Method for selectively phosphating a composite metal construction

Abstract

A multistage method for treatment of composite metal structures containing metallic surfaces of aluminum, zinc and optionally iron, is provide wherein in a first step, selective zinc phosphating of zinc and ferrous surfaces proceeds using a phosphating solution containing a quantity of water-soluble inorganic silicon compounds sufficient to suppress white spot formation on zinc, but less than the quantity where zinc phosphating loses selectivity. In a following second step, aluminum surfaces are passivated with an acidic treatment solution. Also provided is a zinc phosphating solution suitable for said method containing at least 0.025 g/l, but less than 1 g/l of silicon as water-soluble inorganic compounds calculated as SiF.sub.6, wherein the product (Si/mM).Math.(F/mM) of the concentration of silicon [Si in mM] in the form of water-soluble inorganic compounds and the concentration of free fluoride [F in mM] divided by the free acid point number is no greater than 5.

Claims

1. A zinc phosphating solution comprising: (a) 5-50 g/l of phosphate ions, (b) 0.3-3 g/l of zinc(II) ions, (c) a concentration of free fluoride, in g/l which is at least 0.005 g/l, and no greater than 8/T g/l wherein T is the temperature at which the solution is used for coating, (d) at least 0.025 g/l, but less than 1.0 g/l of silicon in the form of water-soluble inorganic compounds calculated as SiF.sub.6, said zinc phosphating solution having a free acid point number of at least 0.4 points, but of no more than 3 points and a pH value in the range from 2.2 to 3.6; wherein a product (Si/mM).Math.(F/mM) of concentration of silicon [Si in mM] in the form of water-soluble inorganic compounds and concentration of free fluoride [F in mM] divided by the free acid point number is no greater than 5, wherein the zinc phosphating solution is effective for coating zinc or iron surfaces to form a continuous coating, but forms no continuous coating on aluminum surfaces at under the same coating conditions used for coating zinc or iron surfaces to form a continuous coating.

2. The zinc phosphating solution according to claim 1, comprising in total no more than 5 ppm of water-soluble compounds of zirconium, measured as zirconium, and/or titanium, measured as titanium.

3. The zinc phosphating solution according to claim 1, comprising a free acid content of at least 0.6 points, but of no more than 2.5 points.

4. The zinc phosphating solution according to claim 1, comprising a total acid content of at least 10 points, but no more than 50 points.

5. The zinc phosphating solution according to claim 1, comprising in total no more than 1 ppm of water-soluble compounds of zirconium and/or titanium measured as zirconium and/or titanium, and having a free acid content of at least 1.0 point, but no more than 2.0 points, and a total acid content of at least 15 points, but no more than 25 points.

6. The zinc phosphating solution of claim 1, wherein from 0.025 g/l to less than 0.9 g/l of silicon is contained in the solution.

7. The zinc phosphating solution of claim 1, wherein the (Si/mM).Math.(F/mM) of concentration of silicon [Si in mM] in the form of water-soluble inorganic compounds and concentration of free fluoride [F in mM] divided by the free acid point number is no greater than 4.5.

8. The zinc phosphating solution of claim 1, wherein the (Si/mM).Math.(F/mM) of concentration of silicon [Si in mM] in the form of water-soluble inorganic compounds and concentration of free fluoride [F in mM] divided by the free acid point number is no greater than 4.0.

9. The zinc phosphating solution of claim 1, wherein the free acid point number is from 1.0 to 2.0.

10. The zinc phosphating solution of claim 1, which provides a continuous coating on iron and/or zinc surfaces in an areal weight of >0.5 g/m.sup.2, while under the same conditions forms no continuous coating on aluminum surfaces under the same conditions, any discontinuous deposition on aluminum surfaces having an areal weight of <0.5 g/m.sup.2.

11. The zinc phosphating solution of claim 10, which is effective to deposit a zinc phosphate layer having a areal weight of 1.0 g/m.sup.2 to 4.0 g/m.sup.2 on zinc and iron surfaces but which, under the same conditions, deposits no more than a discontinuous layer with an areal weight of <0.5 g/m.sup.2 on aluminum surfaces.

12. The zinc phosphating solution of claim 10, which is effective to deposit a zinc phosphate layer having a areal weight of 1.0 g/m.sup.2 to 2.0 g/m.sup.2 on zinc and iron surfaces but which, under the same conditions, deposits no more than a discontinuous layer with an areal weight of <0.5 g/m.sup.2 on aluminum surfaces.

13. The zinc phosphating solution of claim 1, which contains 5 ppm total of water soluble compounds of zirconium and titanium, calculated on the basis of the elements zirconium and titanium.

