Aluminum anodizing is an electrochemical process that takes advantage of a natural oxidation that happens to the surface of aluminum parts. It improves corrosion resistance, wear resistance, and changes some properties of the part for the better. Many steps are involved in the anodizing process, but it is generally a very cost effective process. This article outlines some of the science behind anodizing, as well as the benefits, when it is good to anodize, as well as a few niche cases where it might be best to use a different finishing process.
The Science Behind Anodizing
As mentioned above, anodizing is an electrochemical process, more specifically it is a form of electrolytic passivation. Passivation simply means removing surface contamination while building up a protective outer layer. For aluminum, this protective outer layer is a form of oxidation. Oxidation is the process that creates rust on mild steel in the form of iron oxide, however for aluminum it is aluminum oxide which is a protective coating rather than corrosive. While the aluminum is submerged in an acidic solution, an electrical current is passed through the solution, this accelerates the natural oxidation process and allows control over the final thickness of this desirable oxide layer.
The anodizing process involves many steps in order to achieve a consistent and durable finish. These steps are outlined below:
- Cleaning – Parts are thoroughly washed and rinsed to remove any oils and surface contaminants
- Etching – An extremely miniscule amount of material is removed from the surface to make it uniform and remove very small imperfections
- Anodizing – Parts are submerged into an acidic solution while electricity passes through the solution, this makes a thicker, stronger version of the naturally occurring oxide layer
- Optional: Dye – Parts are submerged into dye if a color is desired
- Sealing – The pores of the parts are sealed with a sealant creating a very hard and uniform surface finish while still allowing any dye to show through
What Are the Benefits of Anodizing
Increased Wear Resistance
Aluminum oxide is a different material from the underlying raw aluminum. This material has a much higher hardness than aluminum. The hardness is so much higher that some anodizing processes result in the top layer of the aluminum having similar hardness to a hardened steel. When two materials come in contact, the softer material wears while the harder material remains undamaged. Thus this outside hard layer preserves the softer underlying aluminum from wear.
Increased Corrosion Resistance
The aluminum oxide layer created by anodization is largely inert, which means it doesn’t react with most chemicals. This lack of reactivity increases the corrosion resistance of an anodized part. One of the most common ways for aluminum to corrode is through galvanic corrosion. This reaction occurs when dissimilar metals come into electrical contact. For example, if an uncoated aluminum part has an uncoated steel fastener installed, the two will corrode in the presence of moisture. Even as little moisture as humidity in the air can cause galvanic corrosion. Anodized aluminum doesn’t conduct electricity, thus this prevents galvanic corrosion. For more information on galvanic corrosion, see our article about it here.
While the anodizing process does impact the final dimensions of parts, it is a negligible amount for most applications. The thickest anodizing process, type 3 hard coat anodizing, only adds approximately one thousandth of an inch (0.001”) to the thickness. Compare this to powder coating, which adds up to five thousandths to final dimensions.
Anodizing allows aluminum parts to accept dye in order to be permanently colored. This allows for organizational benefits like parts identification. This can be used to keep track of parts that look similar but have a functional difference, such as different revisions of a part, if a part is structural or not, minor thickness variations, end user instruction identification, different alloys, etc.
Heat dissipation increases with surface area. When a part is anodized, the aluminum oxide structure creates microscopic pores. These small pores are what accept the dye, and also have the added benefit of increasing the surface area of the part, thus increasing heat dissipation.
When Should Anodizing Be Avoided?
There are a few niche situations when anodizing should be avoided. Generally speaking, it’s best to wait until all processing on a part has been completed to add any finish. If a part will require post processing after being received from SendCutSend, then it’s probably best to skip anodizing, as those processes will remove the anodizing. Processes such as machining, drilling additional holes, reaming, or welding. Also, the anodizing process significantly reduces the electrical conductivity of the part, to the point of being electrically non-conductive depending on the anodizing type. The final situation is if extremely tight tolerances are required. The anodizing process adds a small amount of thickness to the final dimensions of the parts, on the order of one thousandth of an inch. If a design requires tighter tolerances than that, anodizing might be difficult to account for in the design.
The Difference Between Type 2 and Type 3 Anodizing
Type 2 and type 3 anodizing are very similar processes. The main difference is going to be in thickness, resistance, and color. Type 2 adds between 0.0002”-0.0009” of thickness, while type 3 adds 0.0005”-0.001”. This additional thickness increases corrosion and wear resistance for parts that are type 3 anodized. The main benefits to type 2 anodizing are lower cost, and a larger choice of colors, since type 3 anodize is only offered in clear or black and is a more expensive process. Type 3 anodizing is also known as “hard anodizing” due to the increased wear resistance. Think of type 2 anodizing as good protection, while type 3 is great protection.
What Materials are Available for Anodizing? And When Would I Use Each Material?
5052 H32 Aluminum
Sheet metal bending available
Malleability allows for parts to flex slightly without breaking
6061 T6 Aluminum
Good general purpose material
High strength and moderate malleability
Middle cost of available aluminums
7075 T6 Aluminum
Very high strength to weight ratio
Brittle – doesn’t bend much before breaking
Highest cost of available aluminums
Anodizing is an extremely useful finishing process that can drastically improve the utility of aluminum parts. It protects the part from corrosion and abrasion, while insulating it from electrical current. All this is achieved with minimal changes to final dimensions, and at an affordable cost. Anodizing guidelines are available to provide any information needed to order anodizing services on your next order.
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