Shenzhen Alu Rapid Prototype Precision Co., Ltd.
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Short answer: Anodizing is an electrochemical process that grows a hard, corrosion-resistant oxide layer on aluminum. Use Type II for colored, cosmetic parts and Type III (hard anodizing) where wear resistance matters. The layer grows inward and outward roughly 50/50, so dimensions, threads, and bores must be planned for it.
This guide covers the types, color options, design rules, and cost — the things that decide whether your anodized parts come back as expected.
How Anodizing Works
Aluminum naturally forms a thin oxide layer about 4 nm thick. Anodizing, in an acid electrolyte with an electrical current, thickens that layer by hundreds of times, creating a porous structure that accepts dye and then seals. Unlike paint or plating, the oxide is integral to the metal — it cannot chip or peel off.
The Main Types of Anodizing
| Type | Electrolyte | Thickness | Best for |
|---|---|---|---|
| Type I (chromic) | Chromic acid | 0.5–7 µm | Aerospace fatigue-critical parts; thin, non-structural |
| Type II (sulfuric) | Sulfuric acid | 5–25 µm | Colored cosmetic parts, corrosion resistance |
| Type III (hard) | Sulfuric acid, cold | 25–50 µm | Wear surfaces, sliding parts, industrial use |
Type II is the default for most product enclosures and visible parts. Type III is chosen when the surface must resist abrasion — think sliding components, tooling, and wear pads.
Color Options
Because the oxide layer is porous, it can be dyed before sealing. Common options:
Clear (natural) — silvery, keeps the metal look, cheapest and most consistent
Black — the most popular dyed color; slight variation is normal
Blue, red, gold, green, grey — standard dyes, batch-dependent
Custom color matching — possible but requires a sample and may need a minimum quantity
Two practical warnings: color is never an exact match to a Pantone swatch (it depends on alloy, thickness, and bath), and dyed colors fade under prolonged UV exposure. If color accuracy is critical, approve a physical sample first.
How Anodizing Changes Dimensions
This is where most problems come from. The oxide grows both into the aluminum and out from the surface. A Type II layer of 20 µm total builds up approximately 10 µm outward per surface; Type III builds roughly 25 µm outward per surface.
| Feature | Effect of anodizing |
|---|---|
| External dimension | Increases by 2× build-up |
| Internal bore | Decreases by 2× build-up |
| Threads | Tighten — often need masking or re-tapping |
| Sharp edges | Round slightly; can chip or burn |
| Deep engraving | May fill in or become less legible |
Design rule: on any dimension that has a tight fit, leave clearance for the coating, or specify masking. On precision bores and threads, masking is usually the right call.
Alloy Matters
Not all aluminum anodizes equally:
6061 — the best all-round choice: uniform color, clean finish
7075 — accepts dye but color can look darker or uneven due to copper content
2024 — poor anodizing appearance in most cases
5000 / 6000 series — generally good results
Castings and high-silicon alloys — often blotchy; may need a special process
Cost
Anodizing is usually quoted per batch rather than per part, because parts are racked and processed together. Typical added cost:
| Option | Typical added cost |
|---|---|
| Clear Type II | +$10–$40 / part (batch dependent) |
| Dyed Type II (e.g. black) | +$15–$50 / part |
| Hard anodizing Type III | +$25–$80 / part |
| Masking | Usually the largest surcharge — price per masked area |
Masking is often the hidden cost driver. Reducing the number of masked features reduces the price more than switching from clear to black.
Design Checklist
Allow for build-up on fits and bores
Specify masking on threads, bores, and electrical contact surfaces
Avoid razor-sharp edges — they burn or chip
Choose 6061 when color consistency matters
Approve a physical sample before production color runs
State the standard, e.g. "anodize Type II, black, per MIL-A-8625"
For the full range of finishes, see our surface finishing capabilities and our surface finish guide.
FAQ
What is the difference between Type II and Type III anodizing?
Type II is a decorative and corrosion-resistant layer of 5–25 µm that accepts dye well. Type III (hard anodizing) is 25–50 µm and much harder and more wear resistant, with more limited color options.
Does anodizing change part dimensions?
Yes. Type II adds roughly 10 µm outward per surface; Type III roughly 25 µm. Internal bores shrink by the same amounts.
Can anodized aluminum be machined afterwards?
Yes, but you remove the coating locally, exposing bare metal. It is better to mask before anodizing.
Why is my black anodizing not perfectly uniform?
Color depends on alloy, layer thickness, and bath conditions. Some variation is inherent, especially on 7075 or castings.
Does anodizing work on steel or stainless?
No. Anodizing is aluminum-specific. Stainless is passivated, polished, or bead blasted instead.
Get a Quote for Anodized Parts
Tell us the alloy, the type, the color, and which features must be masked, and we will quote machining and anodizing together.