14. The zinc phosphating solution of claim 1, which contains 1 ppm total of water soluble compounds of zirconium and titanium, calculated on the basis of the elements zirconium and titanium.

15. A zinc phosphating solution consisting essentially of: (a) 5-50 g/l of phosphate ions, (b) 0.3-3 g/l of zinc(II) ions, (c) at least 10 ppm, but no more than 100 ppm of free fluoride ions, (d) at least 0.025 g/l, but less than 1.0 g/l of silicon in the form of water-soluble inorganic compounds calculated as SiF.sub.6, (e) optionally, one or more accelerants, and (f) optionally, one or more metal cations other than zinc cations, said zinc phosphating solution having a free acid point number of at least 0.4 points, but of no more than 3 points and a pH value in the range from 2.2 to 3.6; wherein a product (Si/mM).Math.(F/mM) of concentration of silicon [Si in mM] in the form of water-soluble inorganic compounds and concentration of free fluoride [F in mM] divided by the free acid point number is no greater than 5, wherein the zinc phosphating solution is effective for forming a continuous coating on zinc and/or iron surfaces but forms no continuous coating on aluminum surfaces under the same coating conditions.

16. A zinc phosphating solution consisting essentially of: (a) 5-50 g/l of phosphate ions, (b) 0.3-3 g/l of zinc(II) ions, (c) at least 10 ppm, but no more than 100 ppm of free fluoride ions, (d) at least 0.025 g/l, but less than 1.0 g/l of silicon in the form of water-soluble inorganic compounds calculated as SiF.sub.6, (e) optionally, one or more accelerants selected from the group consisting of chlorate ions, nitrite ions, nitroguanidine, N-methylmorpholine-N-oxide, m-nitrobenzene sulfonate ions, m-nitrobenzoate ions, p-nitrophenol, hydrogen peroxide, hydroxylamine, and reducing sugar, and (d) optionally, one or more metal cations selected from the group consisting of nickel (II), cobalt (II), magnesium, calcium, iron (II), lithium, and tungsten, said zinc phosphating solution having a free acid point number of at least 0.4 points, but of no more than 3 points and a pH value in the range from 2.2 to 3.6; wherein a product (Si/mM).Math.(F/mM) of concentration of silicon [Si in mM] in the form of water-soluble inorganic compounds and concentration of free fluoride [F in mM] divided by the free acid point number is no greater than 5, wherein the zinc phosphating solution is effective for forming a continuous coating on zinc and/or iron surfaces but forms no continuous coating on aluminum surfaces under the same coating conditions.

17. A zinc phosphating solution for use in the simultaneous coating of a composite article comprising at least one surface of iron or zinc, and at least one surface of aluminum, comprising: (a) 5-50 g/l of phosphate ions, (b) 0.3-3 g/l of zinc(II) ions, (c) a concentration of free fluoride, in g/l which is at least 0.005 g/l, (d) at least 0.025 g/l, but less than 1.0 g/l of silicon in the form of water-soluble inorganic compounds calculated as SiF.sub.6, said zinc phosphating solution having a free acid point number of at least 0.4 points, but of no more than 3 points and a pH value in the range from 2.2 to 3.6; wherein a product (Si/mM).Math.(F/mM) of concentration of silicon [Si in mM] in the form of water-soluble inorganic compounds and concentration of free fluoride [F in mM] divided by the free acid point number is no greater than 5, wherein the zinc phosphating solution does not deposit a complete-coverage zinc phosphate layer onto aluminum surfaces of said composite article, but does deposit a continuous coating of zinc phosphate onto surfaces of iron or zinc of said composite article.

18. The zinc phosphating solution of claim 17 which is suitable for use in coating composite articles at a temperature T in the range of 20 C. to 65 C., wherein the free fluoride concentration is from 0.005 g/l to 8/T g/l.

19. The zinc phosphating solution of claim 17, wherein the free fluoride concentration is from 0.005 g/l to 8/65 g/l.

Description

(1) In a further preferred embodiment of the method according to the present invention, the zinc phosphating solution in step (I) contains in total no more than 5 ppm, particularly preferably in total no more than 1 ppm of water-soluble compounds of zirconium and/or titanium relative to the elements zirconium and/or titanium.

