Chromatization / Chemical passivation
The non-noble zinc surface covers itself in contact with atmospheric oxygen with a thin passive layer, which then slows down the further oxidation. Unlike other metals such as e.g. Chrome or aluminum does not protect this self generated passive layer from zinc as it would be technically desirable.
Chromating, which are applied after the actual galvanizing process, produce an oxidic barrier layer, which significantly hinder the corrosive attack on the substrate (Zn layer / white rust formation) and thus considerably increase the corrosion protection of the galvanized steel. This is also referred to as passivation, because the base metal zinc is hindered by this protective layer to further oxidation.
It must be distinguished between different chromating types, which have different passivating power and thus lead to differences in the corrosion resistance of the galvanized steel.
Today only chromium (VI) -free passivations are in use.