Patent ID: 8460534

Claim:
A treatment solution for producing substantially chromium(VI)-free black conversion layers on zinc-containing alloy layers, the solution containing: at least one first carboxylic acid having 1 to 8 carbon atoms, the acid containing no polar groups with exception of the carboxylic group and being a monocarboxylic acid, at least one second carboxylic acid having 1 to 8 carbon atoms, comprising at least one further polar group that is selected from —OH, —SO 3 H, —NH 2 , —NHR, —NR 2 , —NR 3 + and —COOH (wherein R is a C 1 -C 6 alkyl group), 20 to 400 mmol/l Cr 3+ and 50 to 2000 mmol/l NO 3 − , and wherein the total concentration of carboxyl groups of the first carboxylic acid(s) is within a range of 5 to 150 mmol/l, the total concentration of carboxyl groups of the second carboxylic acid(s) is within a range of 5 to 150 mmol/l, the ratio of the concentration (in mol/l) of NO 3 − to Cr 3+ is ≧1, and the following prerequisite is met: 0.1 ≤ c ⁡ ( C ⁢ ⁢ 1 ) c ⁡ ( C ⁢ ⁢ 2 ) * c ⁡ ( C ⁢ ⁢ r 3 + ) c ⁡ ( NO 3 - ) ≤ 0.2 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(Cr 3+ ) is the concentration (in mol/l) of Cr 3+ , and c(NO 3 − ) is the concentration (in mol/l) of NO 3 − .