Titanium anodizing is an electrolytic finishing process that manipulates the oxide layer on the surface of titanium via electric current. The titanium item forms the anode (positive electrode) of an electrolytic cell; hence the name “anodize.”
Anodizing has a long history in the aerospace industry, where it was first used in 1923 to protect British seaplane parts from salt-water corrosion. Aerospace companies continue to use anodizing processes today to protect metals from effects of aging, wear and corrosion.
The medical device industry also widely uses the titanium anodizing process, since anodized titanium parts are nontoxic and suitable for biomedical applications, such as orthopedic implants.
Anodizing machines manipulate the oxide layer on the surface of titanium to produce an “illusion of color.” The titanium oxide layer gives the perception of color due to an interference phenomenon, similar to a prism. Light reflects from both the oxide layer and the underlying titanium at different angles and those reflections interfere with each other. Certain wavelengths of light cancel each other out or combine, so that the remaining light is perceived as color. Unlike aluminum anodizing, no dyes are required to produce the color perception, thus adding to the biomedical safety of the finished part.
An oxide layer forms naturally on the surface of titanium upon exposure to the atmospheric conditions, as oxygen reacts with the surface of titanium. This process of oxidation naturally occurs with many elements and the thin protective oxide layer helps protect it from further reactions to air or water.
When it first forms, the oxide layer is about 1-2 nanometers thick (10-20 angstroms), but it will continue to grow in open air. Without anodizing, the oxide layer typically grows to 20-25 nanometers (200-250 angstroms).
In color titanium anodization, however, the thickness of the oxide layer is augmented and manipulated. For example, the color bronze – the thinnest color layer on the titanium color spectrum – can be achieved by building the oxide layer thickness to about 300 to 350 angstroms. At the other end of the titanium color spectrum, the color green – the thickest color layer – can be achieved with an oxide layer thickness of 500 to 550 angstroms.
Since the entire range of color for the titanium oxide layer is within 25 billionths of a meter (a nanometer is 1-billionth of a meter), it is little wonder that the process requires careful precision and high-quality anodizing equipment to reach the best results.
It should be noted that not every color can be produced with titanium anodizing. In particular, red anodized titanium is not possible due to the physics of the optical interference phenomenon. The nearest available colors to red would be magenta, rose (reddish violet) or purple.
Here are the most common achievable colors in titanium anodizing: