Conversion coating

Anodising

An aluminium oxide layer grown out of the metal itself — hard, integral and never peels.

The part is made the anode in an acid electrolyte. Oxygen liberated at the surface converts aluminium to aluminium oxide, so the coating is part of the metal rather than something sitting on top of it. That is why anodising does not chip or flake the way a paint can.

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How it compares

Relative to the other finishes we apply. A selection aid, not a specification.

Corrosion resistance Excellent
Wear resistance Very good
Colour range Very good
Edge and recess coverage Good
UV stability Excellent
Repairability Limited

How the process works

Degrease and etch to remove the natural oxide and any handling marks.
Desmut in acid to strip alloying-element smut left by the etch.
Anodise in sulphuric acid at controlled current density and temperature.
Dye or electrolytically colour the porous oxide layer where a colour is specified.
Seal in hot deionised water or a cold nickel-fluoride bath to close the pores.

Thickness and systems

Option	Range	Note
Type II (sulphuric)	5–25 µm	Decorative and protective. The usual architectural and consumer choice.
Type III (hard anodic)	25–150 µm	Thicker, denser, markedly harder. Used where wear dominates.
Type I (chromic)	0.5–7 µm	Thin and less aggressive on the substrate. Mostly aerospace.

Typical properties

Property	Value
Coating thickness, Type II	5–25 µm
Coating thickness, Type III	25–150 µm
Surface hardness, Type III	400–600 HV
Dielectric strength	Up to 800 V
Maximum service temperature	About 80 °C for dyed, higher for clear

Advantages

Coating is integral to the metal, so it cannot peel or flake.
Keeps the metallic character and, in clear, the original brush or polish texture.
Excellent corrosion and UV performance; the colour does not fade in sunlight.
Thin compared with paint, so tight tolerances survive.
Type III gives a genuinely hard, wear-resistant surface for sliding parts.

Limitations

Colour is not perfectly repeatable between batches. Specify a master sample and accept a tolerance.
Masking is limited — it is hard to anodise selectively without visible drag lines.
Not repairable on site. A scratched part has to be stripped and re-anodised.
Thick Type III grows into the section and reduces thread and bore clearances.
Dark dyed colours fade faster in strong UV than clear or bronze.

Design notes

What costs money when it is discovered after tooling.

Allow roughly half the coating thickness per surface when dimensioning — a 20 µm coating adds about 10 µm per side.
Call out thread class after coating, or specify masking; hard anodising will change a thread fit noticeably.
Sharp outside edges anodise thin and can show the bare substrate. Radius or chamfer them.
Blind holes and deep recesses get thinner coverage — expect some fall-off in colour.
Specify the sealing method: hot sealing is cheaper, cold sealing gives better colour retention on dark dyes.

Colour and effect range

Clear (natural silver)
Black
Bronze and champagne
Gold
Blue, red and green
Custom dyed to sample

Standards

ISO 7599 (anodised aluminium for general applications)
MIL-A-8625 Type II / Type III
Qualanod (architectural)
EN 12373

Typical applications

Architectural window and facade profiles
Consumer electronics housings
LED lighting channels and bezels
Sliding and wear surfaces (Type III)
Furniture and display hardware

Other finishes

Electrodeposited organic coating

Electrophoretic coating

Paint deposited by electric current, so every edge and recess gets the same film.

Surface texture, no coating

Mechanical finishes

Brushing, polishing and blasting change the surface texture without adding a coating.

Applied organic coating

Powder coating

The widest colour and texture range of any finish we apply, with a tough cured film.

Sublimation transfer over powder

Wood grain transfer

A photographic wood or stone texture sublimated into a powder coat — the look of timber…

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