Aluminum anodizing is a surface hardening process in which electrolytic passivation is used to thicken the layer of oxide that naturally occurs on aluminum alloys. Used to increase corrosion and wear resistance, anodizing can be performed on a variety of metals, such as titanium anodizing; however, amongst anodizers, aluminum is typically the metal of choice since it is naturally prone to corrosion due to alloying elements such as copper and iron.
There are three main processes that are used in aluminum anodizing: chromic acid anodization (Type I), sulfuric acid anodizing (Type II) and sulfuric acid hardcoat anodization (Type III), also known as hard anodizing. Other sub-processes fall within these three main categories including clear anodizing and color anodizing, such as black anodizing. Titanium anodizing, which is closely related to aluminum anodizing, is also known for its color variety although it achieves different shades without using dye. In addition, custom anodizing processes are available. These main processes are specifically methods for anodized aluminum; other processes must sometimes be used to achieve other types of anodized metal. Some processes, like chromate conversion have similar methods and results to anodizing, but still have key factors of difference that separate them, such as retaining electric conductivity or not.
Aluminum anodising is utilized in a vast array of applications, in industries such as: electronics, for protective outer casings for items such as cameras, mp3 players and satellites; food and beverage, for resilient cookware such as pots, mixers and pans; architecture, for structural durability in areas such as window frames, roofs and exterior surface panels; automotive, for aesthetic purposes such as trim as well as protective housing of exposed parts in auto shops; and industrial manufacturing, for plant equipment such as scales, electrolytic capacitators and conveyors.Aptly named Type I, chromic acid anodizing is the oldest and most common anodizing process. An electrolytic process using chromic acid, which is a corrosive, oxidizing acid compatible with most aluminum alloys, anodizing occurs when a reactive metal to which an anode is attached is immersed in an electrolytic solution and a direct current is passed through the solution.
While the anode produces oxygen, creating the oxide film, a cathode attached to the tub filled with solution simultaneously produces hydrogen. A downside of the Type I process is that it can reduce the thickness of aluminum by .02 to .4 mils; however, Type II and Type III may reduce the thickness by several times as much. In addition, the Environment Protection Agency (EPA) restricts the use of chromic acid emissions because they can be harmful to the environment. Type I also differs from Types II and III not only in the type of acidic solution used, but also because in Type I the voltage of the DC is ramped up throughout the process. Types II and III both use a sulfuric acid solution, which is a corrosive, dense and oily acid formed from sulfur dioxide. Type III differs from Type II in that it is performed at lower temperatures and at a higher electrical current density, which provides increased anodic growth making for a much harder surface.
With so many sub-processes, aluminum anodizing has a wide range of capabilities. For instance, clear anodizing is a type of sulfuric acid anodizing that is followed by a hot water seal. As the most common anodize coating, clear anodizing is widely used for automotive applications. On the other hand, color anodizing is generally performed for aesthetic purposes and utilizes either organic dyes or metallic salts to achieve a wide range of colors such as black, white and yellow. Also a type of sulfuric acid anodizing, the color is typically added after the anodization has occurred and then sealed. A process that is similar to anodizing, but can be used as an alternative, is chromate conversion. A type of conversion coating, the main aspect of chromate conversion that differs from anodizing processes is that it allows the electric conductivity of the aluminum remain intact.
There are two main types of chromate conversion and they use either hexavalent chromium or non-hexavalent chromium. Hexavalent chromium is a chemical compound that contains chromium in its +6 oxidation state and is considered by the EPA to be a human carcinogen. However, non-hexavalent chromium is an environmentally friendly alternative that meets both the Restriction of Hazardous Substances and the European Union End-of-Life Vehicle recycling directives.Although there are a wide range of processes that fall under the heading of aluminum anodizing, they all contain the essentials of the basic process. Although often compared to applied coatings and electroplating, anodizing is technically an electrochemical conversion process because it converts the surface aluminum to aluminum oxide.
Although aluminum is most popular, there are different metals that may be anodized including titanium, magnesium, zinc, niobum and tantalum. Titanium is most often anodized for jewelry and other decorative applications; in fact, it has recently become a popular choice for wedding bands due to its hardness and low allergic response. Anodized magnesium, comparatively, is often used as a primer for paint and can be sealed with materials such as oil or wax. Although zinc is rarely anodized, a process has been developed by the International Lead Zinc Organization, which produces hard, corrosion resistant anodized zinc in an olive-green color. As lesser-known materials, anodized niobium and anodized tantalum are both used in much the same way as anodized titanium; for decorative applications such as jewelry and even commemorative coins.
The thickness of the oxide film for these two metals depends on the voltage of the anodizing current. This short list of metals that are improved rather then destroyed by the chemical acid bath that is anodization is always topped with aluminum, because it responds the best to the treatment. Common now in numerous industries and applications, anodized aluminum is a material that has been with us since the early nineteen twenties, and will continue to stay for a long time.