Hard anodizing system:
Due to the various item sizes (from small precise mechanical parts to large-surface running plates) surface treatment is carried out manually or crane operated.
The different types of alloys and geometries of semifinished products to be anodized do not allow an automatic manufacture.
Brief Specialist Information

Hard-Coat designates a procedure in which the surface of aluminium articles is provided with a protective layer by electro-chemical processes. It is an option to the regular anodizing process, which is used for a variety of parts made of aluminium.

Basically, in both of these procedures no impurity layer is applied to the surface of an item. Instead, it is a conversion of the aluminium-surface into an aluminium-oxide, which is microporous and therefore dyeable. With the standard anodizing procedure, aside from the protection of the surface against corrosion, an optically attractive appearance can be reached by using a corresponding pre-treatment. Simultaneously, there is a possibility of many different colourings.

Contrary to the standard anodizing process the protective layers, the protective layers obtainable with hard-coating are considerably harder and thicker – comparable to hard chrome layers – and hence they are preferably used for parts where it is specified by their technical application. Hard-coated layers possess a self-colouring capacity depending on the alloy and the layer thickness. Moreover, their porosity (absorbency of colourings) is significantly lower than with the regular anodizing procedure so that the possibility of the additional colouring is heavily reduced and limited to dark colour tones.

Hard-anodized layers possess no levelling effect, too, so that pits, scratches and damages will be visible after anodizing – therefore, as a rule, likewise a mechanical and/or chemical surface treatment is applied before, in order to eliminate such faults if high optical requirements are demanded. Both pre-treatment procedures involve material removal, though. Previously invisible inhomogeneities of the alloys also appear beyond the anodizing process and may cause optically disturbing structures.

Characteristics of Hard-Coating

  • Hardness: 300 to 450 HV
  • Layer thickness: up to 140 µm
  • Material self-colouring capacity
  • Wear-resistant – similar to hard-chrome
  • Corrosion resistant against climatic influences and a multitude of chemicals
Maximum workable size of item
length: 2.900 mm
breadth: 400 mm
height: 800 mm
weight: 200 kg
Schichtaufbau *Zur Einfärbbarkeit der Schicht siehe Text

Layer Build-Up
*For colouring of the layer see text beside.

Layer Build-Up

During the hard-anodizing process the oxide layer grows (different from decorative anodizing) forming columns to about 50% within the base material, the other 50% builds up externally.

The achievable layer thickness tolerance on hard anodized aluminium components is about ± 5 µm on the outer surface and decreases significantly in bores, hollow spaces and other areas based on the physical and geometrical condition.

A simplified diagram of the oxide layer build-up is figured aside.

Härtebereich bei hochfester Aluminium-Legierung AlZn5,5MgCu

Hardness region on high-strength aluminium alloy AlZn5,5MgCu


The hardness achievable with hard-coating depends on the alloy and generally ranges between 300-450 HV.

For example, the illustrated Vickers-hardnesses against the thicknesses for the alloy AlZn5,5MgCu (former DIN-designation AIZnMgCu1.5) were ascertained during accomplished test sequences. This alloy is especially used for high-strength welding parts in centrifuge constructions.

The cluster-shaped distribution gets generated despite equal processing parameters due to fluctuations of tolerances of the alloy. On the alloy mentioned, usually required layer thicknesses of 40-60 µm normally possess a hardness between 375 and 425 HV.

Aussehen der Harteloxal-Schichten

Appearance of Hard-Coated Layers

Appearance of Hard-Coated Layers

All hard-coated surfaces acquire an alloy depending intrinsic material colour during the anodizing-process. With the dependence on the layer-thickness and the treating temperature the following colours can be obtained:

grey to grey-brown
brown to dark brown
natural black

Due to permitted varitions in alloy tolerances these colours are only partionally reproducible.

Colourability of Hard-Coated Layers

Controlled colouring after the hard-anodizing process is likewise alloy-dependent and possible in only a limited way in most cases. Due to the lower working temperatures of the hard-coating process, the obtained pore diameter is less than for the regular anodization. Consequently, the particles of the of the colouring medium usually have a bigger dia- meter, wherefore it is hard to penetrate into the bulit-up oxide layer.

