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Rhodium Plating
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Anodizing is the general name applied to methods of treating metals, where the work piece is made the anode in an electrolytic cell, usually to form oxide coatings for the purpose of increasing the performance of the surface. In the case of aluminum, the anodizing process forms a layer of aluminium oxide - Al203 - or corundum, which isvery hard, relatively inert, electrically insulating and can absorb dyes to colour the film. The anodic film itself grows at the aluminium / aluminium oxide interface by the continuous formation and dissolution of a layer of oxide, this is the so-called barrier layer and its thickness is a function of the process starting voltage. A porous, more structured layer forms on top of the barrier layer making up the rest of the coating.

Sulphuric Acid Anodizing


Embraces anything from heavy duty black dyed coatings for high-tech instruments to cheap coloured ashtrays, expected to last in a pub for a few weeks. It also includes architectural anodising primarily for protectingaluminium window frames etc from the elements. The natural colour of these films is light grey; other colours are achieved by dyeing the film.

Chromic Acid Anodizing

Andoising Chromic Acid

Produces coatings of exceptional corrosion and chemical resistance for its film thickness in an electrolyte, which is non-corrosive towards aluminium. These coatings are thin, and relatively soft and generally only used in specialised applications.

Hard Anodizing (MIL-A-8625F)
Is a branch of sulphuric acid anodizing where process conditions have been pushed in a certain direction to achieve significantly harder, thicker, denser films. Applications involve resistance to wear, corrosion, temperature effects etc.

Color will vary from light tan to black depending on alloy and thickness. Can be dyed in darker colors depending on thickness. Coating penetrates base metal as much as builds up on the surface. Provides very hard ceramic type coating. Good dielectric properties. Corrosion resistance is good, but recommended seal hard anodize in 5% dichromate solution where increased corrosion resistance is required. Where extreme abrasion resistance is required do not seal as some softening is encountered. (As specified on drawing. if not specified normal thickness shall be 0.002”)

Type III: Most aluminum alloys depending on process used. Where maximum serviceability or special properties are required, consult metal finisher for best alloy choice. Thick coatings (over .004”) will tend to break down sharp edges. Typical applications: hydraulic cylinders, wear surfaces, actuating cams, etc... Can be used as an electrical insulation coating. “Flash” hard anodize may be used instead of conventional anodize for corrosion resistance and may be more economical in conjunction with other hard anodized areas.

Class 1: Non-dyed
Class 2: Dyed


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