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Anodizing is one of the most common surface treatment options available for aluminum. We’ve probably all seen anodized pieces used in a variety of applications, ranging from some of Apple’s iPhones and iPods to sprockets on motorcycles and karts. This article will walk you through the design considerations, as well as the applications, for anodizing.
While we can anodize other materials, we will focus on aluminum in this article, as it’s very widely used and holds a large percentage of the market share for anodized parts. The process is well-suited for aluminum parts made from a variety of different manufacturing processes, such as CNC machining, casting, and sheet forming.
Throughout this article, we will reference military specification MIL-A-8625 for many technical details. This is a common specification used in multiple industries, but it’s good to check for other requirements that may drive another specification.
Anodizing is a process that converts the surface of a metal into an oxide layer through means of an electrolytic process. This natural oxide layer thickness is increased through this process to increase part durability, paint adhesion, component appearance, and corrosion resistance. The image below shows a few parts that have been anodized and then dyed different colors.
The process uses an acid bath and an electrical current to create this anodic layer on the base metal. In simple terms, we’re creating a controlled and durable oxide layer on our component, instead of relying on the thin oxide layer that will create itself. It’s similar to bluing, Parkerizing, passivating, and other surface treatments for steels that are used for corrosion resistance and surface hardening.
You may have caught onto the fact that rust is also an oxide, so why are we intentionally “rusting” metal parts? Well, oxide is not always bad as long as it is the controlled and, in the case of steels, correct composition.
As I mentioned before, this article will reference MIL-A-8625 throughout to correlate with an industry specification. In this specification, there are three types and two classes of anodizing. The three types are as follows:
Type I and IB – Chromic Acid Anodizing
Type IC – Non-Chromic Acid Anodizing as a replacement to type I and IB
Type II – Conventional coating from a sulfuric acid bath
Type IIB – Non-chromate alternative for type I and IB coatings
Type III – Hard Anodize
Each type of anodize is used for a specific reason. Some of those reasons are:
All types may help the adhesion of paints and some other bonding agents compared with bare aluminum. In addition to the anodizing process, some parts may be dyed, sealed, or treated with other materials, such as dry film lubricant. If a part is to be dyed, it’s considered Class 2, whereas an undyed part is Class 1.
These factors, as well as others, will be touched on in more detail below.
By now, you have probably gotten the hint that there are some key elements to consider when designing parts to be anodized. These are easy to overlook (and often are) in the design world. However, knowing and sharing this information will give you a leg up when working with anodized parts.
The first factor we need to consider is the dimensional change associated with anodizing components. On a drawing, an engineer or designer may specify that the dimensions apply after processing to compensate for this change, but with rapid prototypes, we rarely have a drawing, especially if we’re taking advantage of a quick-turn service that relies on solid models.
When a part is anodized, surfaces tend to “grow”. When I say “grow”, I mean that outer diameters will get larger, and holes will get smaller. This is because the anodic layer grows both inward and outward from the part surface as it builds up the aluminum oxide.
The dimensional growth can be estimated to be approximately 50% of the total thickness of the anodic layer. The table below details the thickness range of the different types of coatings, per MIL-A-8625.
These thicknesses will vary, depending on the specific alloy used and the process controls. If a designer is concerned about controlling the growth of a high-precision feature, it may need to be masked. In some cases, such as thick type III coatings, the parts can be lapped or honed to the final dimension, but this will add cost.
Another dimensional consideration is the radii of edges and inside corners because anodic coatings don’t form well on sharp corners. This is especially true for type III coatings, where MIL-A-8625 suggests the following corner radii for a given type III thickness:
With thinner coatings, an edge break in the range of .01-.02 may be sufficient, but it’s best to consult with the anodizing shop to verify this.
Given the increased hardness of the anodic layer, we know that the surface hardness goes up. It is not typical to actually specify the hardness of the coating because of the interaction between a softer base metal and a hard anodic layer. MIL-A-8625 specifies a test for abrasion resistance to accommodate these challenges.
As a frame of reference, however, 2024 aluminum base metal has a hardness in the range of 60-70 Rockwell B, where Type III anodize has a hardness of 60-70 Rockwell C. The image below is one of my CNC hold down clamps that has been anodized and dyed red.
The surfaces show almost no wear, despite the tough application of holding down hardwoods, engineered plastics, and non-ferritic metals in a high vibration environment.
As we can see above, anodized coatings can be dyed. This may be done for a variety of reasons, such as aesthetics, stray light reduction in optical systems, and part contrast/identification in assemblies.
When it comes to dying anodize, it’s also important to work through expectations with the shop that is going to anodize the parts. Some challenges to discuss with your suppliers are:
These lists are not intended to be fully comprehensive, but they should give you a great start on making the parts you want the first time.
Anodic layers are good insulators, despite the base metal’s conductivity. For this reason, it may be necessary to apply a clear chemical conversion coating and mask certain areas if there is a need to ground to a chassis or component.
A common way to determine if an aluminum part is anodized is to test the conductivity of the surface with a digital multimeter. If the part is not anodized, it will likely be conductive and give a very low resistance.
Anodized parts can also have secondary process to either coat or treat the anodized surface for increased properties. Some common additions to the anodic coating are:
There are other processes that can be applied to change the properties of anodic coatings, but they’re less common and will likely require a specialized vendor. If you have a special need, it’s best to reach out to a coating specialist.
Anodic coatings have a broad range of applications, as we’ve seen in this article. The most common is probably for aesthetics because of the ability to dye these components. The images below are of thumb drives that have been anodized and dyed, simply for aesthetic purposes.
We can see that the top thumb drive is class 1 (undyed), so the anodic coating appears clean, and the color is almost that of the base alloy, while the bottom has been dyed blue. In another example, we can see a thread adapter that has been anodized and dyed black.
This coating hasn’t been nearly as wear-resistant as the hold-down clamps above, which indicates that it may be a thinner type II, or the process controls were poor. In the third example, we have some anodized heat sinks.
Anodic coatings can actually improve the effectiveness of a heat sink by increasing the surface emissivity an order of magnitude over bare aluminum, which improves the radiation heat transfer.
In the final example below, we can see a matte black valve that has a relatively high-quality anodic layer. To date, the part doesn’t show any signs of wear. Another interesting characteristic of the part below is the laser etching that allows the logo to contrast with the rest of the component. It is common to etch through the anodic layer, instead of printing on the surface of the part. This may be done to increase the durability of the logo, or to save cost and process steps.
While this is a broad range of examples, it still doesn’t come close to capturing the numerous applications for anodized parts. If you have an aluminum part and need any corrosion protection or aesthetic enhancements, I encourage you to look into the anodizing process.
As with any process, there are some warnings that need to be discussed or recapped from the information above. This list is not fully comprehensive but will touch on the top considerations when designing parts for anodize.
Anodizing is a great surface treatment option for corrosion and wear resistance, and the applications extend far beyond that. We can use anodizing to improve paint adhesion, provide a good surface for impregnation, and improve other surface qualities.
Furthermore, it’s a process that can expand into improved aesthetics. As long as the key factors mentioned in this article are adhered to, adding an anodize process to your components should be easy to implement.