The Complete Guide to Anodizing CNC Parts

What Is Anodizing?

Anodizing is a finishing process for aluminum parts. It builds a hard, protective layer on the surface. This layer resists scratches, corrosion, and wear. It also lets you add color.

Unlike paint or plating, the anodized layer is part of the metal. It grows from the aluminum itself. This means it will not peel, chip, or flake off over time.

How Does Anodizing Work?

The part goes into an acid bath. Electric current flows through the bath. This grows a layer of aluminum oxide on the surface. The oxide layer is part of the metal. It will not peel or flake off.

After the oxide layer forms, the part can be dyed. The porous surface absorbs the dye. Then the part is sealed in hot water to lock in the color.

Types of Anodizing

There are three main types of anodizing. Each one uses a different acid and creates a different thickness. The type you pick depends on your needs.

Type I (Chromic Acid)

Type I makes the thinnest layer. It ranges from 0.00002" to 0.0001" thick. This type works best for tight tolerances. Aerospace companies use it often. The thin layer adds very little weight.

Type II (Sulfuric Acid)

Type II is the most common. The layer is 0.0002" to 0.001" thick. It works well for corrosion protection. You can dye it almost any color. Most anodized parts you see use Type II.

Type III (Hardcoat)

Type III makes the thickest layer. It ranges from 0.001" to 0.003". The surface is very hard, equal to 60-70 HRC. Use it for parts that slide or wear against other parts.

Which Metals Can Be Anodized?

Aluminum works best for anodizing. It gives the most consistent results. You can also anodize titanium and magnesium.

Steel, stainless steel, brass, and copper cannot be anodized. These metals need other finishes like plating or powder coating.

Not all aluminum alloys anodize the same way. 6061 gives even, consistent color. 7075 can look uneven or splotchy. High-silicon casting alloys give poor results.

Dimensional Changes

Anodizing adds thickness to your part. This matters for parts with tight tolerances.

Type II grows about 50% outward and 50% inward. If the coating is 0.001" thick, your part grows by about 0.0005" per side. Type III grows mostly outward.

Color Options

Type II anodizing can be dyed almost any color. Common colors include black, blue, red, gold, green, and clear (no dye).

Type III is usually black or dark gray. The thick oxide layer absorbs light. Bright colors do not work well with hardcoat.

Color consistency depends on the alloy. 6061 gives more even color than 7075. If color match matters, use the same alloy and lot of material for all parts.

Cost and Lead Time

Type II anodizing costs $5 to $20 per part for small batches. The exact price depends on part size and quantity. Larger batches cost less per part.

Type III costs more because of the thicker coating. Expect to pay 2 to 3 times more than Type II.

Most anodizers can turn parts around in 3 to 5 business days. Rush services are available from many shops for an extra fee.

How to Spec Anodizing on Your Drawing

Call out the MIL-A-8625 spec on your drawing. Include the type (I, II, or III). Add the class (1 for undyed, 2 for dyed). List the color if dyed.

Example callout: "ANODIZE PER MIL-A-8625 TYPE II, CLASS 2, BLACK"

Place the finish note in the title block or near the affected surfaces. If only some surfaces need anodizing, mark them on the drawing. Use flags or leaders to point to the right areas.

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Frequently Asked Questions

Can you anodize any aluminum alloy?

Most aluminum alloys can be anodized. However, some work better than others. 6061 anodizes very well and gives even color. 7075 can anodize but may show uneven color. High-silicon casting alloys (like A380) give poor results. Always check with your anodizer before picking an alloy.

Does anodizing change part dimensions?

Yes. Anodizing adds thickness to your part. For Type II, about half the coating grows outward and half grows inward. If the total coating is 0.001" thick, your part grows about 0.0005" per side. Always tell your machine shop about anodizing before they cut your parts.

What colors are available for anodizing?

Type II anodizing can be dyed almost any color. Common colors are black, blue, red, gold, green, and clear. Type III hardcoat is usually limited to black or dark gray. Color consistency depends on the aluminum alloy you use.

Is anodizing the same as powder coating?

No. Anodizing converts the surface of the aluminum into aluminum oxide. It becomes part of the metal. Powder coating sprays a plastic layer on top of the metal. Anodizing is thinner and harder. Powder coating is thicker and offers more color options. Anodizing will not peel. Powder coating can chip if hit hard enough.

How do I call out anodizing on my drawing?

Reference MIL-A-8625 on your drawing. Include the type (I, II, or III), the class (1 for undyed, 2 for dyed), and the color. Example: "ANODIZE PER MIL-A-8625 TYPE II, CLASS 2, BLACK." Place this note in your title block or near the surfaces that need anodizing.

Anodizing Surface Finishing Aluminum MIL-A-8625 Type II Type III Hardcoat

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