Anodising is an electrochemical process used to produce durable and decorative finishes on components made of aluminium or aluminium-based alloys.

Aluminium, in an untreated form produces a protective oxide coating on exposure to the atmosphere. This oxide is inherently protective, but being very thin provides little resistance to long term corrosion. The coating can, however, be thickened by anodising.

For more detailed technical information about the anodising process

The 3 main types of anodising process in regular use are:

Sulphuric acid anodising

Hard anodising

Chromic acid anodising

Sulphuric acid anodising
Sulphuric acid anodising which is extensively used for general industrial and architectural applications, produces a coating (typically silver or pale grey) which can be left natural or dyed to produce a wide range of colours.

The translucent nature of sulphuric acid anodic oxide coatings enables them to be used on aluminium in conjunction with a wide range of chemical and mechanical pre-treatment.

For more information on chemical and mechanical pre-treatments

Hard anodising
Hard anodising covers a number of processes which are based on the use of specialist acid solutions and high voltage and current density electrical conditions, to produce particularly hard anodic oxide coating with excellent wear and corrosion-resistant properties. These coatings have particular advantages on aluminium used for specialist engineering components where the properties of lightweight plus a hard surface are of particular benefit.

Chromic acid anodising (not used by Alpha)
Chromic acid anodising is normally used for the protection of aircraft and specialist engineering components that require only a thin anodic coating, or for crack detection in castings or forgings. The dove grey finish is often used as a pretreatment for painting, or to give corrosion protection while allowing electrical contact.