in military or aerospace applications requiring substantial requalification efforts). Refer to the “What elements should I consider…” section for additional details.
  3. How much does anodizing aluminum cost? The cost depends on coating thickness, anodizing type, secondary finishing services, part geometry, and batch size.
  4. Does anodizing wear off? Anodize is long-lasting as it becomes part of the base aluminum material. However, it can wear away due to abrasion, and the color may fade with prolonged outdoor exposure. Type III hard anodize maintains its color and wear resistance for a longer duration than Type II.

Concluding Remarks

Opting for aluminum anodization proves to be a superb choice for enhancing corrosion and wear resistance; however, its utility goes beyond these aspects. Anodization also enhances paint adhesion, offers an optimal surface for impregnation, and improves various surface characteristics, as well as the visual appeal of coated components.

The process of aluminum anodization, as elaborated upon in this article, is intricate. By incorporating our design recommendations and addressing the highlighted considerations, you will pave the way for the successful production of batches of anodized aluminum components.

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