The Anodizing Process

Aluminum anodizing is the electrochemical process by which aluminum is converted into aluminum oxide on the surface of a part. This coating is desirable in specific applications due to the following properties:

• Increased corrosion resistance
• Increased durability / wear resistance
• Ability to be colored through dying
• Electrical insulation
• Excellent base or primer for secondary coatings

The process of anodizing is rather simple. It consists of an anodizing solution typically made up of sulfuric acid. A cathode is connected to the negative terminal of a voltage source and placed in the solution. An aluminum component is connected to the positive terminal of the voltage source and also place in the solution. When the circuit is turned on the oxygen in the anodizing solution will be liberated from the water molecules and combine with the aluminum on the part forming an aluminum oxide coating.

Types of Anodizing

Over the last several decades a variety of anodizing processes have been developed. However, there have been three main variations of aluminum anodizing: chromic anodizing, sulfuric anodizing, and hardcoat anodizing. Each of these has advantages and disadvantages depending on the application.

Chromic Anodizing
Utilizing a chromic acid electrolyte this form of anodizing yields the thinnest coatings, only .05 to .1 mils thick. Chromic anodizing is a good choice when a part is complex and difficult to rinse because chromic acid is less corrosive than sulfuric acid used in other anodizing methods. Chromic anodize also reduces the fatigue strength of the aluminum less than the other methods described.

Sulfuric Anodizing
This form of anodizing yields coatings under 1 mil thick. Although it offers mild abrasion resistance it is more durable than chromic anodize. Like most anodizes corrosion resistance is excellent. The most desirable feature of this form of anodizing would be the excellent results from dyeing yielding deep and rich colors.

Hardcoat Anodizing
Also using a sulfuric acid electrolyte, although at a lower temperature, hardcoat anodize's claim to fame is wear resistance. This will produce a Rockwell C-scale rating of 60-70. This makes it an excellent candidate for many wear situations.