The neccessary closure of the pores at standard hard-coating processes will be carried out in a finishing working-step (sealing) related to each application only.

Thus hard-coated components are usually used only for technical purposes and not for decorative ones.

Anodized layers, produced according to accerted standards, possess an excellent durability against industrial and sea athmospheres.
Anodizing coatings, which are made according the applies norms, have an excellent stability against industrial and sea atmospheres.

Example for surface treatment
Centrifugal item (AlZn5,5MgCu)

Rohteil, gleitgeschiffen

Rough part, vibratory grinded

Harteloxiert mit Eigenfärbung, 60μm

Hard-coated with intrinsic colouring,
60 µm

Zusätzlich schwarz eingefärbt und verdichtet

Additionally black coloured and sealed


The recommendation for the anodizabilty of aluminium alloys aside is given according to the DIN data sheets.

EQ = Anodizing quality acc. to DIN 17611

= Wear-resistant surfaces possible by hardcoating

= at missing characteristics no further literature references available

Signification of characteristics:

1 = excellent
2 = good
3 = satisfactory
4 = inadequate
5 = not recommendable
6 = unsuitable


D = decorative coating
S = protective coating
HC = Hardcoating


Forgeable alloys DIN EN 573-3
EU-Standard DIN
Designation Anodizability
new former D S HC
EN AW-1050A 3.0255 Al99,5 2 (EQ=1) 1
EN AW-1070A 3.0275 Al99,7 1 1
EN AW-1080A 3.085 Al99,8(A) 1 1
EN AW-1098 3.0385 Al99,98 Al99,98R
EN AW-1200 3.0205 Al99,0 3 1
EN AW-1350A 3.0257 EAl99,5(A) E-Al
EN AW-2007 3.1645 AlCu4PbMgMn AlCuMgPb 5
EN AW-2011 3.1655 AlCu6BiPb AlCuBiPb 6 5
EN AW-2014 3.1255 AlCu4SiMg AlCuSiMn 6 3
EN AW-2017A 3.1325 AlCu4MgSi(A) AlCuMg1 6 2
EN AW-2024 3.1355 AlCu4Mg1 AlCuMg2 6 2
EN AW-2117 3.1305 AlCu2,5Mg AlCu2,5Mg0,5
EN AW-3003 3.0517 AlMn1Cu AlMnCu 4 1
EN AW-3004 3.0526 AlMn1Mg1 4 1
EN AW-3005 3.0525 AlMn1Mg0,5 4 1
EN AW-3103 3.0515 AlMn1 4 1
EN AW-3105 3.0505 AlMn0,5Mg0,5
EN AW-3207 3.0506 AlMn0,6
EN AW-5005 AlMg1(B) 3 1
EN AW-5005A 3.3315 AlMg1(C) AlMg1 2 (EQ=1) 1
EN AW-5019 3.3555 AlMg5 4 1
EN AW-5049 3.3527 AlMg2Mn0,8 4 2
EN AW-5041A 3.3326 AlMg2(B) AlMg1,8
EN AW-5052 3.3523 AlMg2,5 2 1
EN AW-5083 3.3547 AlMg4,5Mn0,7 AlMg4,5Mn 4 2
EN AW-5086 3.3545 AlMg4 AlMg4Mn 3 1
EN AW-5182 3.3549 AlMg4,5Mn0,4 AlMg5Mn
EN AW-5241 3.3525 AlMg2 AlMg2Mn0,3 4 1
EN AW-5454 3.3537 AlMg3Mn AlMg2,7Mn 4 2
EN AW-5754 3.3535 AlMg3 2 (EQ=1) 1
EN AW-6005A 3.3210 AlSiMg(A) AlMgSi0,7 2 1
EN AW-5012 3.0615 AlMgSiPb AlMgSiPb bis 10µm 3
EN AW-6060 3.3206 AlMgSi AlMgSi0,5 1 (EQ) 1
EN AW-6061 3.3211 AlMg1SiCu 3 1
EN AW-6082 3.2315 AlSi1MgMn AlMgSi1 3 1
EN AW-6101B 3.3207 EAlMgSi(B) E-AlMgSi0,5
EN AW-7020 3.4335 AlZn4,5Mg1 AlZn4,5Mg1 3 2
EN AW-7022 3.4345 AlZn5Mg3Cu AlZnMgCu0,5 6 2
EN AW-7075 3.4365 AlZn5,5MgCu AlZnMgCu1,5 6 3
EN AW-8011A 3.0915 AlFeSi(A) AlFeSi

