RESOURCE-SAVING METHOD FOR ACTIVATING A METAL SURFACE PRIOR TO PHOSPHATING

Abstract

The present invention relates to a method for phosphating metal surfaces in a layer-forming manner using a colloidal aqueous solution as an activation stage, containing a dispersed particulate constituent, the particulate constituent containing, in addition to dispersed inorganic compounds of phosphates of polyvalent metal cations, polymeric organic compounds as dispersing agents which are composed at least partially of styrene and/or an α-olefin having no more than 5 carbon atoms and are composed at least partially of maleic acid, its anhydride and/or its imide, the polymeric organic compounds additionally comprising polyoxyalkylene units. The cleaning and rinsing stages preceding the activation stage as well as the activation stage itself can be carried out using service water in a resource-saving manner without any loss of activation performance, the colloidal aqueous solution containing at least 0.5 mmol/L of alkaline-earth metal ions dissolved in water.

Claims

1. A method for anti-corrosion pretreatment of a metal material selected from zinc, iron or aluminum or of a component which is composed at least partially of such metal materials, wherein the metal material or the component undergoes firstly activation (i) and then phosphating (ii), in particular zinc phosphating, in consecutive method steps, wherein the activation in method step (i) is carried out by bringing the metal material or the component into contact with a colloidal aqueous solution containing a dispersed particulate constituent (a) comprising: (a1) at least one particulate inorganic compound which is composed of phosphates of polyvalent metal cations at least partially selected from hopeite, phosphophyllite, scholzite and/or hureaulite; and (a2) at least one polymeric organic compound which is composed at least partially of styrene and/or an α-olefin having no more than 5 carbon atoms and is composed partially of maleic acid, its anhydride and/or its imide, wherein the at least one polymeric organic compound additionally comprises polyoxyalkylene units; wherein the colloidal aqueous solution contains an amount of at least 0.5 mmol/L of alkaline-earth metal ions dissolved in water.

2. The method according to claim 1, wherein the colloidal aqueous solution contains at least 1.0 mmol/L and no more than 10 mmol/L of the alkaline-earth metal ions dissolved in water.

3. The method according to claim 2, wherein, in the colloidal aqueous solution, amount of condensed phosphates dissolved in water is less than 0.25, based on phosphate content of the at least one particulate inorganic compound, in each case based on the element P.

4. The method according to claim 2, wherein the colloidal aqueous solution contains at least 1.5 mmol/L, but no more than 10 mmol/L, of the alkaline-earth metal ions dissolved in water.

5. The method according to claim 1, wherein the colloidal aqueous solution has an alkaline pH and contains at least one complexing agent selected from α-hydroxycarboxylic acids, organophosphonic acids and combinations thereof.

6. The method according to claim 5, wherein the complexing agents in the colloidal aqueous solution are present in an amount that is no more than twice the molar amount of the alkaline-earth metal ions dissolved in water.

7. The method according to claim 1, wherein the colloidal aqueous solution in the activation (i), has a pH above 8.0, but below 11.0.

8. The method according to claim 1, wherein phosphate content, calculated as PO.sub.4, in the at least one particulate inorganic compound (a1), based on the dispersed particulate constituent of the colloidal aqueous solution, is at least 25 wt. %.

9. The method according to claim 1, wherein the at least one polymeric organic compound (a2) of the colloidal aqueous solution contain the polyoxyalkylene units in their side chains, the polyoxyalkylene units being present in an amount of at least 40 wt. % but not exceeding 70 wt. %, based on total amount of the at least one polymeric organic compounds (a2).

10. The method according to claim 9, wherein the at least one polymeric organic compounds (a2) of the colloidal aqueous solution further comprise N-heterocycle units comprising one or more of pyridine, imidazole, imidazoline, morpholine, pyrrole and pyrrolidone units; the N-heterocycle units being part of the side chains of the at least one polymeric organic compound (a2).

11. The method according to claim 9, wherein the polyoxyalkylene units of the at least one polymeric organic compound (a2) are at least partially end-capped with an N-heterocycle unit.

12. The method according to claim 1, wherein the colloidal aqueous solution contains at least one thickener as a further component b).

