Process for forming corrosion protection layers on metal surfaces
View Patent ↗The invention relates to a process for producing a corrosion-inhibiting coating for substrates having a surface consisting of zinc, magnesium, aluminum or one of their alloys, wherein the surface to be treated is brought into contact in direct succession with two aqueous treatment solutions containing chromium(III) ions, metal ions of the substrate surface to be treated and at least one complexing agent. The first treatment solution has a pH in the range from 1.0 to 4.0, while the second treatment solution has a pH of from 3.0 to 12.0. The process of the invention produces a smaller amount of wastewater polluted with heavy metals.
1. Process for forming corrosion protection layers on a surface of a metal or metal alloy, wherein the metal is zinc, aluminium or magnesium, which comprises the following steps:
i) contacting of the surface of the metal or metal alloy with a first treatment solution comprising
chromium(III) ions,
metal ions selected from the metal or metal alloy, and
at least one complexing agent selected from the group consisting of carboxylic acids, polycarboxylic acids, hydroxycarboxylic acids, aminocarboxylic acids, alcohols, amines and ethers,
where the first treatment solution has a pH of 1.0-3.0, wherein the contacting the surface of the metal or metal alloy with the first treatment solution forms a passivation layer on the surface;
ii) then direct contacting of the passivation layer without a rinsing operation with a second treatment solution comprising
chromium(III) ions,
metal ions selected from the metal or metal alloy, and
at least one complexing agent selected from the group consisting of carboxylic acids, polycarboxylic acids, hydroxycarboxylic acids, aminocarboxylic acids, alcohols, amines and ethers,
where the second treatment solution has a pH of 3.0-12.0, and
iii) drying directly after contacting with the two treatment solutions without a final rinsing step, wherein the direct contacting in ii) without a rinsing operation with the second treatment solution durably fixes the corrosion protection layers to the surface,
wherein the first treatment solution and the second treatment solution have the same composition but are set to different pH values by addition of acid or base to set pH to the first treatment solution and to the second treatment solution.
2. Process according to claim 1 , wherein the two treatment solutions each contain from 0.5 to 200 g/l of the complexing agent.
3. Process according to claim 1 , wherein both treatment solutions contain at least one water-soluble cobalt compound.
4. Process according to claim 1 , wherein the two treatment solutions each contain at least one phosphate compound selected from the group consisting of orthophosphoric acid, polyphosphoric acids, metaphosphoric acid, the salts of these acids, the esters of these acids with organic radicals having up to 12 carbon atoms and mixtures of these compounds.
5. Process according to claim 1 , wherein both treatment solutions contain fluoride ions in an amount from 0.001 to 10 g/l.
6. Process according to claim 1 , wherein both treatment solutions contain chloride ions in an amount from 0.1 to 10 g/l.
7. Process according to claim 1 , wherein the treatment solution in step i) has a temperature of from 10 to 50° C.
8. Process according to claim 1 , wherein the treatment solution in step ii) has a temperature of from 10 to 80° C.
9. Process according to claim 1 , wherein the treatment time in the treatment solution in both steps is in the range from 10 to 600 s.
10. Process according to claim 1 , wherein soluble chromium(III) ions are added in an amount of from 0.5 to 10 g/l to each of the two treatment solutions.
11. Process according to claim 1 , wherein the contacting in step i) is aided by connecting the substrate as cathode in the first treatment solution.
12. Process according to claim 11 , wherein a cathodic current density applied to the substrate is in the range from 0.05 to 10 A/dm 2 .
13. Process for forming corrosion protection layers on a surface of a metal or metal alloy, wherein the metal is zinc, aluminium or magnesium, which comprises the following steps:
preparing a first treatment solution comprising
chromium(III) ions,
metal ions selected from the metal or metal alloy, and
at least one complexing agent selected from the group consisting of carboxylic acids, polycarboxylic acids, hydroxycarboxylic acids, aminocarboxylic acids, alcohols, amines and ethers,
preparing a second treatment solution comprising
chromium(III) ions,
metal ions selected from the metal or metal alloy, and
at least one complexing agent selected from the group consisting of carboxylic acids, polycarboxylic acids, hydroxycarboxylic acids, aminocarboxylic acids, alcohols, amines and ethers,
wherein the first treatment solution and the second treatment solution have the same composition;
adjusting pH of the first treatment solution to a pH of 1.0-3.0 by addition of acid or base;
adjusting pH of the second treatment solution to a pH of 3.0-12.0 by addition of acid or base, and
wherein the first treatment solution and the second treatment solution are adjusted to different pH values;
i) contacting the surface of the metal or metal alloy with the first treatment solution to form a passivation layer on the surface;
ii) without a rinsing operation, contacting the passivation layer with the second treatment solution;
iii) drying directly after contacting with the two treatment solutions without a final rinsing step, wherein the direct contacting in ii) without a rinsing operation with the second treatment solution durably fixes the corrosion protection layers to the surface.