What Is Anodizing?
Anodizing is an electrochemical process that converts the surface of aluminum into a durable aluminum oxide layer. Unlike paint or plating (which add material on top), anodizing grows the oxide from the base metal. The result is an extremely hard, corrosion-resistant surface that’s integral to the part — it can’t peel or flake.
Aluminum naturally forms a thin oxide layer (about 2–3 nm) that provides some corrosion resistance. Anodizing thickens this layer by 1,000–10,000x, creating a controlled, uniform, and much harder surface.
Types of Anodizing
Type I — Chromic Acid Anodize
| Property | Value |
|---|---|
| Thickness | 0.02–0.1 mil (0.5–2.5 μm) |
| Hardness | Moderate |
| Color | Light gray, not easily dyed |
| Spec | MIL-A-8625 Type I |
| Use | Aerospace fatigue-critical parts, primer adhesion |
Thinnest anodize. Least dimensional change. Preferred where fatigue life matters (the thin layer doesn’t create stress risers). Being phased out due to hexavalent chromium environmental concerns — replaced by Type IC (non-chromic thin film).
Type II — Sulfuric Acid Anodize (Standard)
| Property | Value |
|---|---|
| Thickness | 0.1–1.0 mil (2.5–25 μm) |
| Hardness | ~200–400 HV (Vickers) |
| Color | Clear, black, red, blue, gold — wide range |
| Spec | MIL-A-8625 Type II |
| Use | Consumer products, architectural, general industrial |
The most common anodize. The porous oxide layer absorbs dyes before sealing, enabling a wide color palette. Class 1 is non-dyed (clear or natural), Class 2 is dyed. This is what you see on MacBooks, bike parts, flashlights, and building facades.
Type III — Hardcoat Anodize
| Property | Value |
|---|---|
| Thickness | 1.0–4.0 mil (25–100 μm) |
| Hardness | ~400–700 HV (approaches low-end hardened steel) |
| Color | Dark gray to black (natural), limited dye options |
| Spec | MIL-A-8625 Type III |
| Use | Wear surfaces, pistons, cylinders, military, firearms |
Much thicker and harder than Type II. Grown at lower temperatures and higher current density. The coating penetrates into the base metal (roughly 50% penetration, 50% buildup), so a 2-mil hardcoat grows ~1 mil into the surface and ~1 mil above. Plan your tolerances accordingly.
Dimensional Impact
Anodizing changes dimensions. This matters for precision parts:
- Type II — ~50% growth. A 0.5-mil anodize adds ~0.25 mil per side to external dimensions.
- Type III — ~50% growth. A 2.0-mil hardcoat adds ~1.0 mil per side.
- Holes shrink — anodize grows inward on bore surfaces. Specify masking or post-anodize reaming for critical bores.
Which Alloys Anodize Well?
| Alloy | Anodize Quality | Notes |
|---|---|---|
| 5052 | Excellent | Sheet metal favorite. Clear, bright finish. |
| 6061-T6 | Excellent | The standard. Great color consistency. |
| 6063 | Excellent | Extrusion alloy. Architectural standard. |
| 7075-T6 | Good | Slight yellow tint. Acceptable for most uses. |
| 2024 | Fair | High copper content causes uneven color. |
| A356 (cast) | Fair | Silicon causes dark/mottled appearance. |
| ADC12 (die cast) | Poor | High silicon — gray/blotchy. Avoid cosmetic anodizing. |
Design Tips
- Match alloys — different alloys on the same assembly will anodize to different shades. Specify the same alloy for cosmetic assemblies.
- Mask threads — anodize builds up on threads and can interfere with fit. Specify masking on critical threaded features.
- Weld marks show — anodize reveals weld heat zones as a different color. If cosmetics matter, plan for it.
- Minimum radius — sharp edges anodize thinner (current concentrates at corners). Break edges to 0.010″ min.
- Racking marks — parts hang on racks or fixtures, leaving small contact marks. Specify preferred rack location on cosmetic parts.
Cost
Type II (standard) anodizing is one of the cheapest aluminum finishes — often $5–25 per part for typical CNC parts. Type III hardcoat costs 2–3x more due to slower processing and tighter controls. Black is cheapest (most common). Custom colors may require minimum batch sizes.
Need aluminum parts anodized? Get a quote — we coordinate with local anodize shops for quick turnaround.