An anodized coating is a layer of molecules that is affixed to a metal's surface to help make the metal last longer. This metal becomes easier to maintain and usually is much harder than metal without an anodized coating. To give metal this coat, a sealant, acid and electricity have to be used; otherwise, the whole process will fail to yield a proper coating. Depending on how this is done, the anodized metal may turn out differently. Nearly every metal — except those that refuse to integrate with oxygen or have difficulty doing so — can be anodized.
Unlike many coatings and finishing materials, an anodized coating is not artificial. It is a layer of charged oxygen, or oxide, which forms on the surface of the metal. This thin layer adds some benefits, such as being harder and tougher than the metal was without the coating. Metal that has been anodized also is easier to maintain because of the coating. Without a coating, some metals are prone to break or crack, but this coat keeps the metal smooth and easier to use.
Three things are needed to apply an anodized coating to metal: acid filled with oxygen, electricity from a cathode, and a sealant such as water or a nickel-based solution to keep the layer permanent. After a dip in the charged acid, the metal is removed and placed in the sealant. This is usually easier than other coating methods and typically also is cheaper.
There are four classifications of anodized coating, based on how strong it is and what is done to anodize the metal. Both Type I A and Type I B are thin layers that are exposed to a small amount of electricity and are good for making general parts. With a Type II coating, slightly more electricity is used and the acid is much stronger. The last type, Type III, is the hardest coating and requires going through the anodizing process several times with a solution similar to the Type II coating.
Most metals are capable of taking on anodized coating, but not all. If the metal refuses to interact, or has a very hard time integrating, with oxygen, then the metal cannot be used. Iron, steel and any other ferrous or iron-containing metals are incapable of being anodized because, while iron does react with oxygen, it rusts in its presence.