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Hard Coat Anodizing Services | MIL-A-8625 Type III Aluminum Anodizing

PFI Inc. / November 15, 2018

What is Hard Coat Anodizing?

  • Extremely resistant to marine and atmospheric corrosion
  • Contains a very high lubricity and resistance to flame
  • Provides greater durability with a smoother finish
  • Easier to clean and maintain
  • Compatible with adhesives

Sometimes referred to as an “engineering hardcoat,” the Type III hardcoat anodizing process will form an extremely hard, abrasion resistant outside coating on aluminum. Since this process provides more durability than other anodizing types and also has a much smoother finish, Type III hardcoat anodizing has become extremely popular in a variety of applications.

Aluminum is already a very lightweight and versatile material; hard anodizing not only improves aesthetics but furthers the strength and corrosion-resistant properties aluminum already possesses. Type III hard anodize is commonly performed clear or with a black dye finish as this creates the most uniform finish. Engineered anodizing generally makes the aluminum easier to clean/maintain, improves lubrication, improves flame resistance, and is overall more resistant to peeling and chipping. Type III hardcoat anodize is also non-conductive, can repair a worn aluminum surface, and is non-contaminating which is perfect for medical instruments.

Type II vs. Type III Anodize

Basically how the Type III anodizing process works is a uniform oxide layer is formed in a sulfuric bath which penetrates the material ½ into the aluminum and ½ build up on the surface. This is a similar process as compared to Type II anodizing, just the acid bath is much cooler at around 32 degrees Fahrenheit whereas Type II, for example, has an acid bath temperature of about 70 degrees Fahrenheit.

Though the acid and aluminum concentrations are virtually the same, the differences only become apparent when considering the end-use of the anodized aluminum. Type III is much more abrasive resistant due to the increased density and can have a thicker build-up than type II too. Type II anodize results in a thickness of around .0002” to .0005” but Type III hardcoat anodize can be up to .003” thick and sometimes more depending on the specific alloy of aluminum being anodized.

Hardcoat Teflon for Type III Anodizing

Type III anodize can be performed using polytetrafluoroethylene (PTFE) based formulas, the best-known brand name being Teflon, and is a very durable anodize solution. A Teflon hardcoat will also lower the coefficient of friction, up to 50% lower than that of hard anodizing, making it more slippery. This makes the surface more suitable for aluminum molds as the lubricity translates into superior release characteristics and the coating remains stable at relatively high temperatures as well. These added characteristics make hardcoat/PTFE applications popular in the aerospace, firearms, electronics, military, and marine industries.

How much does anodizing add to thickness?

The Type III anodize coating process forms a thick oxide coating with 1/2 penetration into the substrate and 1/2 build up on the surface. Overall thickness build-up on the surface is about .001” and also .001” into the surface itself +/- .0002”.

What’s the natural appearance of Hardcoat Anodize and can it be dyed?

The undyed appearance can change based on the alloy and thickness of the aluminum being anodized. Due to the dark nature of the undyed coating, other colors are not as decorative as we see in Type II anodizing where many more colors are available. Black is by far the most common color as this will create the most uniform appearance as compared to other colors.

Do you need to seal Type III hardcoat anodize?

Generally speaking no. For most applications, the coating is left unsealed as the primary purpose of Type III hardcoat anodize is to already be corrosion resistant and wear resistant. The only exception is for a dyed hardcoat anodize where a sealant may be needed to limit the dye from fading.  

What are some Type III hardcoat anodize common applications?

  • Sliding parts such as pistons and some valves
  • Hinge mechanisms
  • Gears
  • Wear & mold plates
  • Anything that encounters a lot of wear and tears in a situation where aluminum is preferred over steel

Progress for Industry Inc., A Leader in Aluminum Anodizing

For over 30 years, Progress for Industry, Inc. has created excellent metal finishing services for the molding industry. Using expert craftsmanship and extensive knowledge to construct precision molds and components, many of PFI’s workers have been there since it’s first establishment in 1982. Learn more about all the services we have to offer.

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