Hard anodizing, also known as hard coat anodizing or Type III anodizing, utilizes a similar process as standard anodizing, but results in harder and thicker coatings. Whereas standard anodized coatings can reach a Rockwell surface hardness rating of 55, hard anodized coatings can be as high as 70 on the same scale; untreated aluminum tops out at 44 on the Rockwell scale. Standard anodized coatings may be as much as 25 μm thick; Type III coatings can be made up to 150 μm in thickness.
Like standard anodizing, hard coat anodizing uses a sulfuric acid solution to boost aluminum’s natural oxidation. Hard anodized coatings can greatly improve the wear resistance, corrosion resistance, and hardness provided by aluminum’s auto-passivating oxide layer. A wide range of aluminum alloys can be treated with Type III coatings, including 1100, 3000, 5000, 6000, and 7000 series alloys.
Benefits of Hard Anodizing
Hard anodizing offers numerous benefits. Because the resulting coatings can be made thicker than standard anodized coatings, hard coat anodizing provides even greater wear resistance and corrosion resistance, as well as excellent lubricity and electrical and thermal insulation.
Type III coatings can withstand up to 1000 hours of 5% salt spray exposure, per ASTM B 117, and continuous operating temperatures up to 500°C or higher. In general, hard anodized aluminum provides ten times the durability of untreated aluminum.
Dyeing & Sealing
Hard anodized coatings can be dyed or undyed. The color of an undyed coating will vary depending on the alloy of the aluminum material, and may range from pale gold to dark gray. Type III coatings are most commonly dyed in black or other dark colors.
Sealing hard anodized coatings will further improve their corrosion resistance, though this will reduce the coatings’ abrasion resistance.
Applicable standards for Type III anodizing include MIL-A-8625 Type III Class 1 (for undyed coatings) and Class 2 (for dyed coatings), AMS 2469, BS 5599, and BS EN 2536.