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What Is an Anodized Finish?

Lori Kilchermann
Lori Kilchermann

An anodized finish is a clear or colored finish that is chemically applied to the surface of many metals. Used for a variety of purposes, from reducing thread galling on some fasteners to preventing corrosion on others, an anodized finish adds not only protection to the surface of the metal, it adds strength as well. The color of the anodized finish is not always created by colors or dyes, rather the colors are often directly related to the amount of time the piece was left exposed to the anodizing chemicals. There are, however, a large number of dyes that make it possible to create nearly any color on an anodized product.

Softer metals, such as aluminum, can often benefit from the protective properties of an anodized finish. On surfaces that will be removed and replaced in excess, the anodized finish allows lubricants to be infused into the surface of the threads, adding to the ease of installation. A small amount of dye can be introduced into the chemicals in order to give the appearance of something special to an anodized part, and most anodized parts can be anodized into any color desired. The anodizing film is thick enough that it can require a screw hole to be opened up before attempting to drive the screw into the hole.

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Woman holding a book

Any holes that may be machined into the component being anodized should be bored or machined slightly oversize to take into consideration the anodized finish adding slightly to the inside dimensions of the hole. Failure to do this could result in a hole that is too tight to allow the pin or component into the hole once the part is assembled. For anodized surfaces that are not critical to any lubrication properties, sealing is often required for the surface once the anodizing has been completed. Immersion in very hot water is usually all that is required to close the small holes left in the surface of the anodized metal.

By placing the aluminum into a bath of liquid chemicals that are often nothing more than acetone and applying an electrical current through the aluminum component, a chemical reaction occurs that is similar to a type of rust forming on the aluminum. This rust-like material is the anodized finish and actually works to increase the strength of the aluminum's surface. Similar to gold or silver plating, the color of the anodized finish is dependent on the types of metal sheets immersed into the liquid chemicals.

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