Type II vs Type III Hardcoat Anodizing: Dimensional Growth

Type II vs Type III Hardcoat Anodizing: Dimensional Growth & Specs

Anodizing is an electrochemical process converting aluminum surfaces into a durable, corrosion-resistant aluminum oxide layer with controllable dimensional build-up and hardness up to 65 HRC.

Engineering Specifications & Parameters

Dielectric Strength	Up to 1500 V
Specification Standard	MIL-A-8625 / ISO 7599
Dimensional Growth Ratio	50% Penetration / 50% Outward Build-up
Hardcoat Surface Hardness	60 - 65 Rockwell C (HRC)
Type II Coating Thickness	8 - 18 microns (0.0003 - 0.0007 in)
Type III Hardcoat Thickness	25 - 50 microns (0.0010 - 0.0020 in)

Type II vs Type III (Hardcoat) Comparison

Type II (MIL-A-8625 Type II):
- Thickness: 5 to 20 microns (0.0002 to 0.0008 in).
- Primary Purpose: Decorative colors, moderate corrosion resistance, cosmetic appearance.
- Dimensional Growth: 50% penetration into substrate / 50% surface growth.
Type III Hardcoat (MIL-A-8625 Type III):
- Thickness: 25 to 50 microns (0.001 to 0.002 in).
- Hardness: 60-65 HRC (Glass-scratching wear resistance).
- Operates in refrigerated acid baths (-2 C to 0 C) with high current density.

Critical Engineering Drawing Callouts

CNC drawings must state whether dimensions apply BEFORE or AFTER anodizing, especially on tight-tolerance reamed dowel holes and bearing bores.

Frequently Asked Questions

How much does Type III hardcoat anodizing increase part dimensions?

For a 50 micron (0.002 in) total coating, the part surface expands outward by 25 microns (0.001 in) per side, increasing outer diameter by 50 microns (0.002 in).

Should precision dowel pin holes be masked before anodizing?

Yes, dowel holes with +/-0.005mm tolerance should be masked with silicone plugs or reamed to size post-anodizing.