Chromium(vi)-free black passivation of surfaces containing zinc

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)), 0.05 ≤ c  ( C   1 ) c  ( C   2 ) * c  ( Cr 3 + ) c  ( NO 3 - ) ≤ 0.5 wherein c(C1) is the total concentration (in mol/l) of carboxyl groups of the first carboxylic acid(s), c(C2) is the total concentration (in mol/l) of carboxyl groups of the second carboxylic acid(s), c(Cr3+) is the concentration (in mol/l) of Cr3+, and c(NO3−) is the concentration (in mol/l) of 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.

Dependencies are inferred from the claim text.