Anodized aluminum is aluminum that has been treated with a sulfuric acid bath while a low-voltage current runs through the solution. Hard anodized aluminum has been treated in a similar manner, except the electrical current is substantially higher and the temperature of the solution is lowered to 32°F (0°C). Depending on the process, the solution may also contain oxalic acid. The process creates a material that is thicker, harder and smoother than aluminum that has not been anodized.
Although a number of different metals may be anodized, aluminum reacts differently to the process. Anodizing aluminum produces a thin coating that protects from rust, but certain processes also produce a much thicker rust barrier. The final product has many different uses and is an important part of many automobiles and aircraft. Architects may use anodized aluminum for exterior decorations, and electronics manufacturers value it for its ability to insulate sensitive parts from electrical currents.
Automakers have long been major supporters of the anodizing process for aluminum. Although stainless steel provides some of the benefits of hard anodized aluminum, steel is heavier and more expensive. Anodized products can withstand high temperatures and do not conduct electricity.
Professional cooks and chefs often favor anodized pans for more than the material's ability to withstand heat. Hard anodizing produces a smooth surface that provides a non-stick surface even without the application of a non-stick coating. Delicate foods may be prepared so that they release easily and are more attractive. Cleaning hard anodized aluminum pans typically requires little effort, but they should not be placed in a dishwasher.
The medical field is another area to adapt hard anodized aluminum for its needs. Prosthetics and artificial joints made from anodized aluminum are stronger than those made from regular aluminum, yet weight is not substantially increased. By using hard anodized aluminum, prosthetics designers can create devices that are more durable and operate more smoothly. Implanted devices and joints made from anodized aluminum are more resistant to damaging effects of blood, which can cause such devices to corrode.
Manufacturers can easily add a dye during the anodizing process to produce a variety of colors. The solution penetrates deep into the coating, carrying the dye with it. As a result, the finished product resists fading. Some colors, however, are more fade-resistant than others, with certain shades of red and blue being more prone to fading and black being the least likely to fade. Without the addition of a dye, the color of hard anodized aluminum ranges from dark brown to dark gray.