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A00132.gifGreen chromating (Topic: 14083)

Green chromating is also called phosphochromating. The main components of the phosphochromate baths are phosphoric acid and/or an acid phosphate, hydrofluoric and chromic acid or another source of hexavalent chromium. The pH in the bath is commonly between 1,7 and 1,9, but sometimes lower. pH is maintained by adding phosphoric acid and a buffer component. The temperature in the bath should be between 25 and 30ƒC.

Investigation of a phosphochromate coating shows that the bulk of the coating consists of a hydrated form of chromium phosphate with smaller amounts of chromium oxide present. The chromium oxide is concentrated towards the aluminium surface, and fluorides and aluminium oxides are present in the immediate surface regions as before. See also figure below.

Structure of chromate-phosphate pre-treatment [27]

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Phosphochromate conversion coatings are green in colour. The intensity of the colour varies from iridescent green to dark green, as the coating weight increases from 0,4 to 1,2 g/m2. The coating should be as uniform as possible, adhering to the substrate and free from powder. However, at a visual examination, the colour and its uniformity may vary depending on the material and its surface conditions.