Aqueous post treatment composition and method for corrosion protection

Method for corrosion protection of an iron containing substrate comprising, in this order, the steps of

• i. providing an iron containing substrate,
• ii. electrolytic depositing a zinc layer on at least a part of said iron containing substrate by using a zinc deposition bath,
• iii. depositing a passivation layer on at least a part of said zinc layer being on at least a part of said iron containing substrate by using a passivation deposition bath,
• wherein the method is characterized by an additional method step iv, which is subsequently executed after method step iii, namely
• iv. depositing a post treatment layer on at least a part of said passivation layer of method step iii. by making use of an aqueous post treatment composition, which comprises at least one chromium (III) ion source, does not comprise hexavalent chromium, and at least one compound containing the chemical element silicon;
• wherein a molar ratio of silicon versus chromium is given in said composition with the proviso, that said molar ratio is ranging from 2600:1 to 20:1 for using an acid zinc deposition bath in method step ii.;
• or that said molar ratio is ranging from 5200:1 to 20:1 for using an alkaline zinc deposition bath in method step ii, wherein the at least one compound containing the chemical element silicon is an anionic colloidal silica, or the at least one compound containing the chemical element silicon is a silane.