Abstract: The invention relates to a treatment solution for producing substantially chromium(VI) free black conversion layers on alloy layers containing zinc, the solution comprising the following: (i) at least one first carboxylic acid having 1 to 8 carbon atoms, the acid containing no polar groups with exception of the carboxyl group and being a monocarboxylic acid, (ii) at least one second carboxylic acid having 1 to 8 carbon atoms, comprising at least one further polar group that is selected from —OH, —SO3H, —NH2, —NHR, —NR2, —NR3+, and —COOH (wherein R is a C1-C6 alkyl group), (iii) 20 to 400 mmol/l Cr3+ and (iv) 50 to 2000 mmol/l NO3?. The invention further provides a method for the black passivation of surfaces containing zinc, wherein the surface to be treated is immersed into such a treatment solution.

Abstract: The invention relates to a treatment solution for producing substantially chromium(VI)-free black conversion layers on alloy layers containing zinc, the solution comprising the following: (i) at least one first carboxylic acid having 1 to 8 carbon atoms, the acid containing no polar groups with exception of the carboxyl group and being a monocarboxylic acid, (ii) at least one second carboxylic acid having 1 to 8 carbon atoms, comprising at least one further polar group that is selected from —OH, —SO3H, —NH2, —NHR, —NR2, —NR3+, and —COOH (wherein R is a C1-C6 alkyl group), (iii) 20 to 400 mmol/l Cr3+ and (iv) 50 to 2000 mmol/l NO3?, and wherein (a) the total concentration of carboxyl groups of the first carboxylic acid(s) is within a range of 5 to 150 mmol/l, (b) the total concentration of carboxyl groups of the second carboxylic acid(s) is within a range of 5 to 150 mmol/l, (c) the ratio of the concentration (in mol/l) of NO3? to Cr3+ is ?1, and (d) the following prerequisite is met (formula (I)), 0.