Aluminum alloys are especially weak in terms of corrosion resistance because the introduction of certain alloying elements, such as copper, iron and silicone, to natural aluminum weakens the overall resistance of the metal to eroding external forces. This weakening can also happen if the aluminum already contains any impurities. Of the various series of aluminum alloys, the two-thousand, four-thousand and six-thousand series are the most prone to corrosion.
Also spelled anodised, anodized aluminum is protected from corrosion through the thickening of the outer surface layer through the process of oxidization. Although this process actually weakens the structure of many other metals, such as iron that corrodes into rust, aluminum is actually strengthened by the formation of aluminum oxide.
Thus, anodized aluminum is utilized in a wide range of industries including: electronics, for protective casings of mp3 players, cameras and computer systems; construction, for door and window trim, siding and roofing; commercial, for items such as cookware, appliances and furniture frames; and industrial manufacturing, for equipment and parts such as actuators, electrolytic capacitors and scales.
As the most commonly anodized metal, anodized aluminum can be formed using a variety of anodizing processes. The three main processes used are: Type I anodizing, or chromic acid anodizing, Type II anodizing, or sulfuric acid anodizing, and Type III anodizing, or sulfuric acid hardcoat anodizing (also known as simply hard anodizing).
However, these are not the only anodizing processes used, just the most common. Other types include boric/sulfuric acid anodizing, thin film sulfuric acid anodizing, organic acid anodizing and phosphoric acid anodizing. For mass production of anodized aluminum, a method referred to as bath anodizing is commonly used. In bath anodizing, a tub is filled with the anodizing acid and the aluminum is immersed, with anodes, or electrochemical cells, attached to the surface layer.
Next, a power supply is turned on, introducing an electrical current that moves through the acidic solution and reacts to the anodes, thus producing oxygen. A cathode is also attached to the tub’s side, producing hydrogen as oxygen is produced. The oxygen reacts to the surface layer of the aluminum and changes it into a thin film of aluminum oxide, the thickness of which depends upon the voltage of the electrical current.