Method for coating zinc die-cast parts, multi-layered coating for the protection of zinc die-cast parts, and coated zinc die-cast part

1 . A method for coating zinc die-cast parts comprising the steps:

i) treating the zinc die-cast parts with a first liquid containing at least one builder and at least one surfactant;

ii) treating the zinc die-cast parts with a second liquid to form a first layer on a surface of the zinc die-cast parts, where the second liquid contains at least one chromium(III) complex and at least one sulfate;

iii) treating the zinc die-cast parts with a third liquid to form a second layer on the first layer, where the third liquid contains inorganic nanoparticles; and

iv) drying of the treated zinc die-cast parts,

wherein the at least one sulfate in the second liquid is selected from the group consisting of magnesium sulfate, sodium hydrogen sulfate, potassium hydrogen sulfate, and combinations thereof, and

wherein the chromium(III) complex is a chromium(III) fluoride complex.

2 . The method according to claim 1 , wherein the third liquid further comprises a polymer dispersion.

3 . The method according to claim 1 , wherein the third liquid comprises the inorganic nanoparticles as a colloidal dispersion in a range of 1.5% by weight to 10% by weight based on weight of the third liquid.

4 . The method according to claim 1 , wherein the inorganic nanoparticles contain silicon dioxide or consist of silicon dioxide.

5 . The method according to claim 1 , wherein the builder is a phosphate and/or wherein the surfactant is a non-ionic surfactant.

6 . The method according to claim 1 , wherein the first liquid further comprises an amphoteric surfactant.

7 . The method according to claim 1 , wherein the treating in step i) is carried out at a pH in the range of 11-12.

8 . A multi-layered coating for protection of zinc die-cast parts, constructed by the method according to claim 1 , wherein the coating comprises a first layer containing chromium(III) and a second layer on the first layer containing inorganic nanoparticles.

9 . A coated zinc die-cast part produced by the method according to claim 1 , wherein the surface of the zinc die-cast part has a first layer with chromium(III) and a second layer on the first layer containing inorganic nanoparticles.