Anodising

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:

Good for tight tolerance parts: will not change dimensions

Can be black dyed - other colors not practical

Good for Bonding

Non-Conductive

Good for welded parts and assemblies

Chromic Acid Anodise Applications:

Precision Machined Components

Aerospace Components

Welded components and assemblies

As a paint/prime base

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:

Good for tight tolerance parts: will not change dimensions

Corrosion protection

Good for Bonding

Non-Conductive

Boric-Sulphuric Acid Anodise Applications:

Precision Machined Components

Aerospace/Aircraft Components

As a paint/prime base

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:

Less expensive than other types of Anodise with respect to chemicals used, heating, power consumption, and length of time to obtain required thickness.

More alloys can be finished.

Harder than chromic anodize.

Clearer finish permits dying with a greater variety of colors.

Waste Treatment is easier than chromic anodize, which also helps to reduce cost.

Sulphuric Acid Anodise Applications:

Optical components

Hydraulic valve bodies

Military weapons

Computer and electronic enclosures

Mechanical hardware

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:

Improved wear resistance

Non-conductive

Can repair worn surfaces on aluminum

Improve parts surface for slide applications

Can be black dyed; other colors less decorative

Finish is harder than tool steel

Can be ground or lapped

Hard Anodise Applications:

Valves

Pistons

Sliding Parts

Hinge Mechanisms

Cams

Gears

Swivel Joints

Insulation Plates

Blast Shields

Other Anodises:

Titanium Anodise:

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.

Used for coloring of titanium

Used to aid in looking for stress cracks in base material

Improved corrosion resistance

Improve adhesion of dry film lubricant or paint application

Titanium Anodise Applications:

Aircraft parts

Medical devices

Spacecraft components