(2) It is known from WO 2008/055726 that the presence of water-soluble compounds of these elements in a phosphating step is likewise capable of effectively suppressing the formation of crystalline phosphate layers on aluminum surfaces. It has become apparent, however, that in the presence of water-soluble compounds of zirconium and/or titanium, an inhomogeneous amorphous zirconium- and/or titanium-based conversion coating is more often produced on the aluminum parts, in particular when the phosphating solution is applied by spraying; this leads to the occurrence of mapping in the context of a subsequent organic painting operation. Mapping is understood by one skilled in the art of dipcoating metallic components as a speckled visual impression of the paint coating, due to an inhomogeneous paint layer thickness after stoving of the dipcoating paint. The addition in particular of water-soluble compounds of zirconium and/or titanium in phosphating solutions is consequently entirely avoided in the method according to the present invention. It is additionally necessary, when applying phosphating solutions that contain water-soluble compounds of zirconium and/or titanium, to correspondingly increase the free fluoride proportion in the phosphating bath in order to avoid inhibiting the formation of a phosphate layer on iron surfaces resp. steel surfaces of the metallic component. Such an increase in the free fluoride proportion promotes the formation of phosphate crystal clusters on the aluminum parts, however, and at the same time increases the pickling rate, so that the elevated sludge formation has a disadvantageous effect on the cost-effectiveness of the method. The presence of the water-soluble compounds of zirconium and/or titanium in a method according to the present invention therefore either produces comparatively lower zinc phosphate layer weights on steel surfaces, or produces aluminum surfaces on which local defects in the form of phosphate crystal clusters interfere with a homogeneous paint structure and potentially promote corrosive paint delamination. For an optimum phosphating outcome on metallic components that comprise not only aluminum surfaces but also surfaces made of steel and of galvanized and/or alloy-galvanized steel, zinc phosphating solutions that contain no more than 5 ppm, particularly preferably in total no more than 1 ppm of water-soluble compounds of zirconium and/or titanium relative to the elements zirconium and/or titanium, and particularly preferably no water-soluble compounds of zirconium and/or titanium, are therefore preferred in step (I) of the method according to the present invention.

(3) The zinc phosphating solution contains, in step (I) of the method according to the present invention, by preference at least 0.3 g/l, particularly preferably at least 0.8 g/l, but preferably no more than 3 g/l, particularly preferably no more than 2 g/l of zinc ions. The proportion of phosphate ions in the phosphating solution in this context by preference amounts to at least 5 g/l, but is preferably no greater than 50 g/l, particularly preferably no greater than 25 g/l.

(4) The zinc phosphating solution of the method according to the present invention can additionally contain, besides the zinc ions and phosphate ions recited above, at least one of the following accelerators:

(5) TABLE-US-00001 0.3 to 4 g/l chlorate ions, 0.01 to 0.2 g/l nitrite ions, 0.05 to 4 g/l nitroguanidine, 0.05 to 4 g/l N-methylmorpholine-N-oxide, 0.2 to 2 g/l m-nitrobenzenesulfonate ions, 0.05 to 2 g/l m-nitrobenzoate ions, 0.05 to 2 g/l p-nitrophenol, 1 to 150 mg/l hydrogen peroxide in free or bound form, 0.1 to 10 g/l hydroxylamine in free or bound form, 0.1 to 10 g/l reducing sugars.

(6) Such accelerators are usual in the existing art as components of phosphating baths and perform the function of hydrogen catchers, by directly oxidizing the hydrogen resulting from acid attack on the metallic surface and thereby being themselves reduced. The formation of a homogeneous crystalline zinc phosphate layer on the steel surfaces and on the galvanized and/or alloy-galvanized steel surfaces is substantially facilitated by the accelerators, which decrease the occurrence of gaseous hydrogen on the metallic surface.

(7) The corrosion protection and paint adhesion of crystalline zinc phosphate layers produced with an aqueous composition according to the present invention are improved according to the present invention if one or more of the following cations are additionally contained:

(8) TABLE-US-00002 0.001 to 4 g/l manganese(II), 0.001 to 4 g/l nickel(II), 0.001 to 4 g/l cobalt (II), 0.002 to 0.2 g/l copper(II), 0.2 to 2.5 g/l magnesium(II), 0.2 to 2.5 g/l calcium(II), 0.01 to 0.5 g/l iron(II), 0.2 to 1.5 g/l lithium(I), 0.02 to 0.8 g/l tungsten(VI).

(9) Aqueous compositions for conversion treatment that contain, besides zinc ions, both manganese and nickel ions are known to one skilled in the art of phosphating as trication phosphating solutions, and are also well-suited in the context of the present invention. A proportion of up to 5 g/l, by preference up to 3 g/l nitrate, as is usual in the context of phosphating, also facilitates the formation of a homogeneous and continuous crystalline phosphate layer on the steel surfaces and galvanized and alloy-galvanized steel surfaces.

