METHOD FOR THE SELECTIVE REMOVAL OF ZINC IONS FROM ALKALINE BATH SOLUTIONS IN THE SERIAL SURFACE TREATMENT OF METAL COMPONENTS

Abstract

The present invention relates to a method for the serial surface treatment of metal components that have zinc surfaces, wherein the method comprises an alkaline pretreatment, and a method for the selective removal of zinc ions from an alkaline bath solution for the serial surface treatment of metal surfaces that have zinc surfaces. According to the invention, in order to perform each method, part of the alkaline aqueous bath solution is brought in contact with an ion exchange resin that bears functional groups selected from —OPO.sub.3X.sub.2/n and/or —PO.sub.3X.sub.2/n, wherein X is either a hydrogen atom or an alkali metal and/or alkaline-earth metal atom to be exchanged having the particular valency n.

Claims

1. A method for the selective removal of zinc ions from an alkaline aqueous bath solution for the serial surface treatment of metal components that have surfaces of zinc, said alkaline aqueous bath solution being stored in a system tank, wherein the alkaline aqueous bath solution contains: a) at least 50 mg/kg of iron(III) ions; b) at least 50 mg/kg of zinc(II) ions; and c) a complexing agent Y in the form of water-soluble condensed phosphates; water-soluble organic compounds that have at least one functional group selected from —OPO.sub.3X.sub.2/n, —PO.sub.3X.sub.2/n, and combinations thereof, wherein X is either a hydrogen atom or an alkali metal and/or alkaline-earth metal atom having a particular valency n; and combinations thereof; wherein the alkaline aqueous bath solution has a molar ratio of the complexing agent Y, with respect to the element phosphorus, to a total amount of the iron(III) ions and the zinc(II) ions that is greater than 1.0; the method comprising steps of: contacting a part of the bath solution with an ion exchange resin that bears functional groups containing —OPO.sub.3X.sub.2/n and/or —PO.sub.3X.sub.2/n, wherein X is either a hydrogen atom or an alkali metal and/or alkaline-earth metal atom to be exchanged having a particular valency n.

2. The method according to claim 1, wherein the alkaline aqueous bath solution has a molar ratio of the complexing agent Y, with respect to the element phosphorus, to the iron(III) ions that is greater than 1.5.

3. The method according to claim 1, wherein the iron(III) ions in the alkaline aqueous bath solution are present in an amount of at least 100 mg/kg, but not more than 2 g/kg.

4. The method according to claim 1, wherein the alkaline aqueous bath solution has a pH value that is at least 9 and a free alkalinity that is at least 0.5 points but less than 50 points.

5. The method according to claim 1, wherein the alkaline aqueous bath solution contains not more than 0.6 g/kg of aluminum dissolved in water.

6. The method according to claim 1, wherein the ion exchange resin has, in total, at least 1.0 mol of the functional groups selected from —OPO.sub.3X.sub.2/n and/or —PO.sub.3X.sub.2/n per kilogram of the ion exchange resin.

7. The method according to claim 1, wherein the ion exchange resin has a polymer backbone based on the monomers styrene, divinylbenzene and/or based on phenol-formaldehyde condensates.

8. The method according to claim 1, wherein the functional groups of the ion exchange resin are selected from aminoalkyl phosphonic acid groups.

9. The method according to claim 8, wherein the aminoalkyl phosphonic acid groups are selected from aminomethyl phosphonic acid groups conforming to —NR.sup.1—CH.sub.2—PO.sub.3X.sub.2/n, wherein X is either a hydrogen atom or an alkali metal and/or alkaline-earth metal atom to be exchanged having the particular valency n and R.sup.1 is a hydrogen atom or an alkyl, cycloalkyl, or aryl residue.

10. The method according to claim 1, wherein the complexing agent Y of the alkaline aqueous bath solution additionally contains, in the α or β position with respect, to an —OPO.sub.3X.sub.2/n and/or —PO.sub.3X.sub.2/n group, an amino, hydroxyl, or carboxyl group.

11. The method according to claim 1, wherein the ion exchange resin is a solid, which is in the form of beads having a bead diameter in the range of 0.2-2 mm.

12. A method for wet-chemical surface treatment of metal components, which have surfaces of zinc and aluminum and which are serially wet-chemically pretreated comprising steps of: A.) contacting metal components having surfaces of zinc and aluminum with an alkaline bath solution, which is stored in a system tank and contains: a) a complexing agent Y in the form of water-soluble condensed phosphates and/or in the form of water-soluble organic compounds, which have at least one functional group selected from —OPO.sub.3X.sub.2/n and/or —PO.sub.3X.sub.2/n, wherein X is either a hydrogen atom or an alkali metal and/or alkaline-earth metal atom having the particular valency n, and b) iron(III) ions, wherein the alkaline bath solution in the wet-chemical pretreatment has a pH value that is greater than 10 and a free alkalinity that is at least 0.5 points, but less than 50 points; wherein a maximum value “Zn.sub.max” for concentration of dissolved zinc in the alkaline bath solution of the system tank is not greater than Zn.sub.max according to Formula I:
Zn.sub.max=0.0004×(pH−9)×[FA]+0.6×[Y]  (I) pH is pH value; Zn.sub.max is the maximum value for concentration of dissolved zinc in mmol/l; [FA] is free alkalinity in mmol/l; [Y] is concentration in mmol/l of complexing agents Y in the form of water-soluble condensed phosphates, calculated as P.sub.2O.sub.6, and/or in the form of water-soluble organic compounds that have at least one functional group selected from —COOX.sub.1/n, —OPO.sub.3X.sub.2/n, and/or —PO.sub.3X.sub.2/n, wherein X is either a hydrogen atom or an alkali metal and/or alkaline-earth metal atom having the particular valency n; and B.) preventing the maximum value Zn.sub.max in the wet-chemical pretreatment from being exceeded by: 1) contacting at least part of the alkaline bath solution of the system tank with an ion exchange resin that bears functional groups containing —OPO.sub.3X.sub.2/n, and/or —PO.sub.3X.sub.2/n, wherein X is either a hydrogen atom or an alkali metal and/or alkaline-earth metal atom to be exchanged having the particular valency n, and 2) returning the part of the alkaline bath solution that was brought in contact with the ion exchange resin to the system tank.

