Clear and hardcoat are types of anodic finish that result from traditional anodizing processes. Hardcoat anodizing produces an oxide coating that is denser and thicker than clear anodize. Clear anodizing producing a nearly clear oxide coating that is usually less than 0.0010” thick and is usually sealed to improve corrosion protection.

Clear anodizing

Also known as sulfuric acid anodizing, clear anodizing provides corrosion protection in mild environments and minimum wear protection. Compared to chromic acid anodizing, sulfuric acid anodizing can provide similar corrosion protection and better wear characteristics. Specification determines the coating thickness.

Process

Sulfuric acid anodizing yields a porous film that readily absorbs dyes. The basics process involves:

  1. Sulfuric acid process anodizes the aluminum substrate
  2. Cold water rinse
  3. Dye
  4. Cold water rinse
  5. Hot water bath to seal the anodic film

Uses

Clear anodizing’s relatively low cost makes it a commonly requested specification on engineered aluminum components. It is used in many architectural applications (e.g. windows, siding, and railings). It is often dyed for decorative purposes.

Hardcoat anodizing

Hardcoat anodizing yields a thicker, denser oxide coating than clear anodizing.In hardcoat anodizing the film thickness ranges from three to 20 times that of clear anodize, its density producing a hardness of 60 to 65 Rockwell on the C scale. The material’s alloy and temper along with the coating thickness determine the color of film – from light gray to dark olive gray – generated from the hardcoat process.

Hardcoat anodize is also dielectric (does not conduct direct current), so may be used to insulate assembly components.

Process

The difference between hardcoat and clear during the same basic process is the recipe of the chemical substrate along with the processing voltage.  The process involves the same steps:

  1. Sulfuric acid process anodizes the aluminum substrate
  2. Cold water rinse
  3. Dye
  4. Cold water rinse
  5. Hot water bath to seal the anodic film

Uses

More expensive than clear anodizing, hardcoat anodizing is often more economical than chromic acid anodizing in terms of the wear resistance achieved. Hardcoat anodizing is often sought for components that require high wear and corrosion resistance. It is typically applied to machine wrought aluminum and is used in hydraulic and pneumatic components, screw threads, heat sinks, and has many other applications.