What Is Anodizing? Types, Thickness and Design Tips
The difference between decorative and hard anodizing, how coating thickness affects dimensions, colouring, masking and design tips before anodizing.
Table of contents 6 sections
Anodizing is an electrochemical process that grows a controlled oxide layer on aluminum parts. It protects against corrosion and wear, can be coloured and provides an electrically insulating surface.
In short: choose standard (Type II) anodizing for appearance and general protection, and hard anodizing (Type III) for wear resistance. Because the layer changes dimensions, account for it on precise fits.
How does anodizing work?
The part is connected as the anode in an acid bath and current is passed through it. The surface aluminum converts into a porous aluminum oxide layer. These pores can be filled with dye and then sealed to make the layer permanent. Anodizing is not a paint or plating but a transformation of the material’s own surface.
Types of anodizing
| Type | Typical thickness | Characteristics | Uses |
|---|---|---|---|
| Type II (standard / decorative) | about 5–25 µm | Can be coloured, good corrosion protection | Enclosures, panels, consumer products, general parts |
| Type III (hard anodizing) | about 25–75 µm | Very hard and wear resistant, usually dark grey/black | Sliding surfaces, pistons, guides, wear parts |
| Type I (chromic) | thin | Thin layer, minimal effect on fatigue strength | Specialised applications such as aerospace |
Effect on dimensions
The anodic layer grows roughly half into the surface and half outward. A 20 µm layer therefore increases each surface by about 10 µm, while a hole diameter shrinks by about 20 µm. The effect is more pronounced with hard anodizing.
Watch out: account for the anodizing allowance on precise fits such as bearing seats, pin holes and threads, or ask for these surfaces to be masked (left unanodized). State on the drawing whether dimensions apply before or after anodizing.
Colouring
Standard anodizing can produce colours such as black, red, blue and gold. The shade can vary slightly between batches depending on alloy, machined surface and bath conditions. Where appearance is critical, request sample approval and ask for all parts in an order to be anodized in the same batch.
Design tips
- Radius sharp edges: the oxide layer does not form evenly on sharp edges and can crack.
- Plan a racking point: the part is held at a contact point in the bath, which leaves a mark. Specify a hidden area as the racking point.
- Mask electrical contact areas: anodizing insulates, so grounding surfaces must be left bare.
- Alloy choice: 6061 gives a uniform, attractive result; alloys high in copper or silicon may show colour variation.
State in your quote request
- Anodizing type (Type II / hard anodizing) and colour
- Surfaces and threaded holes to be masked
- Whether dimensions apply before or after anodizing
- Visible surfaces and acceptable racking point
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