Hard Coat Anodizing

Hard Coat Anodizing as per MIL-A-8625 Type III

Hard coat Anodizing or simply Hard Anodizing is a dense anodic coating of aluminium oxide applied by converting a properly cleaned and deoxidized aluminium alloy component into an oxide film using a suitable electrolyte, typically sulphuric acid cooled to nearly freezing, and applied voltages upwards of 100 volts at an applied current density of 24-36 amps per square foot.

How is Hard Coat Anodizing different than other types of anodizing?

Aluminium anodizing is typically referred to by its three types. Type I is chromic acid anodize (or chromic alternatives) which are very thin on the order of 0.0001” thick. Type II is the conventional sulphuric acid anodizes which can be decoratively dyed nearly any colour and results in a thickness of 0.0002 – 0.0006”. Hard Coat Anodize is Type III which is done under more exacting process conditions resulting in a harder, denser, thicker, and more abrasion resistant coating. Hard coat thicknesses can vary from 0.0005 to 0.0030” and beyond but is dependent on the specific alloy being anodized.

What are the attributes / characteristics of Hard coat Anodizing?

  • Increased abrasion resistance (best when unsealed)
  • Increased wear resistance
  • Increased corrosion resistance (best when sealed)
  • Improved aesthetics (see Appearance below)
  • Dielectric properties (coating is an electrical insulator)
  • Improved lubrication (particularly when PTFE sealed)
  • Readily sterilized, non-contaminating (for medical instrumentation)

What’s the appearance of Hard Coat Anodizing / MIL-A 8625 Type III?

Undyed, that is Class 1, hard coat anodize will change the colour of the aluminium dependent on the particular alloy and the anodic thickness. The colour of the aluminium after being hard coat anodized depends on the alloy and the thickness of the coating. Many of the 6xxx-series of aluminium will take on a deep gray-black colour while most 7xxx and 2xxx series will appear a more bronze-gray colour. On some alloys, the colour of the aluminium after hard coat anodizing will be gray/bronze. For dyed Class 2 coatings, black is by far the most commonly specified and will take on a very pleasing, uniform appearance. Due to the dark nature of the undyed coating itself, other colours are not as decorative and uniform appearing. If a colour other than black is needed, sampling is required prior to production.

How does Hard coat Anodizing affect dimensions of parts?

All anodizing is a conversion coating in that a portion of the base materials surface is converted from raw aluminium into aluminium oxide. In general terms, for Type III anodize ½ of the coating thickness penetrates into the surface of the parts while the other 1/2 builds up on the surface. Thus for a typical 0.002” thickness requirement, there is 0.001” dimensional change per surface. When tight tolerances are required we’ll work closely with the customer to ensure that the thickness of the coating results in the needed final product dimensions.

Can certain surfaces be masked off so they are Hard coat Anodized? All 3 types of anodic coatings are electrical insulators so when an electrical grounding surface is required or in order to maintain close dimensional tolerances, surfaces can be readily masked such that those areas will remain anodize free.

Should Hard coat Anodizing be sealed?

In general when hard coat anodize is applied for wear or abrasion resistance, the coating is left unsealed (other than PTFE sealing) and this is the default in most specifications. If on the other hand, the primary purpose is corrosion resistance or a combination of corrosion and wear resistance, the hard coat anodize should be sealed. In general, all dyed hard coat anodize is required to be sealed in order to ensure the dye won’t fade or bleach out. Sealing can be accomplished using deionised water, sodium dichromate, nickel acetate, PTFE or a combination of two or more of these.

What are governing specifications that cover Hard Coat Anodizing?

  • MIL-A-8625, Type III
  • AMS 2469
  • ASTM B580, Type A
  • ISO 10074
  • MIL-A-63576 ( PTFE Sealed )
  • AMS 2482 ( PTFE Sealed )

Hard Anodize Features:

  • Improved wear resistance
  • Non-conductive
  • Can repair worn surfaces on aluminium
  • Improve parts surface for slide applications
  • Can be black dyed; other colours less decorative
  • Finish is harder than tool steel
  • Can be ground or lapped

Hard Anodize Applications:

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