METHOD FOR ELECTROLYTIC DEPOSITION OF A PHOSPHATE LAYER ON ZINC SURFACES

Abstract

The present invention relates to a method for phosphating metal surfaces, preferably alloy-galvanized steel strip surfaces, by treating them by dipping or spray-dipping with acidic, aqueous solutions which may contain magnesium ions, phosphate ions and nitrate ions and, for further improvement of the layer formation, possibly further ions selected from ammonium ions, alkali metal ions and/or fluoride ions, wherein the workpieces are simultaneously treated cathodically with a direct current.

Claims

1. A method for phosphating metal surfaces comprising dipping and/or spray-dipping the metal surfaces with acidic, aqueous phosphating solutions containing magnesium, phosphate and nitrate ions, wherein: a) phosphating solutions containing the following components are used: PO.sub.4.sup.3 anions in a range of 2 to 50 g/l, NO.sub.3.sub. anions in a range of 0.1 to 60 g/l, Mg.sup.2+ cations in a range of 0.1 to 20 g/l, b) wherein the following conditions are observed: pH of the phosphating solutions in a range of 1.0 to 5.0, temperature of the phosphating solutions in a range of 10 to 80 C., treatment duration in the phosphating solutions is in a range of 1 to 300 sec., c) and furthermore wherein, during phosphating, the workpieces are treated cathodically with a direct current with a density in a range of 1 to 150 mA/cm.sup.2.

2. The method according to claim 1, wherein phosphating solutions are used which additionally contain simple or complex fluoride anions in a range of 0.01 to 2 g/l.

3. The method according to claim 2, wherein phosphating solutions are used which additionally contain alkali metal ions, NH.sub.4.sup.+ cations or a combination thereof, each in a range of 0.1 to 10 g/l.

4. The method according to claim 1, wherein phosphating solutions containing the following components are used: PO.sub.4.sup.3 anions in the range of 10 to 40 g/l, NO.sub.3.sub. anions in the range of 5 to 50 g/l, Mg.sup.2+ cations in the range of 1 to 10 g/l; the following conditions are observed during the phosphating treatment of the workpieces: pH value of the phosphating solutions in the range of 2.0 to 3.0, temperature of the phosphating solutions in the range of 30 to 70 C., treatment duration in the phosphating solutions is in the range of 2 to 90 sec; and during phosphating, the direct current has a density of at least 5 mA/cm.sup.2.

5. The method according to claim 4, wherein phosphating solutions are used which additionally contain simple or complex fluoride anions in a range of 0.1 to 1.5 g/l.

6. The method according to claim 5, wherein phosphating solutions are used which additionally contain NH.sub.4.sup.+ cations in the range of 0.1 to 10 g/l.

7. The method according to claim 1, wherein, during phosphating, the workpieces are cathodically treated with a direct current with a density of at least 10 mA/cm.sup.2, but below 100 mA/cm.sup.2.

8. The method according to claim 1, wherein, during phosphating, the workpieces are cathodically treated with a direct current with a density of at least 20 mA/cm.sup.2, but below 70 mA/cm.sup.2.

9. The method according to claim 6, wherein phosphating solutions are used which additionally contain alkali metal ions in a range of 0.5 to 5 g/l.

10. The method according to claim 5, wherein phosphating solutions are used which additionally contain NH.sub.4.sup.+ cations in the range of 0.1 to 7 g/l.

11. The method according to claim 6, wherein phosphating solutions are used which additionally contain simple or complex fluoride anions in the range of 0.01 to 1.5 g/l.

12. The method according to claim 1, wherein, during phosphating, the workpieces are cathodically treated with a direct current with a density of at least 10 mA/cm.sup.2, but below 70 mA/cm2 for a treatment duration in the phosphating solutions in a range 10 to 90 sec. at a temperature in a range of 30 to 70 C.

13. The method according to claim 1, wherein the workpieces to be phosphated are previously subjected to a known activation pretreatment, optionally a titanium-containing activation pretreatment solution.

14. The method according to claim 1 as a pretreatment for a subsequent painting or coating.

15. The method according to claim 1, wherein the metal surfaces comprise electrolytically or hot-dip galvanized steel strip surfaces.

16. A workpiece comprising at least one metal surface, phosphated by means of the method according to claim 1.

Description

EXAMPLES

Example 1

[0079] Substrate: Hot-dip galvanized (Z) sheet (specification: DX56+Z140MC) [0080] Cleaning: 30 g/l BONDERITE C-AK 2011+3 g/l BONDERITE C-AD 1270, each commercially available cleaners from Henkel AG & Co. KGaA for 30 seconds by spraying (pressure: 2 bar) at 50 C. [0081] Activation: 1 g/l BONDERITE M-AC 50 CF (commercially available from Henkel AG & Co. KGaA) for 5 seconds by dipping at 20 C. [0082] Mg electrolyte: 25 g/l PO.sub.4.sup.3 [0083] 4 g/l Mg.sup.2+ [0084] 6 g/l NO.sub.3.sub. [0085] 1 g/l SiF.sub.6.sup.2 [0086] pH adjusted with NH.sub.3 to 2.8 [0087] Cathodic polarization: 30 mA/cm.sup.2 [0088] Anode: Stainless steel sheet of specification 1.4571 [0089] Duration: 60 seconds [0090] Cathodic dip coating: 20 m (Cathoguard 800 from BASF Coatings GmbH) [0091] Corrosion: Ritz according to Clemen and stone chip corrosion test (PV 1210)

TABLE-US-00001 Phosphating Delamination U/2/mm Stone impact Comment Tri 1* 2.8 4 4 up to 58% chipped surface Mg electrolyte 1.2 3 3 up to 20% chipped surface *Tri 1: electroless tri-cation phosphating with Bonderite M-ZN 958 (Henkel AG & Co. KGaA) with the above-mentioned additional method steps of cleaning, activation and dip coating

[0092] The values given are mean values obtained from three parallel samples.