Anodizing: Practical Guidance for Design Engineers

Anodizing is one of the most common finishes specified on aluminum parts, and also one of the easiest to misuse if its implications aren't well understood. It's often selected for good reasons—corrosion resistance, appearance, durability—but it also changes dimensions, electrical behavior, and surface characteristics in ways that can affect how a part actually functions in an assembly.

This page is intended to give designers a practical understanding of anodizing as it's used in real production. The goal isn't to catalog every standard or chemistry, but to highlight the trade-offs that tend to matter once parts leave CAD and enter the shop.

What Anodizing Actually Does

Anodizing is an electrochemical conversion process that transforms the surface of aluminum into aluminum oxide (Al₂O₃). Unlike paint or plating, this oxide layer is:

• Grown from the base material
• Fully bonded to the aluminum
• Harder and more wear-resistant than the parent metal
• Naturally porous prior to sealing

Because the oxide is part of the material rather than something applied on top, anodizing doesn't peel or flake. That also means you can't “touch it up” later without reprocessing the entire surface.

Materials That Can (and Can't) Be Anodized

Aluminum Alloys

Aluminum is the primary material for anodizing, but not all alloys behave the same. Wrought alloys like **6061** anodize predictably and uniformly. Higher-strength alloys (e.g., **7075**) and cast aluminum can show more color variation and surface inconsistency.

Aluminum is the primary material for anodizing, but not all alloys behave the same. Wrought alloys like 6061 anodize predictably and uniformly. Higher-strength alloys (e.g., 7075) and cast aluminum can show more color variation and surface inconsistency.

Magnesium and Titanium

These materials can be anodized using specialized processes. Titanium anodizing is typically decorative and based on oxide thickness and light interference, not dye absorption.

Materials That Cannot Be Anodized

• Steel and stainless steel
• Copper and brass
• Zinc alloys
• Plastics

These require other finishing processes entirely.

Common Types of Aluminum Anodizing

Type I — Chromic Acid Anodize

• Very thin coating
• Minimal dimensional impact
• Good corrosion resistance
• Muted grey appearance
• Common in aerospace applications
• Increasingly restricted due to environmental regulations

Type I is usually chosen when dimensional control is critical and cosmetic appearance is secondary.

Type II — Sulfuric Acid Anodize

This is the most commonly specified anodize.

• Moderate coating thickness
• Can be clear or dyed
• Good balance of corrosion resistance, cost, and appearance
• Suitable for most industrial and commercial applications

When someone says “anodized aluminum” without further detail, this is usually what they mean.

Type III — Hardcoat Anodize

• Thick, dense oxide layer
• Significantly harder and more wear-resistant
• Natural color ranges from dark grey to bronze
• Most likely to affect dimensions
• Common on sliding, abrasive, or high-wear components

Hardcoat behaves more like a functional surface treatment than a cosmetic finish, and should be specified with care on precision parts.

How Anodizing Affects Dimensions (Where Problems Start)

Anodizing grows both into the base metal and outward from the original surface.

A good rule of thumb:

• ~50% of the coating thickness grows inward
• ~50% grows outward

What this means in practice:

• External dimensions increase
• Internal diameters and slots shrink
• Threads, bores, and press fits are commonly affected

For example, a Type III hardcoat of 0.002" total thickness can reduce a bore diameter by roughly 0.002". That's often enough to turn a slip fit into a press fit—or make assembly impossible.

For tight-tolerance features, anodizing should be considered during the design stage, not after the first batch comes back from finishing.

What Anodizing Does Well

Corrosion resistance

Anodized aluminum performs well in outdoor and industrial environments, especially when properly sealed.

Surface durability

Hardcoat anodizing significantly improves wear resistance and surface hardness.

Decorative flexibility

Type II anodize can be dyed in a wide range of colors while remaining UV-stable.

Electrical insulation

Aluminum oxide is a dielectric. Anodized surfaces are non-conductive unless masked or mechanically removed.

Long-term stability

Because the oxide is integral to the material, anodizing won't peel, blister, or flake over time.

Limitations Designers Should Plan For

Dimensional change is real

Especially with hardcoat anodize, tolerances must be adjusted or features masked.

Appearance depends on alloy and prep

Two parts with identical geometry but different alloys—or different surface prep—can anodize very differently.

Color matching is approximate

Dye absorption varies with alloy, thickness, and batch. Tight cosmetic matching across suppliers or production runs is difficult.

Surface finish is amplified

Anodizing highlights what’s already there. Machining marks don’t disappear; they often become more noticeable.

Surface Finish and Appearance

Anodizing follows the existing surface condition:

• Polished parts become bright and reflective
• Bead-blasted parts become matte and uniform
• Tool marks remain visible
• Welds and cast surfaces stand out

If appearance matters, specify surface prep—not just the anodize type.

Masking, Racking, and Selective Anodizing

Anodizing is applied to all exposed surfaces unless masked.

Common areas that may need attention:

• Threads
• Grounding points
• Bearing and press-fit surfaces
• Electrical contact areas

Masking is possible but adds cost and complexity. Racking points—where the part is electrically connected—will leave small contact marks. These are normal and should be accounted for in cosmetic designs.

Typical Applications

• Enclosures and housings
• Automation and machine components
• Aerospace parts
• Consumer and industrial electronics
• Medical and lab equipment
• Fixtures, tooling, and frames
• Optical and inspection equipment

When Anodizing Is the Right Choice

Anodizing is usually a good fit when you need:

• Corrosion resistance on aluminum
• A durable, non-peeling surface
• Decorative finishes or color coding
• Lightweight components with improved wear performance

For extremely aggressive environments or applications involving heavy abrasion plus corrosion, specialty coatings or plated finishes may be more appropriate.