Casting alloys DIN EN 1706
EU-Standard DIN
Designation Anodizability
new former D S HC
EN AC-21000 3.1371 G-/GK-/GF-AlCu4MgTi 3
EN AC-21100 3.1841 G-/GK-AlCu4Ti 3
EN AC-42100 3.2371 G-/GK-/GF-AlSi7Mg0,3 4
EN AC-42200 AlSi7Mg0,6 4
EN AC-43000 3.2381 G-/GK-AlSi10Mg(a) 5
EN AC-43200 3.2383 G-/GK-AlSi10Mg(Cu) 5
EN AC-43300 3.2373 G-/GK-/GF-AlSi9Mg 5
EN AC-43400 3.2382 G-/GK-/GF-AlSi10Mg(Fe) 5
EN AC-44000 3.2211 G-/GK-AlSi11 5
EN AC-44200 3.2373 G-/GK-AlSi12(a) 5
EN AC-4300 3.2582 GD-AlSi12(Fe) 5
EN AC-45000 3.2151 G-/GK-AlSi6Cu4 4
EN AC-46000 3.2163 GD-AlSi9Cu3(Fe) 5
EN AC-46200 3.2163 G-/GK-AlSi8Cu3 5
EN AC-47000 3.2583 G-/GK-AlSi12(Cu) 5
EN AC-47100 3.22982 GD-AlSi12Cu1(Fe) 5
EN AC-48000 GK-AlSi12CuNiMg 5
EN AC-51100 3.3541 G-/GK-/GF-AlMg3(a) 1
EN AC-51200 3.3292 GD-AlMg9 2
EN AC-51300 3.3561 G-/GK-AlMg5 1
EN AC-51400 3.3261 G-/GK-AlMg5(Si) 2
EN AC-71000 AlZn5Mg 2

Casting alloys DIN EN 1725 (replaced by DIN EN 1706)
EU-Standard DIN
Designation Anodizability
new former D S HC
3.2581 G-/GK-AlSi12 6 4 4
3.2583 G-/GK-AlSi12(Cu) 6 4 4
3.2381 G-/GK-AlSi10Mg 4 3 4
3.2383 G-/GK-AlSi10Mg(Cu) 6 4 4
3.2163 G-/GK-AlSi9Cu3 6 6 4
3.2153 G-/GK-AlSi6Cu4 6 6 4
3.2211 G-/GK-AlSi11 6 4 4
3.2373 G-/GK-AlSi9Mg 6 4 4
3.2371 G-/GK-/GF-AlSi7Mg 6 4 4
3.1841 G-/GK-AlCu4Ti 6 5 4
3.1371 G-/GK-/GF-AlCu4TiMg 6 5 4
3.3541 G-/GK-/GF-AlMg3 1 1 1
3.3241 G-/GK-/GF-AlMg3Si 2 1 1
3.3561 G-/GK-AlMg5 1 1 1
3.3261 G-/GK-AlMg5Si 2 1 1
3.2341 G-/GK-AlSi5Mg 4 2 3
3.2163 GD-AlSi9Cu3 6 6 4
3.2982 GD-AlSi12(Cu) 6 6 4
3.2582 GD-AlSi12 6 5 4
3.2382 GD-AlSi10Mg 6 4 4
3.3292 GD-AlMg9 4 2 2