CONVERSION LAYERS FOR SURFACES CONTAINING ZINC
The invention relates to a process for producing a coating layer which protects against corrosion, in which a surface to be treated is brought into contact with an aqueous treatment solution containing at least one source of chromium(III) ions and also at least one organic compound which oxidizes zinc in weakly acidic or acidic solution. Here, the decorative and functional properties of the surface are retained or improved. In addition, the known problems associated with the use of chromium(VI)-containing compounds are avoided.
1 . A process for producing essentially chromium(VI)-free conversion layers on zinc or zinc alloy layers, comprising
immersing a surface comprising a zinc or zinc alloy layer in an aqueous treatment solution containing Cr 3+ ions and further comprising at least one organic compound selected from:
where R 1 to R 5 are each, independently of one another,
a) a hydrogen atom or an alkyl group which has from 1 to 10 carbon atoms and may be linear or branched or
b) an —NR 2 , —NO 2 , —COOR, —OR, —SO 3 R group where R═—H or an alkyl group which has from 1 to 5 carbon atoms and may be linear or branched,
with the proviso that from 0 to 2 radicals R 1 to R 5 are selected from group b).
2 . The process according to claim 1 , characterized in that the organic compound is selected from the group consisting of
where R 1 to R 5 are each, independently of one another,
a) a hydrogen atom or an alkyl group which has from 1 to 10 carbon atoms and may be linear or branched or
b) an —NR 2 , —NO 2 , —COOR, —OR, —SO 3 R group where R═—H or an alkyl group which has from 1 to 5 carbon atoms and may be linear or branched,
with the proviso that from 0 to 2 radicals R 1 to R 5 are selected from group b).
3 . The process according to claim 1 , characterized in that the organic compound is selected from the group consisting of
where R 1 to R 5 are each, independently of one another,
a) a hydrogen atom or an alkyl group which has from 1 to 10 carbon atoms and may be linear or branched or
b) an —NR 2 , —NO 2 , —COOR, —OR, —SO 3 R group where R═—H or an alkyl group which has from 1 to 5 carbon atoms and may be linear or branched,
with the proviso that from 0 to 2 radicals R 1 to R 5 are selected from group b).
4 . The process according to claim 1 , characterized in that the organic compound is selected from the group comprising m-nitrobenzoic acid, 2-hydroxy-5-nitrobenzoic acid, 3,5-dinitro-salicylic acid, 2,4-dinitrophenol, m-nitrobenzene-sulfonic acid, N-methylmorpholine N-oxide, pyridine N-oxide and p-benzoquinone.
5 . The process according to claim 1 , characterized in that the concentration of the organic compounds I.-V. is in the range from 30 mg/l to 30 g/l of the substance.
6 . The process according to claim 1 , characterized in that the treatment solution contains from 0.2 g/l to 20 g/l of chromium(III) ions.
7 . The process according to claim 1 , characterized in that the treatment solution contains at least one anion selected from the group comprising methanesulfonate, sulfate, hydrogensulfate, borate, acidic boric esters, phosphate, hydrogenphosphate, dihydrogenphosphate, chloride, iodide, fluoride, nitrate, hexafluoro-silicate, hexafluorotitanate, tetrafluoroborate, hexafluoroantimonate, hexafluorophosphate, phosphate, hydrogenphosphate, dihydrogenphosphate and anions of esters of phosphoric acid.
8 . The process according to claim 1 , characterized in that the treatment solution additionally contains at least one metal or metalloid selected from the group comprising Sc, Y, Ti, Zr, Mo, W, Mn, Fe, Co, Ni, Zn, B, Al, Si, P in a concentration of from 0.01 to 10 g/l.
9 . The process according to claim 1 , characterized in that the treatment solution contains from 0.05 g/l to 20 g/l of at least one complexing agent selected from the group comprising polycarboxylic acids, hydroxycarboxylic acids, hydroxypolycarboxylic acids, aminocarboxylic acids and hydroxyphosphonic acids.
