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Hard Anodizing

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Hard anodizing (also known as hardcoat anodizing) is the process of applying a dense anodic coating of aluminum oxide by converting an aluminum alloy component that has been thoroughly cleaned and deoxidized into an oxide film using an appropriate electrolyte to make it thicker and more abrasion-resistant. Read More…

Hard Anodizing Hard anodizing is a variant of anodizing. It produces similar results to standard anodized surfaces, but its thicker and denser hard anodic film limits the coloring results to dark tones.
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Leading Manufacturers

Dexter, MI  |  734-426-2855

Since 1976 Alpha Metal Finishing has been a trusted leader in anodizing, hardcoat and chemical film (chromate conversion) services. In addition, Alpha now provides stainless steel passivation. We are passionate about providing superior quality, rapid turnaround, and friendly, responsive service. Alpha is ISO 9001:2015 certified. Call us today for your finishing needs!

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Alpha Metal Finishing Company $$$

Union, NJ  |  908-687-4646

For 40 years, Foremost Manufacturing has specialized in aluminum anodizing, bright dip anodizing, color anodizing, and clear coat anodizing. We serve various industries and applications, including indoor and outdoor, theatrical and photographic and medical.

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Foremost Manufacturing $$$

Burbank, CA  |  818-845-7251

For over 50 years, K&L Anodizing has provided metal finishing services for manufacturers, machine shops, tool & die, aerospace, medical, government & commercial industries. We provide multicolor anodizing; anodizing per Mil-8625—types I, II, III—dyed & non-dyed; & a variety of related services.

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K & L Anodizing Corporation $$$

Chatham, ON, Chavies, KY  |  877-556-9191

Dajcor Aluminum is the leading Canadian supplier of extruded, fabricated/machined and anodized components and assemblies to the automotive, renewable energy, transportation, building trades, military, recreation, and consumer-product industries.

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Dajcor Aluminum Ltd. $$$
placeholder image Alpha Metal Finishing Company Foremost Manufacturing K & L Anodizing Corporation Dajcor Aluminum Ltd.

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What is Hard Anodizing?

Hard anodizing (also known as hardcoat anodizing) is the process of applying a dense anodic coating of aluminum oxide by converting an aluminum alloy component that has been thoroughly cleaned and deoxidized into an oxide film using an appropriate electrolyte to make it thicker and more abrasion-resistant. The electrolyte usually used for this process is sulfuric acid that has been cooled nearly to absolute zero when voltages up to 100 volts are then applied with a current density of 24-36 amps per square foot.

Oxide Film Formation

What Distinguishes Hard Anodizing From Other Anodizing Processes?

There are three forms of aluminum anodizing available and the terms are frequently, and incorrectly, used interchangeably. The first process is chromic acid anodizing, which produces an extremely thin (about 0.0001 inches thick) coating. The second process available is standard sulfuric acid anodizing, which produces a coating thickness (ranging between 0.0002 to 0.0006 inches thick) which can be decoratively dyed almost any color. A tougher, denser, thicker, and more abrasion-resistant coating is produced by the third process known as hard anodizing, which also uses sulfuric acid (at a higher concentration) and is carried out under more stringent processing conditions. Depending on the particular alloy being anodized, hard anodizing thicknesses surpass the other anodizing processes (thicknesses can range from 0.0005 to 0.0030 and beyond).

Type I Anodizing Aluminum

Distinguishing Features of Hard Anodized Aluminum

The hard-anodizing process of aluminum results in the following advantages:

  • increased resistance to abrasion surfaces
  • increased resistance to wear
  • it creates a more durable finish than steel
  • increased resistance to corrosion and other weather-related factors
  • better aesthetics; it helps to restore tarnished aluminum; it may be readily be colored black (or other colors though they are less ornamental)
  • it enhances dielectric characteristics (the coating serves as an electrical insulator) are enhanced; it is useful for electrical grounding enhanced lubrication is provided
  • sterilization properties are promoted (useful for medical instrumentation)

Anodization Class Levels and Impact on Color

As a general rule, the thicker the coating added to aluminum, the better able it is to absorb more dye. Depending on the specific alloy and anodic thickness, Class 1 anodizing (or undyed) will only slightly alter the color of the aluminum. After being anodized at this level, the color of the aluminum depends on the alloy and coating thickness. While the majority of the 7xxx and 2xxx series will appear a more bronze-gray tone, the 6xxx series of aluminum will frequently take on a deep gray-black tint and aluminum alloys that have undergone anodizing at this level will appear gray or bronze. Black is by far the most popular color requested for dyed Class 2 anodizing coatings and will have a highly appealing, homogeneous appearance. Other colors don't appear as aesthetically pleasing and homogeneous after going through anodizing procedures at this level due to the dark tone of the undyed coating itself. Before manufacturing, sampling should be employed if a color other than black is required for items being anodized at this class.

Type III Anodized Aluminum

Impact of Hard Anodizing on Part Dimensions

Aluminum which has undergone anodization at Class 3 provides the potential for excellent coloring of this material due to the larger holes available within the metal available to absorb dyes. However, Class 3 anodization creates a coating thickness wherein half penetrates through the surface of the parts and the other half builds up on the surface. Therefore, there may be a 0.001" dimensional change per surface for a standard 0.002" thickness requirement. When precise tolerances are needed, it is very advisable to work closely with a customer to make sure that the end product's dimensions are determined by the coating's thickness.

You can Make Anodized-Free Zones in Hard-Anodized Aluminum?

All three levels of anodic coatings are electrical insulators, making it simple to mask surfaces so that they stay anodize-free when an electrical grounding surface is needed or when tight dimensional tolerances must be maintained. Simple rubber plugs, silicone stop-offs that are 3D printed, finely die-cut custom decals, and painted-on lacquer applied by hand or by a CNC-programmed robot can all be used for this.

Sealing of Hard Anodized Aluminum

Except for PTFE (polytetrafluoroethylene) sealing, the coating is typically left unsealed when hard anodizing is applied for wear or abrasion resistance, and this is the default in most specifications. On the other hand, hard anodizing needs to be sealed if the main objective is corrosion resistance or a combination of corrosion and wear resistance. To ensure that the dye won't fade or bleach away, any dyed hard anodizing must generally be sealed. Deionized water, sodium dichromate, nickel acetate, PTFE, or a combination of two or more of these materials can all be used for sealing.

Applications for Hard Anodizing

There are countless applications for hard-anodized aluminum. Here, consider its usefulness for just a few parts created from this material:

  • Valves
  • Pistons
  • Sliding parts
  • Hinge mechanisms
  • Cams
  • Gears
  • Swivel joints
  • Insulation plates
  • Blast shields

Sectors That Gain From Hard Anodizing

Whether it's the enhanced sterilization or lubrication provided, its dielectric properties, or from some other benefit provided, the list of businesses utilizing hard-anodized aluminum is nearly endless. Below is a partial list of industrial sectors benefiting from this incredible material:

  • Aerospace
  • Automation / robotics
  • Automotive
  • Electrical
  • Food manufacturing equipment
  • General manufacturing
  • Hydraulics / pneumatics
  • Machine tools
  • Medical
  • Military / defense
  • Optical devices
  • Sporting goods

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