(10) In addition to the aforementioned cations that become incorporated into the phosphate layer resp. at least have a positive effect on the crystal growth of the phosphate layer, the phosphating solutions in step (I) of the method according to the present invention as a rule also contain sodium ions, potassium ions, and/or ammonium ions which, by way of the addition of the corresponding alkalis, function to adjust the free acid content in the phosphating solution.

(11) In step (II) of the method, bringing the composite metal structure into contact with the acid treatment solution results, according to the present invention, in the formation of a conversion layer on the aluminum surfaces, the zinc phosphate layer on the steel surfaces, galvanized and/or alloy-galvanized steel surfaces being no more than 50%, by preference no more than 20%, preferably no more than 10% dissolved while being brought into contact with the treatment solution. In the context of the present invention, a conversion layer on aluminum is considered to be passivating inorganic or mixed inorganic/organic thin layers that are not continuous crystalline phosphate layers and therefore have a mass per unit area of less than 0.5 g/m.sup.2 phosphate layer, determined by differential weighing after the aluminum surfaces are brought into contact with 65-wt % nitric acid for 15 minutes at 25 C.

(12) While the pH value of the acid treatment solution in the range from 3.5 to 5.5 already substantially guarantees that no more than 50% of the zinc phosphate layer on the steel surfaces, galvanized and/or alloy-galvanized steel surfaces is dissolved, the corresponding conversion layers on the aluminum surfaces of the composite metal structure are typically produced using chromium-free acid treatment solutions that contain water-soluble compounds of the elements Zr, Ti, Hf, Si, V, and Ce, by preference in a quantity of at least 10 ppm in total relative to the respective elements. A method according to the present invention in which the acid treatment solution in step (II) contains in total 10 to 1500 ppm of fluoro complexes of zirconium and/or titanium relative to the elements zirconium and/or titanium, and optionally up to 100 ppm, optionally by preference at least 1 ppm of copper(II) ions, is particularly preferred.

(13) The method according to the present invention, for corrosion-protective treatment of composite metal structures assembled from metallic materials and at least in part also comprising aluminum surfaces, occurs after cleaning and activation of the metallic surfaces, firstly by bringing the surfaces into contact with the zinc phosphating solution of step (I), e.g. using a spray or dip method, at temperatures in the range from 20-65 C. and for a time span coordinated with the manner of application. Experience indicates that white spot formation on the galvanized and/or alloy-galvanized steel surfaces is particularly pronounced in conventional dip-type phosphating methods, so that the phosphating operation in step (I) of the method according to the present invention is also particularly suitable for those phosphating facilities that operate on the dipcoating principle, since white spot formation is suppressed in the method according to the present invention.

(14) Application of the phosphating solution in step (I) is usually immediately followed by a rinsing operation with tap water or demineralized water; after processing of the rinse water enriched with components of the treatment solution, a selective recycling of components of the phosphating solution into the phosphating bath in accordance with step (I) of the method according to the present invention can be performed. With or without this rinsing step, the composite metal structure treated in accordance with step (I) is brought into contact in step (II) with the acid treatment solution, by immersion or by spraying the solution. In a further subsequent step the composite metal structure can be provided with a primer coat, by preference with an organic dipcoating paint, by preference without prior drying of the component treated according to the present invention.

(15) The composite metal structure protected from corrosion in accordance with the method according to the present invention is utilized in automotive production in body construction, in ship-building, in construction trades, and for the manufacture of white goods.

(16) In a further aspect, the present invention relates to a zinc phosphating solution (A) for selective phosphating of steel surfaces, galvanized and/or alloy-galvanized steel surfaces in a metallic composite structure encompassing a portion made of aluminum, the zinc phosphating solution (A) having a free acid content of at least 0.4 points, but no more than 3 points, and a pH value in the range from 2.2 to 3.6, and containing (a) 5-50 g/l phosphate ions, (b) 0.3-3 g/l zinc(II) ions, (c) at least 10 ppm, but no more than 100 ppm of free fluoride ions, and (d) at least 0.025 g/l, but less than 1.0 g/l of silicon in the form of water-soluble inorganic compounds calculated as SiF.sub.6, the product (Si/mM).Math.(F/mM) of the concentration of silicon [Si in mM] in the form of water-soluble inorganic compounds and the concentration of free fluoride [F in mM] divided by the free acid point number being no greater than 5, by preference no greater than 4.5, particularly preferably no greater than 4.0.

(17) In a preferred variant, the zinc phosphating solution (A) according to the present invention contains in total no more than 5 ppm, particularly preferably in total no more than 1 ppm of water-soluble compounds of zirconium and/or titanium relative to the elements zirconium and/or titanium, and in particular no water-soluble compounds of zirconium and/or titanium.