What is anodising?
What is Anodising?
Anodising is a process that was developed to produce a thick oxide coating to metals, primarily to prevent corrosion. The process decreases the metal’s susceptibility to wear and tear and can extend its lifespan. The process can also improve the appearance of metals with the addition of a decorative layer of colour. The process is predominantly used on aluminium or aluminium alloys but also on titanium, zinc, magnesium, and their alloys. This article focuses on aluminium as it is the most widely anodised metal.
Pure aluminium does not easily corrode due to its naturally existing oxide layer, but aluminium alloys are more susceptible to corrosion. To help prevent corrosion, a thick aluminium oxide layer needs to be created, by way of anodising.
Anodised aluminium and aluminium alloys are commonly found in harsh environments like the marine industry – in things like ship hulls, oil rigs and marine structures. This is because of their increased protection against corrosion. They are also used widely in aeronautical, aerospace, retail, and construction due to the attractive appearance and the minimal requirements for maintenance. Anodised metals are often used for electrical insulation applications.
Anodised metals with a decorative layer of colour are used in signage and exhibition displays as well as home appliances and scientific instruments. Anodised aluminium is used across a wide range of industries and is one of the most popular metals in the commercial world.
How anodising is done.
Anodising is a process of controlled oxidation by using an electrochemical process where aluminium or aluminium alloy is submerged in a tank filled with an electrolytic conductive solution. The electrolyte solution used depends on in the planned use of the final product. The metal - the anode - is submerged in the tank and a cathode is also placed in the tank. The electrical current flows through the conductive solution between the anode and cathode. Oxygen ions are released by the electrolytic solution which blend with the surface atoms of the metal being treated. The metal subsequently oxidises and a layer of aluminium oxide forms on the treated metal.
The electrochemical reaction creates pores on the surface of the metal which erode into the substrate rather than bind to the outside of it. As the current continues to be applied, the pores will continue to penetrate the substrate. Once the anodising process is complete, the result is a hard aluminium oxide coating which can withstand much harsher environments. The coating is very scratch-resistant.
Anodising does not increase the strength of the underlying metal; it simply forms a hard barrier to the exterior.
Electrolytic conductive solutions
Sulphuric acid is the most widely used solution for colouring anodised parts and gives the material a high-quality metallic finish.
Chromic acid is highly oxidising and produces a finish that this highly resistant to corrosion and effective for paint adhesion.
As the aluminium oxide layer is porous, aluminium, and aluminium alloy can be dyed. Dyeing is preferable to painting or plating as it is forms part of the structure as the colour is absorbed by the oxide layer. The dye will not chip or peel as paint or plating may do.
Disadvantages of Anodising.
Only certain grades of aluminium can be anodised, and it is not possible to anodise stainless steel at all. When dyeing anodised metals, it is not always possible to get an exact match from batch to batch as the process is subject to colour variation, so consistency is harder to replicate.
Advantages of Anodising.
Anodising offers a rich metallic appearance which can be used in a wide range of industries. Anodised metals are easy to maintain and are UV stable, so the colours will not fade in natural light. The oxide layer becomes part of the metal, so the anodised surface finish will not peel, chip, or scratch off. The structure is porous and allows for a secondary process such as sealing. Anodised aluminium or aluminium alloy is easy to clean and highly scratch-resistant, as are other anodised metals.
Several metals can be anodised including aluminium, zinc, titanium, magnesium, zirconium, and hafnium depending on requirements of use. Anodised metals are considered to be very safe to the environment.