To perform aluminum anodizing, manufacturers employ one of three main processes: chromic acid anodization (Type I), sulfuric acid anodizing (Type II), and sulfuric acid hardcoat anodizing, or hard anodizing (Type III).
Type I is the oldest and most frequently utilized anodizing process. To work, operators match a reactive metal with an electrolytic process that uses chromic acid; chromic acid is a corrosive, oxidizing acid that is compatible with most aluminum alloys. They initiate anodizing by attaching an oxygen-producing anode to the metal being treated and then immersing the metal into an electrolytic solution through which a direct current passes.
Throughout the process, the voltage of this current is increased. As the anode produces oxygen, it creates an oxide film. Type I is a good option for anodizing, but it reduces the thickness of aluminum only by .02 to .4 mils, which is significantly less than the rates of reduction possible with the other two types. Also, because the EPA (Environmental Protection Agency) has marked the emissions of chromic acid as harmful to the environment, the possibility of its use is limited.
Types II and III both use another corrosive solution, a sulfuric acid solution. This solution, an acid formed from sulfur dioxide, is dense and oily. Both processes are quite similar, but they differ in terms of performance temperature and current density; Type III aluminum anodizing is performed at lower temperatures and at a higher electrical current density, which yields increased anodic growth and a much harder surface.
In addition to these, manufacturers may employ a number of sub-processes, such as clear anodizing, color anodizing, black anodizing, titanium anodizing, and a number of custom anodizing procedures. Clear anodizing, the most common type of anodize coating, uses sulfuric acid and adds a hot water seal at the end.
While clear anodizing is often used for automotive applications, color anodizing is usually used for aesthetic applications. This process is usually performed after the piece has been initially anodized and sealed. To create colors like yellow, black, and white, color anodizing processes make use of either metallic salts or organic dyes.
There are many applications for which non-aluminum anodized materials are employed. Titanium, for example, is frequently anodized for the improvement of jewelry and other decorative pieces. Anodized titanium is increasingly used with wedding bands, because it is hard and elicits a low allergic response. Lesser known metals like tantalum and niobium are also anodized for decorative purposes.
Another metal anodized with alternative methods is zinc. Though it is not often anodized, when it is, it is done via a process introduced by the International Lead Zinc Organization that creates an olive-green, hard, and corrosion resistant zinc.
Anodized magnesium is fairly common; it is commonly used as a primer for paint and it may be sealed using oil, wax, and similar materials.
Unfortunately, however, anodizing Types I, II and III are all processes that can only be applied to aluminum and aluminum alloys. For anodizing of non-aluminum metals, manufacturers may engage in a variety of similar processes.
One such process is chromate conversion, the results of which, while quite similar, are achieved differently than anodizing. Chromate conversion mainly differs from anodizing in that it allows the electrical conductivity of aluminum to remain intact, rather that neutralizing it. Most often, chromate conversion uses either hexavalent chromium or non-hexavalent chromium.
The chemical compound hexavalent chromium, which contains chromium in its +6 oxidation state, is, unfortunately, categorized by the EPA as a human carcinogen. To avoid the issues attached to this categorization, many manufacturers use non-hexavalent chromium, an environmentally-friendly alternative that meets the requirements of both the Restriction of Hazardous Substances and the European Union End-of-Life Vehicle recycling directives.