The anodising process
Aluminium components are anodised by placing them in an acid, applying a voltage (the component is made the anode -- hence the term anodising) and artificially growing their own oxide to an even controlled thickness. This anodic thickness is not a surface coating, but is integral with the metal so a cut through it will not allow corrosion to creep between the metal and oxide.
Sulphuric acid anodising
The most common form of anodising is sulphuric acid anodising which offers an anodic film thickness beteen 3 and 25microns, depending on the end use of the components. The lower to middle ranges are ideal for decorative finishes which may be dyed, etched and/or brightened to enhancve the component's appearance. Anodic film thicknesses above 15microns are used for increased durability and external applications, where corrosion reistance and wear are the main criteria.
Chromic acid anodising
Chromic acid anodising produces an anodic film thickness between 3 and 7micorns. It does not have the same wear properties as sulphuric acid, but is more ductile and has frequent aerospace and militatry application, where it also provides a good "key" for paint and bonding. Chromic anodising is opaque in appearance and has the ability to detect cracks in cast components and emphasise grain structure in forgings.
Alocrom
Alocrom is the creation of a chromated conversion coating on aluminium prior to painting or powder coating. It provides a temporary protection and has electrically conductive properties.
Powder coating
Powder coating is an electrostatic process where powdered pigment is sprayed from a gun and is electrically attracted towards a component. Electrostatics ensures pigment is attracted to all exterior faces and no solvent is required. Finishes can range from high-gloss and metallic, to textured or clear.
Wet painting
Wet painting involves spraying a pigment suspended in a solvent onto a component which is rotated to give complete coverage. Almost any shape of object can be coated, including those with cavities.
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http://www.plasticcoatings.co.uk/processes.html