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