Your Basic Guide to Anodizing Aluminum

Leon Huang: Rapid Direct Author

Your Basic Guide to Anodizing Aluminum

In the following guide, we’ll take you through everything you need to know about anodizing aluminum, looking at the what, how to anodize aluminum, and the reasons for aluminum anodization.

What is anodized aluminum?

Anodizing is a common finishing method used with a selection of nonferrous metal materials, including aluminum, titanium and magnesium.

anodizing process

Anodizing consists of an electrochemical process that transforms the outer surface of metal parts into a durable and highly corrosion and scratch resistant layer. The process is also highly decorative, offering a professional shiny finish that comes in a variety of colors.

How does anodizing aluminum work?

Almost any aluminum part can be anodized, whether it has been CNC machined or made using sheet metal fabrication. The process, which may seem complex due to the various electrochemical reactions taking place, is actually quite simple and cost effective, making anodization a popular choice across many industries. Here’s how to anodize aluminum:

anodizing factory

First, the aluminum part must be cleaned before undergoing anodization. It is then placed into a bath of electrolytic solution and a direct electric current is applied.

This creates a positive electric charge in the aluminum and a negative charge in the solution’s electrolyte plates. The resulting electrochemical reaction creates pores on the surface of the aluminum component, which enable the aluminum substrate to bond with the negatively charged O2 ions in the solution to create aluminum oxide.

This ultimately results in the creation of an anodic oxide barrier layer on the part’s surface, which is more resistant and durable than the underlying aluminum.

Available materials for anodizing

Determined by its principle, anodizing is only available for conductive materials like metals, but it does not mean that aluminum is the only option.

In fact, anodized metals also include magnesium and titanium. However, as many people wonder if it’s possible to anodize steel or stainless steel, it is not available because iron oxide (rust) will not form a tight, tenacious, corrosion resistant coating on steel, so it can’t be beneficially anodized.

Different types of aluminum anodizing

There are three commonly used types of anodizing, each of which results in a different set of functional and aesthetic properties.

anodized part

Type I – Chromic acid anodize

Type I anodizing uses chromic acid to create a thin coating on the surface of metal parts (up to 0.0001 inches). Though Type I is the thinnest anodizing coating, it still results in parts with increased corrosion resistance. However, Type I also results in the least color absorption when dyed.

Type II – Sulfuric acid anodize

Type II anodizing uses sulfuric acid instead of chromic acid, which results in a slightly thicker surface layer on the aluminum part. Sulfuric acid anodize has a thickness of 0.0002 to 0.001 inches and is better suited for dyeing parts.

Type III – Hardcoat anodize

Type III, often called “hard anodizing”, also uses a sulfuric acid approach but results in a thicker (> 0.001 inches) anodized layer compared to Type II. Hardcoat anodized parts have the best abrasion resistance and color dyeing potential; however Type III may not be ideal for parts with extremely tight tolerances.

Which type of anodizing should you choose? What’s the difference?

Considering the difference application scenario your parts will be used, deciding which type of anodizing process is quite an important step. Here is a quick compare for your reference.

Type I uses chromic acid to create a thin coating on the surface of metal parts. It is commonly used in applications where corrosion resistance is needed, including for aircraft parts.

Type II anodizing uses sulfuric acid to create a slightly thicker surface layer on the aluminum part. It is commonly used for finishing consumer good products, aircraft components, architectural parts and kitchenware.

difference between types of anodizing

Type III is similar to Type II but results in a thicker corrosion resistant layer, making it well suited for parts that must withstand extreme temperatures and chemical exposure. For instance, Type III anodizing is used by the military for robust metal parts.

If you cannot decide which type of anodizing is more suitable for your project, RapidDirect can help. We provide professional suggestions based on your project. Upload your design files and get in touch with us.

What are the color options for anodized aluminum?

One of the most significant advantages of anodizing is its various colors available. The standard anodizing colors include clear, bronze, champagne, and black. RapidDirect also provide color cards with Pantone number for you to choose the specific color you want. Some of the additional anodized aluminum colors can be found in the image below:

Advantages of anodized aluminum

Aluminum is a broadly used material because of its beneficial properties, Though the metal does not rust, it is still susceptible to the elements and it can undergo wear and tear, especially due to oxygen exposure. In this section, we’ll dive into the benefits of using anodization on aluminum parts.