10 . The process according to claim 1 , characterized in that the treatment solution has a pH in the range from pH 0.1 to pH 7.
11 . A process for the treatment of a zinc or zinc alloy surface,
characterized in that the surface to be treated is dipped into a treatment solution comprising
at least one source of Cr 3+ ions and
at least one organic compound selected from the group consisting of
where R 1 to R 5 are each, independently of one another,
a) a hydrogen atom or an alkyl group which has from 1 to 10 carbon atoms and may be linear or branched or
b) an —NR 2 , —NO 2 , —COOR, —OR, —SO 3 R group where R═—H or an alkyl group which has from 1 to 5 carbon atoms and may be linear or branched,
with the proviso that from 0 to 2 radicals R 1 to R 5 are selected from group b).
12 . The process according to claim 11 , characterized in that the organic compound is selected from the group consisting of
where R 1 to R 5 are each, independently of one another,
a) a hydrogen atom or an alkyl group which has from 1 to 10 carbon atoms and may be linear or branched or
b) an —NR 2 , —NO 2 , —COOR, —OR, —SO 3 R group where R═—H or an alkyl group which has from 1 to 5 carbon atoms and may be linear or branched,
with the proviso that from 0 to 2 radicals R 1 to R 5 are selected from group b).
13 . The process according to claim 11 , characterized in that the organic compound is selected from the group consisting of
where R 1 to R 5 are each, independently of one another,
a) a hydrogen atom or an alkyl group which has from 1 to 10 carbon atoms and may be linear or branched or
b) an —NR 2 , —NO 2 , —COOR, —OR, —SO 3 R group where R═—H or an alkyl group which has from 1 to 5 carbon atoms and may be linear or branched,
with the proviso that from 0 to 2 radicals R 1 to R 5 are selected from group b).
14 . The process according to claim 11 , characterized in that the organic compound is selected from the group comprising m-nitrobenzoic acid, 2-hydroxy-5-nitrobenzoic acid, 3,5-dinitro-salicylic acid, 2,4-dinitrophenol, m-nitrobenzene-sulfonic acid, N-methylmorpholine N-oxide, pyridine N-oxide and p-benzoquinone.
15 . The process according to claim 11 , characterized in that the concentration of the organic compounds I.-V. is in the range from 30 mg/l to 30 g/l of the substance.
16 . The process according to claim 11 , characterized in that the treatment solution contains from 0.2 g/l to 20 g/l of chromium(III) ions.
17 . The process according to claim 11 , characterized in that the treatment solution contains at least one anion selected from the group comprising methanesulfonate, sulfate, hydrogensulfate, borate, acidic boric esters, phosphate, hydrogenphosphate, dihydrogenphosphate, chloride, iodide, fluoride, nitrate, hexafluoro-silicate, hexafluorotitanate, tetrafluoroborate, hexafluoroantimonate, hexafluorophosphate, phosphate, hydrogenphosphate, dihydrogenphosphate and anions of esters of phosphoric acid.
18 . The process according to claim 11 , characterized in that the treatment solution additionally contains at least one metal or metalloid selected from the group comprising Sc, Y, Ti, Zr, Mo, W, Mn, Fe, Co, Ni, Zn, B, Al, Si, P in a concentration of from 0.01 to 10 g/1.
19 . The process according to claim 11 , characterized in that the treatment solution contains from 0.05 g/l to 20 g/l of at least one complexing agent selected from the group comprising polycarboxylic acids, hydroxycarboxylic acids, hydroxypolycarboxylic acids, aminocarboxylic acids and hydroxyphosphonic acids.
20 . The process according to claim 11 , characterized in that the treatment solution has a pH in the range from pH 0.1 to pH 7.
21 . The process according to claim 11 , characterized in that the treatment solution has a temperature in the range from 10° C. to 90° C.