Aluminum has an oxide layer that is resistant to corrosion and wear. Pure aluminum does not easily corrode because of this. However, aluminum that contains other alloying elements can be subject to corrosion unless a thick aluminum oxide layer can be made to protect it. In order to prevent the corrosion and abrasion of an aluminum alloy, techniques have been created to create this thick, protective aluminum oxide layer. One way of forming this oxide layer is through a process called anodizing. But what is anodizing?
What is Anodizing?
Anodizing is a process that is used to promote the formation of an aluminum oxide layer on a base material more rapidly or with greater thickness than it normally would under natural conditions. While anodizing does work for several other base materials, aluminum responds the most effectively to anodizing. Anodizing first became popular in the 1920’s as a means to prevent the corrosion of aluminum components. Since then, it has been used not only for corrosion resistance, but also for wear resistance and dyeing aluminum. Since aluminum oxide is not nearly as conductive as aluminum, it can be also used for electrical insulation purposes. Anodizing has many benefits, but it is important to note that it will not increase the strength of the aluminum underneath the anodized surface.
How is Anodizing Done?
Anodizing aluminum is considered an electrochemical process. It involves taking an aluminum alloy and submerging it in a tank filled with an electrolytic solution. This solution contains acid; the type of acid depends on the application. Once submerged, an electrical current is passed through the aluminum. The aluminum being anodized serves as the anode. A cathode is placed into the tank as well; usually aluminum or lead. The electrical current causes the aluminum to oxidize. The anodizing process leaves a layer of aluminum oxide thicker than what can be achieved through natural oxidation.
What is Anodizing Used For?
Anodizing is most commonly used for improved corrosion resistance on certain types of aluminum alloys. Aluminum alloys that are subject to marine environments typically benefit from anodizing. Ship hulls, dock components, and oil rig structures are common examples of these.
Anodizing is also used for abrasion control. Aluminum that has not been oxidized is a relatively soft material when compared with steel or titanium. Aluminum oxide, on the other hand, is an extremely hard material. In fact, aluminum oxide is often used in sandpapers because of its high hardness. When the anodizing process forms an aluminum oxide layer on the outside of an aluminum alloy, it greatly increases its wear resistance because aluminum oxide is such a hard material. Applications where anodizing is used for wear resistance include aluminum components that are subjected to constant movement and contact with other materials.
Dyeing is another popular application of anodized aluminum. The aluminum oxide layer that is created on an aluminum alloy during the anodizing process is porous. This allows some dyes to be absorbed by the oxide layer. Aluminum alloys that couldn’t be dyed before can now be made to be a variety of colors. Applications of dyeing anodized aluminum include artwork and and aluminum signs.
What Metals Can Be Anodized?
Aluminum is the most commonly anodized material. However, there are several other types of materials that can be anodized. Magnesium can be anodized but its applications are very limited. Titanium is perhaps the second most commonly anodized material, though still nowhere near as popular as aluminum. Some materials simply should not be anodized. Carbon steel will simply corrode if it is brought through an anodizing process.
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