From satellites in space through to iPods and even the world’s tallest skyscrapers, anodised parts are used in many industrial and commercial applications to enhance corrosion and wear resistance.
We will be looking in more detail at the popular process of anodising in this second blog on finishing techniques.
Enhancing corrosion and wear resistance
Anodising is an electrolytic process that thickens the naturally occurring oxide layer that forms on the surface of aluminium when it is exposed to air. This creates a hard, protective layer. The transparent anodic surface is integral to the metal rather than just a coating.
The process was first used in the 1920s to protect Duralumin seaplanes from corrosion but it remains popular to this day in the architectural, building and consumer markets.
Most anodising is conducted on aluminium and aluminium alloys, although it can also be used on magnesium, titanium and other metals.
Anodising offers many benefits, including:
- long-term corrosion resistance
- low maintenance
- high resistance to scratches and chips
- enhanced aesthetic appeal
- an electrically insulating coating
- an excellent base for paint
- low environmental impact
How the process works
The cast part is first cleaned and pre-treated using either chemical or mechanical processes. It is then immersed in an electrolyte containing an acidic solution.
An electric current is passed between the part (the anode), the electrolyte (the acid) and a cathode. Using electrolysis, a thick oxide layer is formed. The part is removed once the required thickness has been reached and it is rinsed to remove any acid. The component is then sealed to remove any porosity and create a smooth finish.
Choosing the right materials
Two different treatments are commonly used in anodising.
Sulphuric acid anodising is the most popular technique and it is often used where high corrosion resistance and enhanced aesthetic appeal are required. It can produce an anodic film thickness of 5 to 20 microns.
Chromic acid anodising produces a much thinner anodic film of between 2 to 3 microns. It is often used for applications where subsequent painting/bonding is required or where crack detection is sought. This process is also the preferred option when the loss of fatigue strength (due to anodising) needs to be minimised.
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