Yellow chromating (Topic: 14082)

Yellow chromating can either be carried out in unaccelerated or accelerated chromate baths. The unaccelerated chromate baths are essentially composed of chromiumthreeoxide (or another source of chromium ions) and of hydrogenfluoride (or complex hydrofluoric acids, or their salts). Nitric acid is added to obtain the desired pH, that is usually around 1,8ñ2,1. Other substances are present in the proprietary formulations to obtain better conditions in the industrial use. The temperature in the bath should be around 25ƒC.

In the accelerated formulations potassium ferricyanide is often added, but many other variations have been developed. Due to environmental considerations cyanides are less used.

Investigation of a chromate layer shows that a thin outer layer of chromium ferricyanide and hydrated chromium oxide covers the bulk of the hydrated chromium oxide coating. Finally, at the interface between the film and the aluminium, small amounts of aluminium oxide and fluoride are present. See also figure below.

Structure of chromate pre-treatment [27].

Chromate conversion coatings lead to a yellow layer on the substrate, and the intensity varies from iridescent yellow to golden tan. The coating weight is comprised between 0,4 and 1,0 g/m2. The limit value of 1 g/m2 has been established because of powdering phenomenon of the conversion coating if exceeded.