This month, I’d like to explore the different types and colors of hardcoat anodize. This popular choice for many end-users features a dense aluminum oxide anodic coating which is applied after converting a cleaned and deoxidized aluminum alloy component into an oxide film. This is completed by using a proper electrolyte, most often sulfuric acid that is cooled to nearly freezing and applied using voltages near 100 volts at an applied current density of 24-36 amps per square foot.

Aluminum anodizing is normally described by one of its three different types:  

Type I – Also known as chromic acid anodize (or chromic alternatives) are very thin coatings that are roughly 0.0001” thick.  

Type II – The conventional sulfuric acid anodize is the type that can be decoratively dyed almost any color. Its thickness is in the range of 0.0002 – 0.0006”.   

Type III – Hardcoat anodize requires more precise conditions to produce. The result is a tougher, denser, thicker, and more corrosion resistant coating.  Thicknesses can vary from 0.0005 to 0.0030” and beyond, but it depends on the specific alloy being anodized. Class 1 or undyed hardcoat anodize will change the original color of the aluminum depending on the alloy type and anodic thickness.

There are many desired characteristics of a hardcoat anodized finish:

  • Improved lubrication especially when sealed with PTFE
  • Increased wear resistance
  • Increased corrosion resistance (best when sealed)
  • It has dielectric properties (meaning the coating is an electrical insulator)
  • Increased abrasion resistance (best when unsealed)
  • Can be readily sterilized (useful for medical instrumentation)
  • Better aesthetics

Customers have asked me some really good questions over the years about this versatile process, like:

Does hardcoat anodize have an effect on parts dimensions?

In short, yes. Any type of anodizing involves converting the raw aluminum surface of the base material into a tougher aluminum oxide coated surface.  For Type III anodize, roughly half of the coating thickness penetrates into the surface of the parts while the other half builds up on the surface. Therefore, for a typical 0.002” thickness requirement, there is 0.001” dimensional change per surface.  

For new parts, Aerospace Metals always recommends that the required specifications are spelled out clearly in the order in advance. If tight tolerances are required, we work closely with our clients to be sure that the thickness of the coating results in the desired final product dimensions.

Should sealants be applied to hardcoat anodize?

If hardcoat anodize is applied for wear or abrasion resistance applications, we leave the coating unsealed (PTFE is the exception). On most of our orders, that’s the case. If the goal is to maximize corrosion resistance, or some combination of corrosion and wear resistance, we recommend applying a sealant. Usually, all dyed hardcoat anodize applications require a sealant to be sure the dye doesn’t bleach or fade.  Sealants are applied by using deionized water, sodium dichromate, nickel acetate, PTFE or any combination of these options.

Can some surfaces be masked off when using hardcoat anodize?

Yes, all three types of anodic coating options can be masked because they are all electrical insulators. This is especially recommended when an electrical grounding surface is required or if you need to keep tight dimensional tolerances. Masked surfaces will remain anodize free.  

There are many different techniques used to mask surfaces:

  • Rubber plugs
  • Precision die cut decals
  • Lacquers applied by hand
  • 3D printed silicone stop-offs, or 
  • CNC programmed robot  

Aerospace Metals offers all of these options in-house using state-of-the-art masking technologies.

Hardcoat anodize is a great choice if you are seeking:

  • Finish is harder than tool steel
  • Non-conductive
  • Can be black dyed; other colors less decorative
  • Improved wear resistance
  • Worn-out aluminum surfaces can be repaired
  • Improved parts surface for slide applications

And don’t forget about the versatility of the hardcoat anodize process. It can be extremely useful for many more applications than these listed below:

  • Sliding Parts
  • Insulation Plates
  • Gears
  • Valves
  • Pistons
  • Blast Shields
  • Hinge Mechanisms
  • Cams
  • Swivel Joints
  • Wear & Mold Plates

Is the anodize process safe?

In the world of metal finishing, anodizing is one of the safest and environmentally friendly technologies out there today. Anodizing speeds up the natural oxidation process and it doesn’t create any dangerous or harmful by-products. Most importantly, anodizing doesn’t damage human health or the environment.

Our anodizing facilities are safe, ventilated, and worker-friendly environments—and our staff is trained and experienced with using anodic materials and processes. When it comes to employee safety, Aerospace Metals ensures that our staff is equipped with only the best quality protective gear and clothing when working with anodizing materials.

The Wide Spectrum of Hardcoat Anodize Applications

Many of the 6xxx-series of aluminum appear deep gray/black in color, while most 7xxx and 2xxx series are a bronze/gray color.  On other alloys, the color of the aluminum after the hardcoat anodize process is a gray/bronze color. For dyed Class 2 coatings, black is the most often specified color. That’s because dyed Class 2 takes on a really appealing and uniform appearance when finished. Undyed coatings are not as decorative and uniform in look. However, if one of my customers specifies a color other than black, I always recommend looking at samples prior to production to be sure the color matches expectations.

Aerospace Metals provides Type II colored anodizing for a variety of industries and applications including military, automotive, and aerospace. When applied as a sulfuric acid bath, this coating can result in a wide spectrum of colors, from a clear non-dyed appearance to any number of colors like black, blue, gold, olive drab, red, and green to name a few. Upon your request, other colors are also available. 

Type III hard coat anodizing is most frequently ordered for industrial coating applications. That’s because this coating process forms a thick oxide coating with .001 substrate penetration and build up. This hard coat anodize option increases the hardness of aluminum and it’s non-conductive after coating. The hardest aluminum oxide coating is in the 65 RC range. Usually Type III black hardcoat anodizing is requested for compressor fittings or aluminum auto parts.

Another benefit of this process is that the engineered hardcoat anodizing process generally makes aluminum surfaces easier to clean/maintain, improves lubrication, improves flame resistance, and is overall more resistant to peeling and chipping. It’s also non-contaminating which is perfect for medical instruments.

If you’d like to learn more about hardcoat anodize or for a quote on your metal finishing project, contact our team at Aerospace Metals by email at, website linked here, or call (800) 398-0790. 

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