Hard Anodizing
Hard anodizing (Type III) grows a much thicker and denser oxide layer on aluminium, typically 25–100 µm at around 350–600 HV, for serious wear and abrasion resistance. It is used on aluminium parts that slide, rub or face abrasive conditions.
What it is
The process is similar to standard anodizing but runs at low temperature and higher current density, producing a thick, hard, dense coating. Its natural colour is dark grey to bronze, depending on alloy and thickness.
Hard anodizing lets lightweight aluminium replace steel in many wear applications, such as pistons, cylinders, gears and guides.
Typical applications
- Pneumatic and hydraulic cylinder bores and pistons
- Gears, cams, pulleys and slides
- Food, textile and packaging machine parts
- Defence and aerospace components
- Insulating surfaces on aluminium parts
Suitable materials
- 6061, 6082 and 7075 (common choices)
- 5000-series alloys
- 2000-series (limited thickness)
- Low-silicon cast alloys
Points to consider
- About half the thickness grows outward, so machine to allow for growth on precision fits.
- Thick hard anodizing reduces fatigue strength; avoid it on highly stressed cyclic parts unless approved.
- Sharp edges should be radiused (around 0.5 mm) to avoid edge burning and chipping.
- Bright dyed colours are not possible because of the dark natural colour.
Frequently asked questions
How hard is hard anodizing?
Typically around 350–600 HV, harder than many tool steels at the surface, giving excellent wear resistance for aluminium parts.
What is the difference between anodizing and hard anodizing?
Standard anodizing is thinner (5–25 µm) and mainly for appearance and corrosion. Hard anodizing is thicker (25–100 µm) and denser, for wear and abrasion resistance.
Can hard anodized parts be coloured?
The natural colour is dark grey or bronze. Black is readily achievable; bright colours are generally not.