13. The method according to claim 1, wherein total amount of the at least one polymeric organic compounds in the dispersed particulate constituent of the colloidal aqueous solution is at least 3 wt. %, but preferably does not exceed 15 wt. %, based on the dispersed particulate constituent.

14. The method according to claim 1, wherein the colloidal aqueous solution has a D50 value below 1 μm, preferably below 0.4 μm.

15. The method according to claim 1, wherein, in the colloidal aqueous solution, amount of condensed phosphates dissolved in water is less than 0.25, based on phosphate content of the at least one particulate inorganic compound, in each case based on the element P.

16. The method according to claim 15, wherein the colloidal aqueous solution contains the dispersed particulate constituents present in an amount of at least 0.05 g/kg, but no more than 10 g/kg, based on the colloidal aqueous solution; the alkaline-earth metal ions dissolved in water present in an amount of at least 1.5 mmol/L, but no more than 10 mmol/L, based on the colloidal aqueous solution; at least one complexing agent present in no more than an equimolar amount of the alkaline-earth metal ions; and wherein total amount of the at least one polymeric organic compounds (a2) in the dispersed particulate constituent of the colloidal aqueous solution is at least 6 wt. %, but does not exceed 15 wt. %, based on the dispersed particulate constituent; and wherein the at least one polymeric organic compounds (a2) of the colloidal aqueous solution contain the polyoxyalkylene units in their side chains, the polyoxyalkylene units being present in an amount of at least at least 50 wt. %, based on total amount of the at least one polymeric organic compound (a2) and the polyoxyalkylene units of the at least one polymeric organic compound (a2) are at least partially end-capped with imidazole and/or imidazoline units.

17. The method according to claim 1, wherein the colloidal aqueous solution is obtained as an aqueous dispersion diluted by a factor of 20 to 100,000, comprising based on the aqueous dispersion, at least 5 wt. % of a dispersed particulate constituent (A), which in turn comprises (A1) at least one particulate inorganic compound which is composed of phosphates of polyvalent metal cations at least partially selected from hopeite, phosphophyllite, scholzite and/or hureaulite, (A2) at least one polymeric organic compound which is composed at least partially of styrene and/or an α-olefin having no more than 5 carbon atoms and is composed at least partially of maleic acid, its anhydride and/or its imide, the at least one polymeric organic compound additionally comprising polyoxyalkylene units, and optionally at least one thickener (B) selected from urea urethane resins, the dilution being carried out with water containing at least 0.5 mmol/L of alkaline-earth metal ions dissolved in water.

18. The method according to claim 15, wherein the alkaline-earth metal ions are present in an amount of at least 1.5 mmol/L.

19. The method according to claim 1, wherein the phosphating in method step (ii) is carried out by contact with an acidic aqueous composition containing 5-50 g/kg of phosphates dissolved in water, calculated as PO.sub.4; 0.3-3 g/kg of zinc ions; and an amount of free fluoride; which contains a total of less than 0.1 g/kg of ions of the elements nickel and cobalt.

Description

PRACTICAL EXAMPLES

[0061] In the following, the properties of activation involving city water for subsequent zinc phosphating are presented with regard to the phosphate layer weights and corrosion protection results achieved under the same conditions.

Preparation of the Pigment Paste

[0062] To prepare a pigment paste for providing a dispersion for activation, 15 parts by mass of Edaplan® 490 (Münzing Chemie GmbH) were predispersed as dispersing agents in 25 parts by mass of fully deionized water (κ<1 μScm.sup.−1) and then mixed with 60 parts by mass of zinc phosphate of quality level PZ 20. This phase was transferred to a KDL type Dyno®-Mill bead mill and the zinc phosphate particles were continuously milled for two hours (milling parameters: 75% bead fill level, 2000 revolutions per minute, 20 L volumetric flow per hour, temperature of the milled material 40-45° C.). The result was an average particle size of approximately 0.35 μm determined using a Zetasizer Nano ZS from Malvern.

