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Hard anodizing

Hard anodizing

Hard anodizing is the term used to describe the functional refinement of aluminum components as anode in special, cold electrolytes. This process is sometimes also referred to as hard coat anodizing. Hard anodizing creates very thin, non-metallic conversion coatings with very thin film thickness. Parts of the base material become fully integrated with the coating, which results in an extremely high bonding strength of the anodic layers. Aalberts surface technologies offers two hard anodizing processes: CompCote® and HART-COAT®.

CompCote® stands for hard-anodized aluminum oxide-polymer composite coatings. The hard coating process has been patented, among others, in Europe and in the USA. The U.S. Food and Drug Administration (FDA) have rated CompCote® coatings as safe for food contact.

When materials are finished with CompCote®, the base material is transformed into hard-anodized layers through anodic oxidation and the aluminum oxide layer's simultaneous molecular bonding to polymers. This process creates an ideal bond between the coating and the base material. Due to the molecular polymer content, CompCote® offers chemical bond bridges when combined with specifically matched top coats so that an excellent bonding strength is created here as well.

Thanks to the very hard aluminum oxide, the CompCote® surface treatment technique scores with high, alloy- and process-dependent strength. When subjected to abrasion resistance testing (Taber Abraser test), the coating excels with outstanding wear resistance. Often, it scores better than conventional, anodized coatings. CompCote® coatings are corrosion-resistant. When subjected to notched-bar impact testing, they show a fibrous fracture pattern, whereas conventional oxide layers burst in a brittle way similar to glass.

CompCote® creates an optimized micro structure on the surface: In friction pairings and friction tests, the composite layers show excellent gliding properties – so-called “seizures” as a result of poor lubrication could be excluded. Apart from that, CompCote® coatings can be easily dyed. Our standard colors include black, titan grey, blue, red, gold and green. The colored surfaces are more resistant to light than dyed conventional anodized coatings. CompCote®-H is the variant to go for when it comes to special requirements. This coating is even harder and offers even better wear and corrosion protection.

CompCote® is suitable for almost all aluminum alloys.

The combination of low-density aluminum alloys and CompCote® opens up new possibilities when it comes to technical and decorative applications with increased quality requirements. CompCote® does not affect the base material's flexural fatigue strength, which makes it particularly interesting for applications in the aviation sector.

Further areas of application in which CompCote® excels:

One of our core competencies, HART-COAT® laid the foundation for hard anodizing in Germany as early as at the beginning of the 1960s. Today, we serve customers from all key industries. Every year, more than 500 million components are finished with our reliable hard anodizing processes.

The advantages you benefit from when partnering with Aalberts surface technologies:

  • Cutting-edge process technology for aluminum components in lengths of up to eight meters
  • Great variety of processes for individual component properties
  • Maximum precision with our own electrolytes
  • HART-COAT® conforms to the ISO 10074 standard
  • We are the market leader for hard anodizing processes in Germany and, thanks to many locations in Europe and China, we are well positioned when it comes to the implementation of international projects
Hard anodizing of aluminum materials

Hard anodizing is a surface coating process for the functional refinement of aluminum materials. Using electric current, we transform the workpiece surface into a hard, ceramic-like aluminum oxide layer. The new layer is created in the form of regular, hexagonal cells which, like honeycombs, are perpendicular to the workpiece surface. This columnar arrangement creates a particularly stable cell structure; each cell has a pore channel. As a result, HART-COAT® protects workpieces against wear and corrosion. Available in different variants and suitable for many post-treatment processes, HART COAT® is a very versatile type of hard coating.

As a result of the aluminum oxide's metamorphosis, HART-COAT® coatings are extremely hard and wear-resistant. During the hard anodizing process, half of the aluminum oxide grows into the base material, whereas the other half grows out of it. This creates a particularly strong bond between the coating and the base material. The ceramic-like HART-COAT® layer is electrically insulating. It has low thermal conductivity and high dielectric strength. When applied as a protective layer to the bottom ends of motor pistons, it is able to withstand extremely high temperatures for a short time. Depending on the requirements profile, special post-treatment processes can be implemented to further increase the wear and corrosion resistance or the gliding behavior.

