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A guide to anodizing aluminum and other metals

When manufacturing metal components, it’s important to take the surface finish into consideration. Metal finishes improve the part’s overall appearance and can help boost part performance by increasing factors such as corrosion resistance and wear resistance.

Anodizing is a popular finishing choice for aluminum and titanium parts as well as other non-ferrous metals. In this article, we’ll cover the basics of anodization, its benefits, and key considerations for product teams.

What is anodizing?

Anodizing is a process that thickens the natural oxide layer on a metal part’s surface into an anodic oxide film. This extra thickness protects the part’s surface and improves aesthetics.

Aluminum and titanium are the most commonly anodized metals, but other nonferrous metals can also undergo the process. According to the anodizing standard, MIL-A-8625, there are three different types of anodizing:

  • Type I: Uses chromic acid to produce a very thin oxide layer.
  • Type II: Uses sulfuric acid to produce a thicker oxide layer that’s suitable for coloration.
  • Type III: Uses the same process as Type II but produces an even thicker oxide layer. This is often called “hard anodizing.”

Within these MIL-A-8625 types, there are two classes — Class 1 (non-dyed anodization) and Class 2 (dyed or colored anodization).

Inside the anodizing process

Anodizing is a process in which metal is submerged in an electrolytic bath. A submerged cathode (negatively charged electrode) causes hydrogen to be released from the solution while oxygen is released at the surface of the aluminum part (the anode, positively charged). This process results in a protective layer of aluminum oxide being built up on the surface of the part.

The aluminum oxide layer is bound directly to the surface of the part, resulting in a very durable, strong finish. While there are other methods of applying color treatments to aluminum parts, such as painting, they result in a much weaker bond with the surface, relatively speaking.

anodizing purple screws
Anodizing is a widely-used finishing process for metal parts. It can increase a metal component’s lifespan by improving corrosion resistance, wear resistance, and more.

Benefits of anodizing aluminum and other metals

There are many benefits to anodizing aluminum, titanium, and other metals. These benefits can be broken down into four core categories:

Durability

Anodizing a metal component extends its lifespan thanks to improved abrasion resistance and corrosion resistance. This increased durability makes it take much longer for anodized metal parts to wear down during installation, handling, and regular use.

Aesthetics

A clear anodized finish can be used to protect an already acceptable appearance of a part or it can be used to supplement the aesthetics with the addition of color. While there is a substantial amount of expertise required for precise color accuracy and repeatability, the results can be extremely desirable and look professional.

Health and safety

Anodized finishes are chemically stable, non-toxic, and produce very few harmful or dangerous effects on land, air, or water. For this reason, anodizing is considered an environmentally safe procedure. Anodized aluminum can still be recycled like untreated aluminum, further reducing its environmental risks.

Costs

Anodizing metal components can result in significant economic impact down the line. Anodizing typically reduces the amount of long-term maintenance when compared with untreated aluminum, which results in substantial cost savings. Additionally, while the chemical reactions behind the anodizing process are complicated, the process itself is relatively low-cost.

Common applications and considerations for anodizing

Anodizing is used throughout the construction, aerospace, and consumer goods industries. Anodized metals can be found in building exteriors like curtain walls and roofs, interiors like ceilings and floors, plus staircases and escalators. Satellites also commonly have anodized components, which helps them function in the harsh environment of outer space. Consumer electronics such as laptops, smartphones, and smartwatches also commonly utilize anodized finishes.

There are a few challenges and considerations to keep in mind when designing for metal anodizing.

Dimensions

The anodizing process adds to the surface of an object, meaning there will be dimensional changes to the bare metal part. Dimensional growth changes depending on the type of anodization but is typically around 50 percent of the total anodic layer thickness. The thickness of the oxide layer must be considered when determining tolerances, particularly with holes or any mating surfaces. One way this can be mitigated is with chemical or physical masks, as they can be used to keep areas of the raw metal untreated.

Hardness

Anodizing increases the surface hardness of a part far beyond what is typical of the base metal. For example, the surface hardness of anodized aluminum approaches the Rockwell hardness of some hardened steels.

Color matching

It can be difficult to reach a true color match, particularly if the components are not all treated in the same batch or if the goal is a true black finish. Color fading is also a common issue for anodized coatings, which highlights the importance of carefully dialing in the process for each part.

Conductivity

Anodizing a material increases its thermal and electrical resistance. If thermal or electrical insulation is a problem, a mask can also be used in this case to retain the bare-metal finish where full conductivity is needed.

Anodizing metal parts with Fast Radius

Anodizing is a widely-used finishing process for metal parts. It can increase a metal component’s lifespan by improving corrosion resistance, wear resistance, and more. Anodization also simplifies long-term maintenance, creates beautiful and identifiable colored components, and even promotes environmentally friendly manufacturing.

While anodizing your metal parts is incredibly important, knowing how to go about anodizing can be a challenge. Fast Radius, a seasoned manufacturing partner, can help you achieve the best anodized finish for your components. Contact us today to get started.

To learn more about corrosion-resistant materials and metal treatments, visit the Fast Radius resource center.

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