Comparing Type II and Type III Anodizing for Machined Parts

When it comes to enhancing aluminum parts for strength, corrosion resistance, and appearance, anodizing is one of the most widely used surface treatments in precision machining. Two of the most common types of anodizing—Type II and Type III—serve different functional and aesthetic roles.

If you're designing custom machined parts or sourcing aluminum components, understanding the differences between Type II and Type III anodizing is crucial to balancing performance, cost, and appearance.

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What Are Type II and Type III Anodizing?

Type II Anodizing: Sulfuric Acid Anodizing

Type II anodizing is the standard sulfuric acid anodizing process¹. It's commonly used for decorative and protective applications. The oxide layer is thinner (typically 0.0002”–0.0009”) and can be dyed in various colors.

Key characteristics:

• Thickness: ~5–25 µm
• Colorable: Yes (wide range of dyes)
• Primary use: Appearance, moderate corrosion resistance

Type III Anodizing: Hardcoat Anodizing

Type III anodizing, also known as hard anodizing or hardcoat anodizing², uses the same sulfuric acid bath but with lower temperatures and higher voltages, forming a much thicker, denser oxide layer. This layer is harder and more wear-resistant than Type II.

Key characteristics:

• Thickness: ~25–150 µm
• Colorable: Limited (usually dark grey or black)
• Primary use: Industrial durability, wear & corrosion resistance

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Key Differences Between Type II and Type III Anodizing

Dive Deeper: A Side-by-Side Technical Comparison

Feature	Type II Anodizing	Type III Anodizing
Oxide Thickness	5–25 µm	25–150 µm
Hardness	200–300 HV	400–600 HV
Temperature	Room temperature (~20°C)	Near freezing (~0–5°C)
Voltage Used	12–18 V	60–100 V
Dye Absorption	High	Low
Color Options	Many	Few (typically grey, black)
Corrosion Resistance3	Moderate	Excellent
Wear Resistance	Moderate	High
Common Applications	Consumer goods, enclosures	Aerospace, military, tools

The primary difference lies in the functionality: Type III creates a thicker, harder oxide layer, making it suitable for harsh environments and demanding use.

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How to Choose the Right Anodizing Type for Your Application

When selecting between Type II and Type III anodizing, you must align the coating’s properties with the application’s requirements.

Considerations:

1. Mechanical Wear and Abrasion⁴

   • Type III is best for moving parts or parts subject to mechanical stress, like pistons, gears, or hydraulic components.

2. Corrosion Resistance

   • Both types improve corrosion resistance, but Type III is superior in marine or high-humidity environments.

3. Aesthetic Requirements⁵

   • Type II is ideal for parts requiring vibrant or consistent colors, such as electronics casings or consumer products.

4. Tight Tolerances

   • Type II’s thinner layer is less likely to affect dimensional tolerances. Type III’s buildup must be factored into part design.

5. Budget and Volume

   • Type II is more cost-effective for high-volume, low-wear applications. Type III costs more due to stricter controls and longer cycle times.

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Performance Comparison: Corrosion, Wear, and Appearance

Corrosion Resistance

Both processes significantly improve resistance to corrosion by sealing the aluminum’s surface. However, Type III’s thicker layer provides longer-lasting protection, especially in salt spray or marine environments.

• Type II withstands ~336 hours of salt spray (ASTM B117).
• Type III often exceeds 1,000+ hours in salt spray tests.

Wear Resistance

Type III anodizing⁶ is commonly used where extreme wear is a concern. It’s especially popular in:

• Firearm components
• Aircraft landing gear
• Valve bodies

Its high hardness (approaching 60+ Rockwell C) gives it wear resistance comparable to case-hardened steel.

Appearance and Color Options

• Type II: Clear, gold, red, blue, green, black, and more. The oxide is porous and ideal for dye absorption.
• Type III: Dense oxide makes it hard to dye evenly. Most Type III parts are black or dark grey.

For decorative applications, Type II is far more versatile.

Surface Finish

• Type II: Smoother surface finish
• Type III: May result in a slightly rougher finish due to thicker oxide

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Cost and Processing Considerations for Each Type

Type II Anodizing

• Setup: Simpler and faster setup
• Processing time: Shorter (5–30 minutes)
• Power requirements: Low
• Cost per part: Lower
• Environmental impact: Easier wastewater treatment

Type III Anodizing

• Setup: Requires special chilling systems, high-voltage power supplies
• Processing time: Longer (up to 2 hours)
• Power requirements: High
• Cost per part: Higher
• Post-processing: Often requires tight quality control and sealing

Note: Sealing is common in Type II anodizing to improve corrosion resistance⁷ and lock in dye. Type III parts may or may not be sealed depending on whether maximum wear resistance is needed (unsealed) or corrosion resistance is preferred (sealed).

Conclusion

Choosing between Type II and Type III anodizing depends on a balance of appearance, durability, cost, and application requirements.