Be it wrought alloys, cast alloys or die-cast alloys, almost all technically interesting aluminum alloys may be finished with HART-COAT® coatings. Where possible, the following must be observed should you decide to have your components coated by us:

  • No material, processing or surface defects
  • No oil residues
  • Protective, parts-specific packaging
  • Machining before coating
  • No foreign particles on the workpiece surface
  • Thoroughly deburred and, if possible, rounded edges
  • End distances must be taken into account
  • Bleeder holes might become necessary 
Hard anodized components

Thanks to its good wear resistance, hard-coated aluminum can replace more expensive, wear-resistant steel or other corrosion-resistant materials.

Hard-anodized aluminum is used to prevent contact corrosion, improve the gliding properties and increase the electrical or thermal insulation.

The density of aluminum alloys is equal to one third of the value of common iron materials and non-ferrous metals. When switching from steel or brass to aluminum, it is possible to significantly reduce the inertial forces of fast spinning machine parts. This results in new constructive possibilities and energy cost savings. HART-COAT® creates the required surface resistance.

Examples of components which can be finished with HART-COAT®:


Iron bottoms

Surgical instruments

Damping chambers


Guided cylinders


Holders and fixtures


Heating plates


Camera parts

Rocker arms

Clamping devices


Clutch parts

Bearing rollers

Bearing shells

Baffle plates

Guide rollers

Mounting plates

Cam discs

Pneumatic and hydraulics cylinders



Transport levers


Compressor wheels

Toothed wheels and racks


Cylinder heads

Cylinder liners

Customers from the following industries use components finished with HART-COAT®:

  • Can hard-anodized aluminum alloys replace steel?

    Yes, given that the aluminum alloy has the right mechanical properties.

    The aluminum component's mechanical properties (such as the tensile strength) must be suitable for the respective application.

    Compared to steel, aluminum offers the following advantages:

    • Aluminum is about two thirds lighter than steel
    • Use of aluminum components in vehicles
      • Fuel savings and thus less pollutant emissions
      • Realisation of higher payloads
      • Increased driving ranges of electric vehicles
    • Aluminum is a good thermal conductor (Al: 170-220 W/mK, steel: 47-58 W/mK)
    • Aluminum can be processed and shaped in countless ways
  • Why do components have to befree from oil and residues before being hard-anodized?

    Usually, oil and residues compromise the quality of the coating. They can have an insulating effect that prevents the formation of a layer. Since hard-anodized coatings are conversion coatings, residues may later be visible on the surface. When oil or other residues come off during the coating process, the anodizing bath becomes soiled. Oil and residues inside the bath may reattach to the components when these are being removed from the bath.

  • What are the advantages of hard anodizing?

    Hard anodizing improves the wear and corrosion resistance of aluminum components. Hard-anodized coatings are thermal and electric insulators. Since they are electrically insulating, they can be used for electrical isolation. Components treated with hard anodizing may be temporarily exposed to high thermal loads. As the high temperatures reach the base material evenly and with delays only, they will not damage the components. Hard anodizing is the finishing method to go for when it comes to aluminum components for mass applications.

  • Are there any weight limits with regard to the workpieces to be coated?

    The maximum weight depends on the capabilities of the coating plant. As aluminum has a low density, it is not the weight, but rather the dimensions of the component which have to be considered.

  • What is so special about these coatings?

    These coatings are so-called conversion coatings. Parts of the base material become fully integrated with the coating, which results in an extremely high bonding strength of the anodic layers. With hard coat anodizing, the workpiece surface is transformed into a hard aluminum oxide layer, allowing for an exceptionally strong bond between the coating and the base material. The produced film thickness is also very thin.

Product enquiry

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