Passivation composition and method for depositing a chromium-comprising passivation layer on a zinc or zinc-nickel coated substrate

1 . An aqueous passivation composition for depositing a chromium-comprising passivation layer on a zinc or zinc-nickel coated substrate, the aqueous passivation composition comprising:

(i) trivalent chromium ions, present in a total concentration from 0.5 g/L to 10 g/L, based on the total volume of the aqueous passivation composition;

(ii) at least one complexing agent for the trivalent chromium ions selected from the group consisting of monocarboxylic acids, dicarboxylic acids, salts thereof, and mixtures thereof, and

(iii) at least one corrosion-inhibiting agent, which is different from the at least one complexing agent for the trivalent chromium ions and comprises

(A) one or more than one substituted azole compound and/or salts thereof, together in a total concentration below 10 mg/L, comprising at least 3-mercapto-1,2,4-triazole in a range from 3.0 mg/L to 9.4 mg/L, based on the total volume of the aqueous passivation composition,

(B) one or more than one unsubstituted or substituted aliphatic organic acid with at least one mercapto-group and/or salts thereof, together in a total concentration in a range from 0.001 mg/L to 100 mg/L, comprising at least 3-mercapto-propionic acid and/or salts thereof in a range from 2 mg/L to 35 mg/L, based on the total volume of the aqueous passivation composition,

or both (A) and (B), and

wherein the aqueous passivation composition has a pH in a range from 1.0 to 4.0.

2 . The passivation composition according to claim 1 , wherein the at least one complexing agent for the trivalent chromium ions is selected from the group consisting of unsubstituted monocarboxylic acids, hydroxyl-substituted monocarboxylic acids, amino-substituted monocarboxylic acids, unsubstituted dicarboxylic acids, hydroxyl-substituted dicarboxylic acids, amino-substituted dicarboxylic acids, salts thereof, halogen ions, and mixtures thereof.

3 . The passivation composition according to claim 1 , wherein the at least one complexing agent for the trivalent chromium ions has a total concentration in a range from 1.0 wt.-% to 15.0 wt.-%, based on the total weight of the passivation composition.

4 . The passivation composition according to claim 1 , further comprising

(v) iron ions in a total concentration from 0 mg/L to 500 mg/L, based on the total volume of the passivation composition.

5 . A method for depositing a chromium-comprising passivation layer on a zinc or zinc-nickel coated substrate, the method comprising the following steps:

(a) providing the zinc or zinc-nickel coated substrate,

(b) providing a passivation composition for depositing a chromium-comprising passivation layer on the zinc or zinc-nickel coated substrate, comprising the aqueous passivation composition according to claim 1 ;

and

(c) contacting the zinc or zinc-nickel coated substrate with said aqueous passivation composition such that a chromium-comprising passivation layer is deposited on the zinc or zinc-nickel coated substrate.

6 . The method according to claim 5 , wherein step (c) is performed at a temperature in a range from 20° C. to 50° C., wherein step (c) is performed for a time period from 10 sec to 180 sec, or wherein step (c) is performed at a temperature in a range from 20° C. to 50° C. and for a time period from 10 sec. to 180 sec.