Anodising sounds simple in principle, it is an electrical method of producing oxide layers on metal surfaces. However it can be used to impart an extraordinary range of properties. Applications include the creation of colourful finishes, increased durability, corrosion resistance, and electrical properties.
By altering porosity, anodising can make metal surfaces absorb dyes or prepare them for additional surface treatments that would otherwise be impossible. Here are just six interesting facts about anodising.
What does anodising do?
The current causes a stream of positive ions to escape from the metal anode (hence the name). These gradually bore a hole into the surface while depositing a hard structured oxide above. (More details here http://knowledge.electrochem.org/encycl/art-a02-anodizing.htm).
How can you colour metal?
By filling those ionic pores with pigment you can produce a multitude of colours beneath a surface almost as hard as diamond. Light interacts with metal and pigment layers in fascinating ways – penetrating differently according to its wavelength and creating the unique iridescence of anodised surfaces.
Can only aluminium be anodised?
Anodising is also often performed on zinc, titanium, zirconium, magnesium, niobium, hafnium, tantalum, and even on conductive plastics.
Does aluminium really need corrosion protection?
Because aluminium readily forms an oxide layer on exposure to air, many people assume it doesn’t need much more. However, natural oxidation doesn’t provide the same benefits. Anodised surfaces can be made harder, more durable, and certainly more attractive. Components like pistons, chain rings and hydraulic gears are anodised to make them extra hardwearing and corrosion resistant.
Different types of anodising
Anodising can be honed by varying the current, temperature, electrolyte (acid), and treatment time.
A variation called Type 1B usually employs chromic acid and uses a low voltage. It produces a thin coating with high corrosion resistance often needed in aerospace. Type 2 is the most standard method and uses sulphuric acid at room temperature. Phosphoric acid is often chosen for surface preparation before adhesive bonding. Type 3 is performed at low temperature for a longer duration and produces a very hard coat. (Also see https://www.poeton.co.uk/treatments).
Do you need to redesign your product for anodising?
The only possible issue is that the item has to be suspended. Where it contacts the hanger it won’t anodise properly. Usually you can find an inconspicuous attachment point but if not you might want to design one in.
