The following are answers to the most commonly asked questions about Anodizing. Please contact us with any further questions or specific problems you are experiencing.

1. WHAT IS ANODIZING?

Anodizing is a means of building an aluminum oxide film on the surface of aluminum. The process consists of making the aluminum part electrically positive, or the anode, in a suitable electrolyte. The most common electrolyte is sulfuric acid. In sulfuric acid bath through hydrolysis, oxygen is released in an accelerated and uniform manner which reacts with the aluminum to form an oxide film.

2. WHAT DOES ANODIZING ACCOMPLISH?

Anodizing changes the physical and chemical characteristics of the aluminum. The anodic film which is formed on the surface:
a) Renders the surface harder and more abrasion resistant
b) Insulates the aluminum surface against passage of electricity
c) Improves the natural corrosion resistance of aluminum
d) Ability to color aluminum by absorbing dye into the anodic film

3. WHAT IS HARD COATING OR HARD ANODIZING?

Hard coating is a modification of the sulfuric acid anodizing process, which produces a more dense and abrasion resistant oxide film. The process consists of sulfuric acid anodizing performed at high current densities and at low temperatures with additives sometimes added to the electrolyte. The oxide film formed by hard coating is opaque and will vary in color from gold to bronze or gray to black depending on the alloying elements and film thickness.

4. HARD COAT OR ANODIZE-WHICH SHOULD I CHOOSE?

Regular or color anodizing is used mainly for corrosion resistance or decorative purposes. Coating thickness ranges from 0.0002" to 0.0007" and can be dyed in a variety of colors.

Hard coat anodizing is used when wear resistance or higher dielectric strength is the main objective. Coating thickness ranges from 0.0005" to 0.0020" in production processing.

5. CONSIDERATIONS WHEN DESIGNING A PART FOR HARD COAT?

a) Growth and penetration-On the whole, all alloys show a uniform growth equal to that penetrated below the surface of the base metal. Hard coating a shaft 0.002" thick will increase the diameter by only 0.002".
b) Corners-The aluminum oxide film grows perpendicular to the surface of the metal. Sharp corners either concave or convex are left void and becomes more pronounced as thickness increases.
c) Surface micro finish-Surface roughness is increased by hard coating. Normally a 10 micro-inch surface on a wrought aluminum alloy will read 16 micro-inch and a 60 micro-inch finish on a sand cast will read 100 micro-inch after hard coating.
d) Threads-The pitch diameter of a threaded diameter can increase up to four times that of the surface coating thickness dependant upon the pitch angle.

6. WHAT IS CHEMICAL (CHROMATE) CONVERSION COATING?

Chemical conversion coating is an oxide film applied to aluminum. It is considerably thinner than the oxide coating produced by anodizing. Its main function is to act as a base for organic finishes, provide mild corrosion resistance or provide an electrically conductive surface on aluminum. It is sometimes referred to as Alodine or Iridite.