Anodizing is a protective and decorative surface treatment used to enhance the working qualities and visual appeal of items made from a range of metals including aluminum alloys, zinc and titanium. The treatment involves manipulation of the natural oxide layers on the metals to produce thicker and more durable films. These enhanced oxide layers lend the items increased resistance to wear and corrosion and provide surfaces which are more receptive to paints, dyes, and adhesives. When applied thinly, anodized films also tend to cause light interference resulting in attractive surface patterns and multicolor effects. In addition to the improved wear and corrosion resistance offered by anodizing, treated parts are also less inclined to exhibit galling of friction surfaces.
Oxide formation on the surfaces of metals is a naturally occurring phenomenon that results from exposure to oxygen and moisture in the air. Although oxidation on ferrous metals, also known as rust, can cause the eventual destruction of the material, metals such as aluminum alloys, zinc, titanium, magnesium and tantalum can benefit from an oxide layer. If manipulated to be thick enough, these oxidative layers can offer corrosion and wear resistant properties to these metals. This is the principle that underpins the anodizing process used to impart protective and attractive finishes on many non-ferrous metal items.
The anodizing process involves passing an electric current through an electrolyte solution between a positively charged anode, in this case the anodized item, and a negatively charged cathode. This resultant reaction changes the crystal structure of the anode surface and causes a layer of oxide to be deposited on it in what is known as an electrolytic passivation process. The characteristics of this oxide film can be manipulated during this process, thereby allowing for a high degree of control over the end result. Generally the synthesized layers are more robust than those occurring naturally. As a matter of interest, the anode role played by the product is the source of the anodizing name.
Anodized oxide layers are generally fairly porous by nature and require the application of sealant to ensure maximum corrosion and wear resistance. The film's adhesion to the metal is far stronger than conventional plating or painting films though, thus making anodized finishes particularly durable. This durability offers an excellent base for the post-treatment application of paints and dyes, with colored anodized finishes exhibiting outstanding longevity even with continuous use. Anodizing also helps prevent galling, or adhesive wear, of threaded or sliding parts at their points of friction.