Aluminum Anodizing Guide: Type II Decorative and Type III Hard
Anodizing Types
Aluminum anodizing: Type II (sulfuric, 5-25μm) for decorative and moderate wear. Type III (hard, 25-100μm) for high wear. Both increase surface hardness. Sealing: hot water or nickel acetate for corrosion resistance.
Dyeing for color on Type II only. PTFE impregnation for reduced friction on Type III. Chromate conversion (<5μm) for primer/adhesion.
| Type | Thickness | Hardness | Use |
|---|---|---|---|
| Type II Decorative | 5-25μm | 200-300 HV | Consumer/automotive |
| Type III Hard | 25-100μm | 400-600 HV | Industrial/wear |
| PTFE Impreg | Per Type III | 300-500 HV | Low friction |
| Chromate | <5μm | Soft | Primer |
Process Parameters
Type II: 15-20% H2SO4, 18-22°C, 12-15 A/ft², 30-60min for 10-15μm. Type III: 10-15% H2SO4, 0-5°C, 20-30 A/ft², 60-120min for 50μm. Close temperature control critical for Type III coating quality.
Rack contact secure for consistent current. Thread masking for critical dimensions. Step-down current on edges to prevent burning.
| Parameter | Type II | Type III |
|---|---|---|
| Temperature | 18-22°C | 0-5°C |
| H2SO4 | 15-20% | 10-15% |
| Current | 12-15 A/ft² | 20-30 A/ft² |
| Growth | 0.3-0.5μm/min | 0.5-0.8μm/min |
Quality Control
Seal quality: dye stain per ASTM B136. Thickness: eddy current. Hardness: micro-indentation. Abrasion: Taber test. Salt spray: 300-1,000h depending on thickness. Visual for uniform color. Growth: 50% inward, 50% outward.
Thread gaging after anodizing for fit check. Repeatability verification through regular process capability studies.