Anodising is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts.
The aluminum parts are the anode (thus the term “anodising”) and current is passed between them and a cathode, usually flat aluminum bars, through the above mentioned electrolyte (sulphuric acid is most commonly used).
There are different types of anodise, most commonly referred to as Type I-Chromic Acid Anodise, Type II-sulphuric Acid Anodise, and Type III Hard Anodise or Hardcoat from the Mil-A-8625 designation.
Other less common types are phosphoric acid and titanium anodise.
The anodise is a porous structure that grows out of the base aluminum and absorbs colored dye very well. This is done through a secondary operation with either an organic or inorganic colouring. Any coloured anodise is required to be sealed following processing in order to preserve the color.
Chromic Acid Anodise:
Chromic acid anodising or type I anodise results in the thinnest anodic coat of the principal three types; typically on the order of 0.5 µm to 18 µm (0.00002" to 0.0007") which are more opaque films that are softer and ductile, and to a degree self-healing.
While thin, when properly sealed chromic anodize affords the aluminum equal corrosion protection to the thicker sulphuric and hardcoat type anodise.
Chromic anodise appears much greyer in color and being thinner absorbs less color when dyed.
This limits chromic acid anodise as a decorative finish, however, it can be dyed black for use as a non-reflective, protective coating on housings for optical components. Even black dyed chromic anodise is lighter in appearance (greyer), than conventional sulphuric black anodize.
Chromic Acid Anodise Features:
Chromic Acid Anodise Applications:
Boric-Sulphuric Acid Anodise (BSAA):
Boric-Sulphuric Acid Anodise (BSAA) is an alternative to chromic acid anodise (CAA) due to the environmental, worker safety and health concerns and the related costs associated with continued use of hexavalent chromium-bearing processes such as CAA. Primary applications include aircraft and aerospace components.
It is covered by MIL-A-8625, Type Ic and aerospace specifications including Boeing's BAC 5632. Coatings of moderate thickness 1.8 µm to 25 µm (0.00007" to 0.001").
It is used for corrosion protection and paint adhesion. Paint adhesion is equal or superior to chromic acid, and the process is more energy-efficient than chrome-based processes.
Boric-Sulphuric Acid Anodise Features:
Boric-Sulphuric Acid Anodise Applications:
Sulphuric Acid Anodise:
The sulphuric acid process is the most common method for anodising. The sulphuric acid anodise process films range from .0001"-.001" thick. The overall thickness of the coating formed is 67 percent penetration in the substrate and 33 percent growth over the original dimension of the part. It is particularly suited for applications where hardness and resistance to abrasion is required.
However, where parts are subjected to considerable stress, (such as aircraft parts), the possible presence of the corrosive acid residue is undesirable.
The porous nature of Sulphuric acid films prior to sealing is used to particular advantage in the production of colored surface finishes on aluminium and its alloys.
The porous aluminum oxide absorbs dyes well, and subsequent sealing helps to prevent color loss in service. Although dyed anodised films are reasonably colorfast, they have a tendency to bleach under prolonged direct sunlight. Some of the colors are: Black, Red, Blue, Green, and Gold. Parts can be treated chemically or mechanically prior to anodizing to achieve a matte (non-reflective) finish.
Sulphuric Acid Anodise Benefits:
Sulphuric Acid Anodise Applications:
Hard Anodise (Hardcoat):
Hardcoat anodise, while usually done in a sulphuric acid based electrolyte, is much thicker and denser than the more conventional Sulphuric anodise.
Hardcoat is specified for aluminum components subject to extreme wear applications where superior abrasion resistance is needed, or corrosive environments where a thicker, harder, more durable coating is necessary.
It can also be valuable where enhanced electrical insulation is required.
Since hardcoat anodize can be built up to several microns thick in some cases, it makes this type of anodising a candidate for salvaging worn or mis-machined components.
Hard Anodise Features:
Hard Anodise Applications:
Anodised titanium is used in a recent generation of dental implants. An anodised oxide layer has a thickness in the range of 30 nanometers (1.2 x 10-6 in) to several micrometers. Standards for titanium anodising are given by AMS 2487 and AMS 2488.
Anodising titanium generates an array of different colors without dyes, for which it is sometimes used in art, costume jewellery, body piercing jewellery and wedding rings. The color formed is dependent on the thickness of the oxide (which is determined by the anodising voltage); it is caused by the interference of light reflecting off the oxide surface with light travelling through it and reflecting off the underlying metal surface.
Titanium Anodise Applications: