Anodizing

Anodizing is an electrochemical conversion process that forms an oxide film, usually on aluminium, in an electrolyte (chemical solution). The aluminium parts are the anode (thus the term “anodizing”) and current is passed between them and a cathode, usually flat aluminium bars, through the above mentioned electrolyte (sulphuric acid is most commonly used). There are different types of anodize, most commonly referred to as Type I-Chromic Acid Anodize, Type II-Sulphuric Acid Anodize, and Type III Hard Anodize or Hard coat from the Mil-A-8625 designation. Other less common types are phosphoric acid and titanium anodize. The anodize is a porous structure that grows out of the base aluminium and absorbs coloured dye very well. This is done through a secondary operation with either an organic or inorganic colouring. Manideep processes black, blue, red, gold, as well as small lots of green and gray using standard organic dyes. We also do electrolytic colouring which produces a gray-black colour using inorganic metallic salts. Any coloured anodize is required to be sealed following processing in order to preserve the colour.

Chromic Acid Anodize

Chromic acid anodizing or type I anodize results in the thinnest anodic coat of the principal three types; typically on the order of .00002”-.0001” (20 to 100 micro inches) per surface. While thin, when properly sealed chromic anodize affords the aluminium equal corrosion protection to the thicker sulphuric and hard coat type anodize. Chromic anodize appears much grayer in colour and being thinner absorbs less colour when dyed. This limits chromic acid anodize 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 anodize is lighter in appearance (grayer), than conventional sulphuric black anodize. In order to get chromic anodize to accept black dye, the temperature of the chromic acid must be raised, therefore, it isn’t done every day and must be scheduled accordingly.

Chromic Acid Anodize

Good for tight tolerance parts: will not change dimensions
Can be black dyed – other colours not practical
Good for Bonding
Non-Conductive
Good for welded parts and assemblies

Chromic Acid Anodize Applications:

Precision Machined Components
Aerospace Components
Welded components and assemblies
As a paint/prime base

Boric-Sulfuric Acid Anodize (BSAA)

Boric-Sulphuric Acid Anodize (BSAA) is an alternative to chromic acid anodize (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 I c and aerospace specifications including Boeing’s BAC 5632. 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 Anodize Features:

Good for tight tolerance parts: will not change dimensions
Corrosion protection
Good for Bonding
Non-Conductive

Boric-Sulfuric Acid Anodize Applications:

Precision Machined Components
Aerospace/Aircraft Components
As a paint/prime base

Sulfuric Acid Anodize

The sulphuric acid process is the most common method for anodizing. The sulphuric acid anodize 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 coloured surface finishes on aluminium and its alloys. The porous aluminium oxide absorbs dyes well, and subsequent sealing helps to prevent colour loss in service. Although dyed anodized films are reasonably colourfast, they have a tendency to bleach under prolonged direct sunlight. Some of the colours are: Black, Red, Blue, Green, Urban Grey, Coyote Brown, and Gold. Parts can be treated chemically or mechanically prior to anodizing to achieve a matte (non-reflective) finish.

Sulphuric Acid Anodize Benefits:

Less expensive than other types of Anodize 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 colours.
Waste Treatment is easier than chromic anodize, which also helps to reduce cost.

Sulfuric Acid Anodize Applications:

Optical components
Hydraulic valve bodies
Military weapons
Computer and electronic enclosures
Mechanical hardware

Hard Anodize (Hard Coat)

Hard coat anodize, while usually done in a sulphuric acid based electrolyte, is much thicker and denser than the more conventional sulphuric anodize. Hard coat is specified for aluminium 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 hard coat anodize can be built up to several thousands in some cases, it makes this type of anodize a candidate for salvaging worn or mis-machined components.

Hard Anodize Features:

Improved wear resistance
Non-conductive
Can repair worn surfaces on aluminium
Improve parts surface for slide applications
Can be black dyed; other colours less decorative
Finish is harder than tool steel
Can be ground or lapped

Hard Anodize Applications:

Valves
Pistons
Sliding Parts
Hinge Mechanisms
Cams
Gears
Swivel Joints
Insulation Plates
Blast Shields