Preparation of the Dispersion for Activation

[0063] 2.5 parts by mass of a urea urethane resin solution containing 40 wt. % of the resin based on an amine-modified prepolymer of TDI/XDI and PEG-16 (amine value<1 mg KOH/g; hydroxyl number approximately 40 mg KOH/g) in approximately 64 parts by mass of fully deionized water (κ<1 μScm.sup.−1) were then supplied as a thickener, homogenized, and adjusted to pH 9 using 10% sodium hydroxide solution. Then, approximately 33 parts by mass of the pigment paste were added while stirring, adjusted to pH 9 using 1 wt. % NaOH solution and stirred to the point of complete homogenization.

Preparation of a Colloidal Aqueous Solution for Activation for Zinc Phosphating

[0064] In a 5 L beaker, 5 liters of [0065] A) fully deionized water (κ<1 μScm.sup.−1) containing 5 grams of an additive solution consisting of 10.3 wt. % potassium pyrophosphate and 25.3 wt. % potassium phosphate was provided and brought to pH 10.5 using phosphoric acid while stirring, and 7.5 grams of the above-described dispersion was added. The pH was then adjusted to 10.5 using 1% sodium hydroxide solution while stirring. [0066] B) Düsseldorf city water (dissolved alkaline-earth metal ions: 14 mg/L Mg; 96 mg/L Ca; κ=726 μScm.sup.−1) was provided, and 7.5 grams of the above-described dispersion was added. The pH was then adjusted to 8.0 using 1% sodium hydroxide solution while stirring.

Method Sequence for Zinc Phosphating

[0067] For layer-forming phosphating by activation based on the colloidal aqueous solution, sheets of cold-rolled steel (CRS), hot-dip galvanized steel (HDG) and aluminum (AA6014) were:

a) firstly alkaline cleaned while stirring in Düsseldorf city water (pH: 10.2-10.9; 55° C.) by being immersed for 5 minutes in a degreasing bath containing 4 wt. % of Bonderite® C-AK 1565 A and 0.6 wt. % of Bonderite® C-AD 1561, each of which is available from Henkel AG & Co KGaA;
b) subjected to rinsing with Düsseldorf city water and then with fully deionized water (κ<1 μScm.sup.−1) for approximately 30 seconds in each case;
c) in a water-wetted state, brought into contact with the activation solution A or, according to the invention, with the activation solution B by immersion for 60 seconds;
d) and immediately afterwards, and without further rinsing steps, immersed into a hydroxylamine-accelerated phosphating bath having a free acid content of 0.9-1.4 points (titrated to a pH of 3.6), a total acid content of 25-30 points (titrated to a pH of 8.5) and a free fluoride content of approximately 150 mg/kg, containing 4.6 wt. % of Bonderite® M-ZN 1994, 0.8 wt. % of Bonderite® M-AD 565, 0.24 wt. % of Bonderite® M-AD 338 and 0.38 wt. % of Bonderite® M-AD 110, each of which is available from Henkel AG & Co KGaA, in fully deionized water (κ<1 μScm.sup.−1), for 3 min while stirring at 52° C.;
e) subjected to rinsing with fully deionized water (κ<1 μScm.sup.−1) for approximately 30 seconds; and
f) provided with an approximately 20 μm thick layer of an electrocoat of the type Cathoguard® 800 (BASF SE) and then cured at 180° C. for 35 min.

[0068] Table 1 summarizes the results of zinc phosphating with regard to layer weight and after aging in the corrosion test. It is apparent that, when activation occurs using city water (B) by comparison with an approach using deionized water (A), homogeneous, closed zinc phosphate coatings are achieved which have a lower layer weight and also improved corrosion protection results.

TABLE-US-00001 TABLE 1 Layer weight .sup.1/gm.sup.−2 Corrosion .sup.2, 3/mm Substrate A B A B CRS 1.9 1.5 0.5 .sup.2 0.5 .sup.2 HDG 2.4 1.6 2.5 .sup.2 1.9 .sup.2 AA6014 1.6 1.4 3.3 .sup.3 2.4 .sup.3 .sup.1 differential gravimetric determination after detaching the phosphate layer in aqueous 5 wt. % chromic acid solution .sup.2 delamination at the scratch after aging in a VW PV 1210 alternating climate test for 6 weeks over 30 cycles .sup.3 longest filiform corrosion thread according to DIN EN 3665