Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on thesurface of metal parts. But what does that mean?
The name comes from the process itself; the part to be treated acts as the anode electrode of an electrical circuit. This produces an oxide layer on the piece that is thicker than what is usually found on the bare metal. Anodization actually changes the texture of a material at a microscopic level as well as its crystal structure near the surface. Did you know that anodic films can be much stronger and more adherent than most types of paint and metal plating?
Titanium, niobium, zinc, magnesium, aluminium alloys, and tantalum are all metals that can be anodized. However, there are metals that don’t react well to the process like iron or carbon steel. These metals exfoliate when oxidized, which means the new layer will just flake off.
Anodizing a piece of metal will help protect it against corrosion and was first used in 1923 to protect seaplane parts that were constantly exposed to salt water. Anodization also increases wear resistance and provides better adhesion for paint primers or glues than bare metals would. Other uses of anodizing are more cosmetic; body jewelry companies like Anatometal use this process to create colorful pieces of wearable art.
There are quite a few different practiced processes for anodizing and they vary from metal to metal. Titanium and niobium however are quite similar; through anodization these metals generate an array of different colors without the use of any dyes. The color that is formed is dependent on the thickness of the oxide (this is controlled by the anodizing voltage). The interference of light reflecting off the oxide layer with the light actually travelling through said layer and reflecting off the metal underneath is what determines the color you see.
Copper, gold, green, purple, fuchsia, and yellow are just a few of the colors that can be achieved through the anodization of titanium; you can even create an “oil slick” effect with enough practice.
So to review: anodizing a piece of metal changes the texture of that metal which in turn changes how you see it (ie. the color). The process of anodizing both protects and beautifies the material. Let us round this science lesson out with one more anodizing fun fact; anodizing is one of the more environmentally-friendly metal finishing processes as the by products do not contain any heavy metals, volatiles, or halogens. Pretty amazing!