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

Abstract

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 dispersed inorganic compounds of phosphates of polyvalent metal cations; plus 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. In the activation stage of the method according to the invention, the addition of condensed phosphates can be dispensed with such that the content of dissolved condensed phosphates in the colloidal aqueous solution is less than 0.25, based on the phosphate content in the particulate constituent thereof, in each case based on the element P.

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, comprising steps of: (i) activating the metal material or the component; and then (ii) phosphating, optionally zinc phosphating, the metal material or component of step (i), in consecutive method steps, wherein activation in activating step (i) is carried out by contacting the metal material or the component with a colloidal aqueous solution containing, in a dispersed particulate constituent (a) of the colloidal aqueous solution: (a1) at least one particulate inorganic compound 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 composed at least partially of styrene and/or an α-olefin having no more than 5 carbon atoms; at least partially of maleic acid, its anhydride and/or its imide, wherein the at least one polymeric organic compound further comprises polyoxyalkylene units; wherein, in the colloidal aqueous solution, content 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.

2. The method according to claim 1, wherein, in the colloidal aqueous solution, the content of condensed phosphates dissolved in water is less than 0.20, based on the phosphate content of the at least one particulate compound, in each case based on the element P.

3. The method according to claim 2, wherein, in the colloidal aqueous solution, the content of condensed phosphates dissolved in water, calculated as P, is less than 20 mg/kg, based on the colloidal aqueous solution.

4. The method according to claim 1, wherein the colloidal aqueous solution contains an amount of at least 0.5 mmol/L, but no more than 10 mmol/L, of alkaline-earth metal ions dissolved in water.

5. The method according to claim 4, wherein, the colloidal aqueous solution contains at least one complexing agent selected from a-hydroxycarboxylic acids and/or organophosphonic acids.

6. The method according to claim 5, wherein the at least one complexing agent in the colloidal aqueous solution is present in an amount of no more than twice the amount of alkaline-earth metal ions in mmol/L.

7. The method according to claim 1, wherein the colloidal aqueous solution in the activating step (i) has an alkaline pH.

8. The method according to claim 1, wherein content of phosphates, calculated as PO.sub.4, contained 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 contains the polyoxyalkylene units in side chains of the at least one polymeric organic compound, and the polyoxyalkylene units are present in an amount of at least 40 wt. % but not exceeding 70 wt. %, based on total amount of the polymeric organic compounds (a2).

10. The method according to claim 9, wherein the polyoxyalkylene units in the side chains of the polymeric organic compounds (a2) are at least partially end-capped with an N-heterocycle unit.

11. The method according to claim 1, wherein the colloidal aqueous solution contains at least one thickener as a further component b), selected from urea urethane resins.

12. The method according to claim 1, wherein the at least one polymeric organic compounds are present in the colloidal aqueous solution in an amount of at least 3 wt. %, but do not exceed 15 wt. %, based on the dispersed particulate constituent of the colloidal aqueous solution.

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

14. The method according to claim 1, wherein the particulate constituents of the colloidal aqueous solution are present in an amount of at least 0.05 g/kg, but no more than 10 g/kg, in each case based on the colloidal aqueous solution.

15. The method according to claim 1, wherein the colloidal aqueous solution is obtainable 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 polymeric organic compound additionally comprising polyoxyalkylene units, and optionally at least one thickener selected from urea urethane resins.

16. The method according to claim 1, wherein the phosphating 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; said acidic aqueous composition containing a total of less than 0.1 g/kg of ions of the elements nickel and cobalt.

Description

PRACTICAL EXAMPLES

[0068] In the following, the properties of activation without additives—that is, containing no condensed phosphates—are presented both with regard to the bath life and also the phosphate layer weights and corrosion protection results achieved in the subsequent zinc phosphating.

Preparation of the Pigment Paste

[0069] To prepare a pigment paste for providing a dispersion for activation, 15 parts by mass of Edaplan® 490 (Munzing 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

[0070] 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

[0071] 5 liters of [0072] A) fully deionized water (κ<1 μScm.sup.−1) or [0073] B) 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 in a 5 L beaker 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.

Method Sequence for Zinc Phosphating

[0074] 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: [0075] a) firstly alkaline cleaned while stirring in Dusseldorf city water (pH: 10.2-10.9; 55° C.) by being immersed for 4 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; [0076] b) subjected to rinsing with Dusseldorf city water and then with fully deionized water (κ<1 μScm.sup.−1) for approximately 30 seconds in each case; [0077] c) in a water-wetted state, brought into contact according to the invention with the activation solution A or with the activation solution B containing additives by immersion for 60 seconds; [0078] 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.; [0079] e) subjected to rinsing with fully deionized water (κ<1 μScm.sup.−1) for approximately 30 seconds; and [0080] 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.

[0081] 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 in the absence of condensed phosphates (A) by comparison with an approach involving adding corresponding additives (B), zinc phosphate coatings are achieved which have a significantly lower layer weight on CRS and also have, on this substrate, improved corrosion protection results, while layer application on zinc and aluminum is not adversely affected by the lack of additives and homogeneous closed zinc phosphate coatings can also be provided. Only on aluminum do the results in the Filiform test drop slightly by comparison with activation with additives, but often still meet the corrosion values usually required in the automotive industry. It is noteworthy that, even in the absence of the condensed phosphates, the activation performance in the method according to the invention is retained for months.

TABLE-US-00001 TABLE 1 Layer weight .sup.1/gm.sup.−2 Corrosion .sup.2, 3/mm Substrate A B* A B* CRS 1.6.sup.# (1.8**) 2.1 0.3.sup.# (0.5**) .sup.2 0.7 .sup.2 HDG 2.2.sup.# (2.1**) 2.3 2.1.sup.# (2.3**) .sup.2 1.7 .sup.2 AA6014 1.6.sup.# (1.7**) 1.8 2.8.sup.# (4.2**) .sup.3 1.7 .sup.3 .sup.# after 6 weeks bath life * after 10 weeks ** after 18 weeks bath life .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