In the not too distant past, one of the most common compounds used in enhancing the corrosion resistance of zinc plating was yellow hexavalent chromate. This produced the characteristic yellow or gold tint to the surface of the substrate after the plating was completed.
To correct this issue, a clear zinc plating method was developed using either a clear or a blue hexavalent chromate, or sometimes a combination of both. The result was a clear surface finish to the steel or other metal, with a brighter, shinier surface after finishing. At the same time, some of the corrosion resistance was lost, which varied based on the specific metal finishing company and process used.
When there was very little need for corrosion resistance in the environment the part was going to be used clear or blue trivalent chromate was selected as it is safer to use and considered the better option environmentally.
It is important to keep in mind that not all clear zinc plating is completely free of yellow hexavalent chromate or the clear or blue hexavalent chromate. The only way to ensure that the final product is free of the hexavalent chromate compound in the process is to specially ask the finishing company.
Currently throughout much of the world, there is a ban on the use of hexavalent coatings because of their impact on the environment, and also because they are a potential carcinogen.
The development of RoHS or the Restriction of Hazardous Substances Directive was adopted in 2003 throughout all countries in the European Union. While not just directed at yellow zinc plating, it did specifically include the ban on hexavalent chromate, forcing all companies exporting to the European Union or producing goods in those countries to move to using clear zinc plating and passivation to develop the same corrosion resistance and finishing qualities.
In fact, although you will see yellow zinc plating today, this is actually a dye that is used in the actual plating process. It can also be created through the use of a chromate conversion for aluminum that can create a variety of colors as well as to make the surface more workable when it comes to applying paint or glue. These coatings can be very thin or much thicker, and with an experienced company handling the process, tight tolerances can be maintained to any required standards.