Hardcoat anodize or hard anodize is a dense anodic coating of aluminum oxide. There are three types of aluminum anodizing. Type I is chromic acid anodize and has a thickness of 0.0001”. What is often referred to as sulfuric acid anodize are Type II in North America, as named by MIL-A-8625; it has a moderate thickness of 1.8 μm to 25 μm (0.00007″ to 0.001″). Standards for thin (Soft/Standard) sulfuric anodizing are given by MIL-A-8625 Types II and IIB, AMS 2471 (undyed), and AMS 2472 (dyed), BS EN ISO 12373/1 (decorative), BS 3987 (Architectural).
Type III is hard coat anodize; hard anodizing or engineered anodizing can be made between 13 and 150 μm (0.0005″ to 0.006″) thick. It involves an electrochemical process that involves sulfuric acid and low bath temperatures. Standards for thick sulfuric anodizing:
- MIL-A-8625, Type III
- AMS 2469
- ASTM B580, Type A
- ISO 10074
- MIL-A-63576
- AMS 2482
Sulfuric acid is the most widely used solution to produce an anodized coating. The sulfuric acid bath typically contains 180-200 grams per liter of acid with a small amount of dissolved aluminum. The sulfuric acid is cooled to near freezing, and a current of upwardly of 100 volts is applied. Current densities usually range from 24-40 amps per square foot (ASF). The current is applied until the desired oxide thickness is achieved; it can then be colored or sealed.
The temperatures are controlled, typically between 28-32º F and in some instances, higher temperatures. The process forms an abrasion-resistant oxide film that is extremely hard. Thick coatings require more process control and are produced in a refrigerated tank near the freezing point of water with higher voltages than the thinner layers.
Characteristics and Features of Hardcoat Anodize
- Increased corrosion resistance
- Non-conductive
- Enhanced abrasion resistance
- Can be dyed black
- Increase wear resistance
- Improved aesthetics
- Sterilized, non-contaminating attributes
Anodizing thickness increases wear resistance, corrosion resistance, ability to retain lubricants and PTFE coatings, and electrical and thermal insulation. Because of the wear- and corrosion- resistant film that hard anodizing creates, its applications are ideal for industries such as aerospace, aviation, robotics, food manufacturing, military, medical, and oil, and petrochemical. Its ideal for firearms, machinery, cookware, optical devices, and electronics. It’s also often used in sporting goods. And because it’s readily sterilized and has non-contaminating properties, it’s ideal for medical instrumentation.
Hardcoat Anodize Appearance by Class
- Class 1 – undyed: The color of hard coat anodize bases on the specific alloy and the anodic thickness. Some will take on dark gray or black color while others are more bronze-gray in color.
- Class 2 – dyed: The most common dyed coating for hard coat anodizing is black.
Aerospace Metals is proud to offer Hardcoat Anodizing. If you have questions or want further info about our processes, call us today at 800.398.0790 or email us at info@aerospacemetalsllc.com.