13. The method according to claim 12, wherein the iron(III) ions in the alkaline aqueous bath solution are present in an amount of at least 50 mg/kg, but not more than 2 g/kg.

14. The method according to claim 12, wherein the serial wet-chemical surface treatment of the metal components occurs at least for such a quantity of metal components that a total area of only zinc surfaces of the metal components in square meters that is greater than the following term is wet-chemically pretreated with the alkaline bath solution of the system tank: $\frac{V_B\times {\mathrm{Zn}}_{\max }\times M_{\mathrm{Zn}}}{\Delta .Math..Math.m_{\mathrm{Zn}}}$ wherein: V.sub.B is bath volume in m.sup.3; Zn.sub.max is maximum concentration of dissolved zinc in mmol/l M.sub.Zn is molar mass of zinc in g/mol Δm.sub.Zn is area-standardized pickling removal with respect to the zinc surfaces of the metal components in g/m.sup.2.

15. The method according to claim 12, wherein the alkaline aqueous bath solution contains not more than 0.6 g/kg of aluminum dissolved in water.

16. The method according to claim 12, wherein the ion exchange resin has, in total, at least 1.0 mol of the functional groups selected from —OPO.sub.3X.sub.2/n and/or —PO.sub.3X.sub.2/n per kilogram of the ion exchange resin.

17. The method according to claim 12, wherein the ion exchange resin has a polymer backbone based on the monomers styrene, divinylbenzene and/or based on phenol-formaldehyde condensates.

18. The method according to claim 12, wherein the functional groups of the ion exchange resin are aminomethyl phosphonic acid groups conforming to —NR.sup.1—CH.sub.2—PO.sub.3X.sub.2/n, wherein X is either a hydrogen atom or an alkali metal and/or alkaline-earth metal atom to be exchanged having the particular valency n and R.sup.1 is a hydrogen atom or an alkyl, cycloalkyl, or aryl residue.

19. The method according to claim 12, wherein the complexing agent Y of the alkaline aqueous bath solution additionally contains, in the α or β position with respect to an —OPO.sub.3X.sub.2/n and/or —PO.sub.3X.sub.2/n group, an amino, hydroxyl, or carboxyl group, preferably a hydroxyl group, especially preferably a hydroxyl group but no amino group.

Description

EXAMPLES

[0087] An alkaline iron-coating treatment solution was prepared and sent across columns having different ion exchange resins in parallel. The specific load per column was 5 BV/h (20° C.), wherein the resin volume was 0.1 l at a layer height of 30 cm. [0088] The iron-coating treatment solution was composed as follows: [0089] free alkalinity (FA): 16 points; [0090] bound alkalinity: 46 points; [0091] pH value: 11.7; [0092] Fe(III) ion concentration: 0.35 g/l; [0093] Zn(II) ion concentration: 1.0 g/l; [0094] HEDP: 12.0 g/l; [0095] P.sub.2O.sub.7: 1.5 g/l; [0096] PO.sub.4: 3.0 g/l;

[0097] The separating performance of different ion exchange resins was examined and is presented in Table 1. To determine the separating performance, the concentration of the elements zinc and iron was examined in effluent samples of the iron-coating treatment solution during a throughput of 10 BV (bed volumes) of the iron-coating treatment solution at 20° C. by means of ICP-OES.

TABLE-US-00001 TABLE 1 Ion Exchange Resins A B C Functional group —NH—CH.sub.2—PO.sub.3H.sub.2 —NH—C(═S)—NH.sub.2 Polyamines Number density* [eq./l] 1.15 1.0 1.15 Matrix Polystyrene Polystyrene Acrylate- divinylbenzene copolymer Particle size [mm] 0.55 0.55 0.7 Selectivity.sup.1 ⊕⊕ Ø ⊙ Zn load.sup.2 [g/l] 20-25 <1 2 *with respect to the particular functional group in the dry resin material .sup.1determined after the throughput of 2 BV and determined as the quotient ΔZn/ΔFe from the concentration difference of the elements Zn and Fe ⊕⊕more than 1000 ⊕between 100 and 1000 ⊙between 5 and 100 Øless than 5 .sup.2determined after 10 BV and with respect to the dry resin material