What we call as phosphating to steel an iron is called chromating for aluminium surfaces. There are yellow, green and transparent chromating types. Yellow chromate is hexavalent coating and green chromate is trivlent coating. Yellow chromate coats Cr+6, green chromate coats Cr+3. Drying temperature must not pass over 65 º C for yellow chromate and 85 º C for green and transparent chromate coatings. Both provide high corrosion resistance and paint adherence.
It is important to provide a clean, grease-free surface before chromate application. If hot degreasing bath is prepared, following nitric acid bath can be used for pickling. On the other hand, acidic degreasing baths have pickling ability with it selves. Chromating and paint adhesion will be much better on pickled and degreased aluminium surface.