1. Improve material properties

First, the finishing process significantly improves material properties on the part’s surface, including increasing corrosion, scratch and weather resistance.

Further, because the process is electrochemical, the barrier layer created using anodizing becomes a part of the component, meaning that it can’t peel or chip like paint coatings.

In terms of properties, it should be noted that the outer anodized layer of a part has insulating properties, meaning parts may have lower electrical conductivity than before.

2. Better surface effect

The second key reason that many customers choose to anodize their aluminum products is the decorative anodized coating finish.

Anodizing can also apply a color finish to metal parts. There are practically infinite colors to choose, including clear anodized aluminum, black anodized aluminum, blue, gold, grey, red, etc.

Design tips for anodizing aluminum

1. Watch for tolerances

If you know you want to apply the anodizing process to your aluminum component, be aware that the process does add some thickness to the part, which can (albeit marginally) affect part tolerances.

If tight tolerances are critical, consider option for Type I or Type II anodizing or take the extra layer into account in the design stage.

2. Edges and corners

With anodizing, an important design tip is to ensure that all edges and corners of the workpiece have radii of at least 0.5 mm. Part designs should also not integrate any burrs.

The reason for these design considerations is that they help to prevent overheating (and even burning) of the workpiece due to a high concentration of electric current.

3. Consider using other finishing steps

Because anodizing is an electrochemical process, it doesn’t have the same effect as bead blasting or polishing. That is, if a machined part goes straight to anodizing, it is likely that some machine marks or scratches will remain on the finished part’s surface.

For this reason, if a completely uniform surface finish is required, it can be beneficial to use polishing, bead blasting or another mechanical finishing process beforehand. That being said, anodizing will make the surface of a part smoother than before.

4. Work with batches

If you are coloring your aluminum parts or products, it is advisable to anodize them in small batches. This ensures a greater degree of color uniformity, as it can be difficult to exactly match a color from one batch to the next. The ideal scenario for color consistency is to anodize a small batch of small parts at once.

Applications for anodizing aluminum

 Anodizing is a high quality and affordable finishing process which has made it popular for a number of applications across a wide variety of industries. So broad is its use that it is likely you encounter an anodized metal part in the course of your day.

anodized parts

Some of the industries that use anodizing regularly are the aerospace, automotive, architecture, consumer goods and homeware sectors. And while it is impossible to list all the specific applications that use anodized aluminum, here are a few: kitchen equipment, duct covers, light fixtures, food preparation products, photo equipment, radio equipment, electronic casings and more.

How to identify if the anodizing is successful?

There are ways to tell if a part has undergone anodizing. First, you can usually tell by the matte finish that anodizing creates. Beyond that, you can use a simple scratch test. Scrape a coin on the surface of the aluminum part: if a scratch is visible, the part has likely just been polished, not anodized. An anodized part will be totally scratch resistant.

Good anodizing will result in a uniform surface with even color dispersal. Anodizing defects to watch out for on your finished product include anodizing burns, which are caused by high current densities and not enough agitation in the anodizing process.

Conclusion

At RapidDirect, anodizing is one of our integral finishing solutions for metal parts, along with bead blasting, brushing, polishing, electroplating, powder coating and painting. Our expert team is highly knowledgeable in the anodizing process and guarantees high quality aluminum parts to our customers.

To see if anodizing is the best finishing solution for your part or product, or for a quote, just get in touch with a member of the RapidDirect team. We’re at your service!

FAQ

How much does anodizing cost?

One of the reasons anodizing is a popular finishing process is that it is highly cost effective. The cost of the process is dependent on several factors, including part quantity, part dimension and shape, anodizing type (i.e. coating thickness) and color. In short, a complex part that needs to be dyed will cost more to anodize than a simple part with no color finish. Please get in touch with us at RapidDirect for a customer-specific anodizing quote.

Does anodizing wear off? How long can it keep?

The anodizing process creates a barrier layer on the surface of aluminum parts that is bonded on a molecular level. This means that it cannot peel or chip away, unlike paint coatings. A properly anodized part should not wear for many decades. Similarly, dyed anodized parts that are properly sealed should not fade for at least five years, often more. It should also be noted that the thicker the anodized layer (Type III being the thickest), the less wear the part will experience.

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