Hard anodizing
Description
Anodizing is an electrochemical process of oxidation in which the growth of a hard aluminium oxide (alumina) layer takes place over the surface of aluminium components. The resulting layer is not superficially stuck, but as it is a chemical reaction it becomes part of the metal itself.
The reaction has place in an electrolyte medium base sulfuric acid, at low temperature (0 to 5ºC). This way the dilution of the layer that takes place in the conventional or clear anodizing is avoided, obtaining a low porosity, dense, very hard and compact aluminium oxide (Al₂O₃) layer.
This process is executed according to the rule MIL-A-8625 F (Type III, class 1 & 2).
Properties
HARDNESS
Similar to steel.
Values over 430 Vickers (43HRc) are reached.
ABRASION RESISTANCE
10 times upper the resistance of the conventional anodizing.
Layer’s loss of weight lower than 15 mg / 10.000abrasion cycles (s / FED-STD-141).
CORROSION RESISTANCE
No corrosion evidence within more than 850 hours exposure to salt spraying at 5% (s/ ASTM B.117).
HIGH THERMAL AND ELECTRICAL INSULATION
Similar to porcelain. (The anodic layer resists up to 2000 ᵒC direct flame)
DIMENSIONAL VARIATIONS
The oxide layer grows up 50% in depth and the remaining 50% stays superposed on the surface. That is why it is used as a technique for re-dimensioning and recovery of pieces as well.
Properties are altered according to the alloy and the thickness of the layer applied.
Layers over 150 microns may be obtained in alloys containing up to:
- Al-Mg: 6% magnesium
- Al-Cu: 5% copper
- Al-Zn: 8% zinc
- Al-Si: 13% silicon
| WROUGHT ALLOYS | HARD ANODIZING | FINAL COLOUR |
|---|---|---|
| Serial 1XXX (1050-1070-1080-1100-1200-1300) | Good / Very good | Bronze / Grey |
| Serial 2XXX (2011-2014-2017-2024-2030) | Regular / Good | Bronze |
| Serial 3XXX (3003-3004-3005-3105) | Good | Grey |
| Serial 5XXX (5005-5050-5052-5056-5083-5086-5154-5454-5657-5754) | Very good | Grey / Brown |
| Serial 6XXX (6005-6061-6060-6063-6082-6261-6262-6351-6082) | Very good | Grey / Black |
| Serial 7XXX (7015-7049-7075-7020) | Good / Very good | Bronze / Brown |
| CASTING ALLOYS | HARD ANODIZING | FINAL COLOUR |
|---|---|---|
| L-2051-2331-2332-2341 | Very good | Grey |
| L-2570-2571-2610-2620-2710 | Good | Grey |
| L-2140-2150-2630-2651 | Regular | Grey |
A specific process in order to get a hard anodizing of high quality in alloys with high copper content (Serial 2XXX) is also available.
Colour
The colour of the anodic layer is based on the aluminium alloy and on the thickness of the layer, turning to bronze, brown and grey-black tones. In any case, a black-stain may be made by means of the introduction of a pigment in the pores of the anodic layer before its sealing. Our metalwork workshops are available in order to anodize in hard black pieces up to 3m long.
Sealing
Being the resultant anodic layer porous, it is mandatory to accomplish a sealing after the anodizing, so as to protect the substrate from external elements and let the aluminium oxide layer inert and free from porosity.
There are different sealing techniques. The most common one is made at high temperatures, however this type of sealing involves two inconveniences: the hardness of the layer is reduced and encourages cracking creation.
That is why from the very beginning ANESDUR carries out the final sealing known as “cold sealing” that consists in plunging the anodized pieces into a bath containing fluoride and nickel salts at 28 – 30 ᵒC. With this “cold sealing”, besides optimizing the process by working at “low temperature”, a faultless pores occlusion of the anodizing layers is obtained with no reduction in hardness properties and wear resistance properties of the anodic layer.
Masking
In those cases in which the treatment of the whole piece is not required, the masking will be imperative on the areas where the increase of the anodic layer is not demanded. For the aforementioned performance plugs, adhesive tapes, lacquers, etc., are used, depending on the surfaces to be covered.
