It is well known that aluminium and some of its alloys develop a natural oxide film which gives a degree of protection against corrosion. Under many conditions of exposure, particularly damp and humid conditions, this natural protection is inadequate and corrosion occurs.
Aluminium, even when thoroughly cleaned, does not present an ideal surface for painting. Extrusions emerging from the die at about 500oC immediately oxidise on contact with the air. The oxides of the alloying elements (magnesium, silicon, copper, manganese, etc.), swarf and extrusion 'fines' (mechanical linishing can be offered as an additional service), damage caused during handling and organic materials (dirt and oils) must all be removed to ensure adequate paint adhesion.
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Oil |
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Metal inclusions, swarf. |
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Alloying metals. |
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Thin section through an extruded surface covered with contaminants.
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Metal pretreatment, fundamentally, has two well-established aims:
Protection against corrosion and the preparation of a surface to which paint will stick.
After inspection of the metal for any obvious defects which might affect the quality of the finish, the aluminium extrusions, pressings or sheet are loaded into specially designed baskets. They are then taken, under computer controlled conditions through the following 11-stage immersion process.
1. |
Alkaline Degreasing
This removes oil, dirt and some oxide.
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Degreasing lifts organic contaminants and a little oxide from the surface.
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2. |
Alkaline Etching
Removes swarf and the natural aluminium oxide layer to provide a chemically abraded surface for the paint to bond onto. Metal removal should be between 8 and 10 g/m2
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Etching solution removes swarf (provided that it is not too deeply embedded in the substrate) and a layer of aluminium.
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3. |
Rinse
Overflowing and recirculating warm water rinse.
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4. |
Rinse
Also overflowing and recirculating but at ambient temperature.
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5. |
Desmutting
The removal by the etching bath of a layer of aluminium means that more of the alloying oxides which are not soluble in strong alkaline conditions will now be left on the surface as a grey 'smut'. The smut and aluminates formed from the etching operation are soluble in dilute acids and this solution removes them.
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Desmutting removes alloying oxides to produce a 'pure' aluminium layer.
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6. |
Rinse
Overflowing water rinse.
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7. |
Conversion Coating
Provides an inert, amorphous, iridescent green, chromium-phosphate film which is integral with the prepared aluminium. The coating weight should be between 0.6 and 1.2 g/m2.
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Chromate layer deposited on the surface.
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8. |
Rinse
Overflowing water rinse.
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9. |
Rinse
Deionised water rinse.
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10. |
Rinse
Deionised water is controlled so that the conductivity does not exceed 30µS/cm2. The effectiveness of the previous chemical treatments can be reduced by poor rinsing. The use of high purity, deionised water in the final rinse ensures that the aluminium will be in optimum condition for painting.
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11. |
Drying
Dried at temperatures below 85oC. The use of excessive heat to dry components whilst the chromate conversion coating is still 'soft' may result in 'mud cracking'. This can reduce adhesion of the paint coating and its long term durability.
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The aluminium has now finally been cleaned down to pure metal, coated with a chromate film and carefully dried, providing a substrate to which paints bond remarkably well.
