Anodized Metal
Although the
anodizing process can be very helpful for some metals, it can also be harmful to others; for instance, iron should not be anodized because it produces iron oxide, otherwise known as rust, on the surface of the iron. This exposes the inner layers of the metal to additional corrosion, rather than protecting them. The most common metal that is anodized is
aluminum and its various alloys, particularly because the alloyed elements of aluminum decrease the corrosion-resistance of the metal. Other metals that benefit from anodization include well-known metals such as titanium, tungsten, magnesium, zinc and zirconium, as well as some lesser-known metals such as niobium and tantalum. Since there are multiple metals that can be anodized, there are a variety of industries that utilize them including: jewelry, in which
anodized titanium and tantalum are particularly popular and can be
color anodized as well as left their natural shade; architecture, for siding, decorative panels and various areas requiring trim such as windows and doors; medical, in which anodized titanium is becoming increasingly popular for dental implants; and automotive and aviation, in which anodized magnesium is used in paint to coat the body of the vehicle or plane for protection as well as aesthetics.
There are three main processes of anodizing: Type I chromic acid anodizing, Type II
sulfuric acid anodizing and Type III sulfuric acid
hard anodizing. However, there are other less common processes as well, including boric/sulfuric acid anodizing, thin film sulfuric acid anodizing, nitric acid anodizing and oxalic acid anodizing. The processes are all pretty much the same, except that the acid in each one of them is changed and some may require sealing after anodization while others may not. There are two main ways that metal is anodized: bath anodizing and anodic painting. The more common of the two is bath anodizing, which involves fully immersing the metal in a tub of the anodizing acid. Once the metal is submerged, a power supply is turned on that introduces an electrical current, typically direct current, to the bath. The current moves through the acid and reacts to the anode that is also introduced in the bath, resulting in the production of oxygen. The oxygen reacts to the surface of the metal and changes it to a thin film of metal oxide, such as aluminum oxide or titanium oxide. In addition, a cathode is attached to the side of the bath, which simultaneously produces hydrogen as the anode produces oxygen. A very different anodizing process, in anodic painting there is an anode lead and a cathode output from the power supply. The anode lead is attached to the metal, while the metal ferrule of a paint brush is connected to the cathode output, the tip of which is then dipped in an electrolytic solution. When the power supply is turned on, and the paint brush touches the surface of the metal, anodization occurs.
