Chromium layers are passivated by the presence of a mixture of air and oxygen. The potential of the layers of chromium is hence displaced into the positive range – the surface hence becomes nobler. This is the basis of the corrosion mechanism for Cu-Ni-Cr and Ni-Cr layers.
The hexavalent chromium layer deposited considered separately, is passivated additionally in the chromium plating electrolyte consisting of chromic acid anhydride. The hexavalent chromium plating is deposition from the acid form, whereas trivalent chromium plating is carried out from the salt form. Here this type of passivation is missing! That is why it is necessary to form a protective film or a corrosion-inhibiting conversion layer. In the following diagram you can see the progressive formation of the passivation layer with SAPHIR 2000 EPT.