Chromate coating for aluminium surface
Aluminium and aluminium alloys are treated by a corrosion resistant conversion coating that is called “chromate coating” or “chromating”. General method is to clean the aluminium surface and then apply an acidic chromium composition on that clean surface. Chromium conversion coatings are highly corrosion resistant and provide excellent retention of subsequent coatings. Different type of subsequent coatings can be applied to the chromate conversion coating to produce an acceptable surface.
What we call as phosphating to steel an iron is called chromating for aluminium surfaces. Also it is knows as alodine coating. There are yellow, green and transparent chromating types. Yellow chromate coats Cr+6, green chromate coats Cr+3. Coating weight can vary according to application time and coating type. Drying temperature must not pass over 65 º C for yellow chromate and 85 º C for green and transparent chromate coatings.
It is importat to provide clean, grease free surface before chromate application. If hot degreasing bath is prepared, caustic bath and following nitric acid bath can be used for pickling. On the other hand, acidic degreasing baths have pickling ability with itselves. Chromating and paint adhesion will be much better on pickled and degreased aluminium surface.
Along with providing high corrosion resistance and paint adhesion properties to aluminium surface, it is well known that the visual desirability may be improved by forming a chromate coating by contacting the surface with an aqueous conversion coating solution containing chromium ions and other additives.
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