Anodizing aluminum design guide: types, dimensions and callouts

Anodizing is an electrolytic coating process run in tanks. A shop wires the part to a rack, submerges it in an acid electrolyte, and makes the part the anode. Electrical current converts the outer metal into a hard, porous oxide layer grown from the substrate itself. That porous oxide can then take a dye and is sealed to lock in corrosion resistance and color.

Aluminum is rated excellent for anodizing, and the process is its signature finish. Alpha-beta titanium is rated good for color anodizing used to identify parts, though it does not form a hard wear coat. Zinc and magnesium can also be anodized. Steel cannot be anodized because the process requires a metal that grows a stable surface oxide; steel parts are plated, painted, or conversion coated instead.

Use anodizing on aluminum when your part needs corrosion resistance, a hard wear surface, electrical insulation, or a dyed cosmetic finish without the thick buildup of paint or powder. Use titanium anodizing when you need color coding for part identification.

Do not anodize a surface that must conduct electricity or carry a ground. As published guidance from Hubs notes, an anodized layer is a very poor electrical conductor and is relatively brittle compared with powder coating. If an anodized part has a grounding point, you must mask that area bare or penetrate the oxide layer during assembly.

When anodizing is not the right fit, consider these alternatives:

• Chromate conversion coating when you need electrical conductivity and paint adhesion more than cosmetic appearance.
• Powder coating when you want a thick, tough color coat across any metal, including steel.
• Painting for general color and coverage on metals that do not anodize.

MIL-A-8625 is the governing specification for anodic coatings on aluminum and aluminum alloys. Calling it out on your drawing invokes standard requirements for coating weight and thickness, sealing, corrosion resistance, and paint adhesion.

The standard organizes coatings by acid bath and thickness into the following types:

• Type I and Type IB: Chromic acid anodizing.
• Type IC: Non-chromic acid anodizing.
• Type II: Standard sulfuric acid anodizing.
• Type IIB: Thin sulfuric acid anodizing.
• Type III: Hard anodic coating for heavy wear resistance.

Each type is paired with a class that specifies whether dye is added:

• Class 1: Undyed (natural oxide appearance).
• Class 2: Dyed.

Because the anodic oxide grows partly into the metal and partly outward from the original surface, every anodized face grows by a fraction of the coating thickness. External diameters and thicknesses get slightly larger, while internal bores and threads get tighter. Type III hard anodizing produces the largest dimensional change. For precision features, threads, and close-fitting bores, either mask the feature or size the pre-anodize dimensions to allow for growth. Confirm exact growth allowances with your anodizer.

The aluminum alloy you select sets the visual outcome:

• Wrought aluminum grades anodize to a clear, even finish and take dye well.
• Cast alloys and grades high in copper or silicon turn out gray or blotchy and dye poorly.

Anodizing reproduces whatever texture is underneath it. Machining marks, scratches, and handling dents look more visible after anodizing, not less. Polish or bead blast the part before anodizing if you want a uniform appearance. Bead blasting before anodizing produces an even matte finish, though blasting alters critical dimensions and should be kept off tight-tolerance faces. If a part is welded before anodizing, the weld bead and heat-affected zone will show in a different shade.

• Leaving sharp corners on Type III parts: The anodized oxide is hard but brittle, and it chips at sharp edges under a hard coat. Add radii to edges before anodizing.
• Anodizing with steel hardware installed: Steel inserts, threaded inserts, and any dissimilar metals are attacked by the acid bath and contaminate the tank. Mask them completely or install hardware after anodizing.
• Bolting a ground strap straight onto an anodized face: The layer insulates. Mask the pad on the print or use hardware that bites through the coating.
• Expecting a color name to guarantee a match: Dye shade shifts across alloys, coating thicknesses, and tank batches. An unsealed coating also stains and fades, so always require sealing and specify critical cosmetic colors against a physical limit sample.
• Forgetting the rack contact mark: Every part must be wired or clamped to an electrified rack in the tank, leaving a small bare spot at the contact point. Tell the shop where that bare contact mark is allowed to sit.

Give the anodizer a complete note on the drawing rather than writing a generic callout. A clear specification includes:

• The governing standard, type, and class, for example: Anodize per MIL-A-8625, Type II, Class 1 (undyed) or MIL-A-8625, Type III, Class 2 (dyed).
• Whether part dimensions and tolerances apply before or after anodizing.
• Masking callouts showing every thread, precision bore, mating datum, or electrical ground pad that must stay free of coating.
• Allowed rack contact locations where a bare electrical contact mark is acceptable.
• Pre-treatment and sealing notes, such as bead blasting for a matte texture, sealing requirements, and a target color sample for Class 2 dyed parts.