PROCESS FOR SELECTIVE PHOSPHATING OF A COMPOSITE METAL CONSTRUCTION

Abstract

The present disclosure relates to a method of chemical pretreatment and selective phosphation of a composite metal construction comprising at least a portion made of aluminum and at least a portion made of zinc and optionally a further portion made of iron, which includes (I) treating the composite metal construction with an aqueous zinc phosphation composition that results in the formation of a surface-covering crystalline zinc phosphate layer and thenwith an intervening water rinse operation (II) applying an aqueous acidic passivation composition,

The present disclosure also relates to a corresponding zinc phosphation composition, to a concentrate for production thereof, to a corresponding composite metal construction and to a method of using thereof.

Claims

1. A method of chemical pretreatment and selective phosphation of a composite metal construction comprising at least a portion made of aluminum and at least a portion made of zinc and optionally a further portion made of iron, wherein the method comprises: (I) treating the composite metal construction with an aqueous zinc phosphation composition that results in a formation of a surface-covering crystalline zinc phosphate layer having a coat weight in a range from 0.5 to 5 g/m.sup.2 on the parts made of iron and zinc, but does not produce a zinc phosphate layer having a coat weight of at least 0.5 g/m.sup.2 on the aluminum parts, and thenwith an intervening water rinse operation (II) applying an aqueous acidic passivation composition having a pH in a range from 2.0 to 5.5 to the composite metal construction, where the aqueous acidic passivation composition removes not more than 50% of the crystalline zinc phosphate on the parts made of zinc and iron, but forms a passive layer on the aluminum parts that does not constitute a surface-covering crystalline phosphate layer, wherein the aqueous zinc phosphation composition in step (I) comprises a content of sodium and/or potassium ions and of aluminum ions, wherein the aqueous zinc phosphation composition in step (I) has a temperature in a range from 20 to 65 C. and comprises a free fluoride content of at least 5 mg/l but not greater than 200 mg/1, where wherein at least 0.04 g/l but not more than 3.2 g/l of boron in a form of water-soluble inorganic compounds, calculated as BF.sub.4, is present in the aqueous zinc phosphation composition, and wherein a points number of free acid with added KCl in the aqueous zinc phosphation composition is at least 0.6 points.

2. The method according to claim 1, wherein the aqueous zinc phosphation composition in step (I) has a free fluoride content in a range from 10 to 200 mg/l.

3. The method according to claim 1, wherein the aqueous zinc phosphation composition in step (I) has a concentration of boron in the form of water-soluble inorganic compounds calculated as BF.sub.4 in a range from 0.08 to 2.5 g/l.

4. The method according to claim 1, wherein the content of sodium and/or potassium ions (calculated as sodium) in the aqueous zinc phosphation composition in step (I) is in a range from 1 to 4 g/l, and the free fluoride content is in the range from 50 to 150 mg/l.

5. The method according to claim 1, wherein a content of inorganic compounds comprising silicon (calculated as SiF.sub.6) in the aqueous zinc phosphation composition in step (I) is below 25 mg/l.

6. The method according to claim 1, wherein the aqueous zinc phosphation composition in step (I) has a points number of free acid with added KCl in a range from 0.6 to 3.0 points.

7. The method according to claim 1, wherein for the aqueous zinc phosphation composition in step (I) a dimensionless quotient
(A valueT)/(F.sub.tot[B]1000) is in a range from 0.13 to 22.5.

8. The method according to claim 1, wherein the aqueous zinc phosphation composition in step (I) additionally includes one or more of the following cations: 0.3 to 2 g/l of manganese(II), 0.3 to 2 g/l of nickel(II), 0.001 to 0.2 g/l of iron(III).

9. The method according to claim 1, wherein a pH of the aqueous acidic passivation composition in step (II) is in a range from 2.5 to 5.5.

10. The method according to claim 1, wherein the aqueous acidic passivation composition in step (II) comprises Ti, Zr and/or Hf in complexed form and optionally further metal ions in a form of water-soluble compounds.

11. The method according to claim 1, wherein the aqueous acidic passivation composition in step (II) additionally comprises at least one organosilane and/or at least one hydrolysis product thereof and/or at least one condensation product thereof.

12. A zinc phosphation composition for selective phosphation of steel, galvanized steel and/or alloy-galvanized steel surfaces in a metallic composite construction comprising a portion made of aluminum according to claim 1, wherein the zinc phosphation composition has a points number of free acid with added KCl of at least 0.6 points, and (a) 5 to 50 g/l of phosphate ions calculated as P.sub.2O.sub.5, (b) 0.3 to 3 g/l of zinc ions, (c) at least 5 mg/l but not more than 200 mg/l of free fluoride, and (d) at least 0.04 g/l but not more than 3.2 g/l of boron in the form of water-soluble inorganic compounds, calculated as BF.sub.4, where the zinc phosphation composition additionally comprises a content of sodium and/or potassium ions and of aluminum ions.

13. A concentrate from which the zinc phosphation composition according to claim 12 can be obtained by diluting with a suitable solvent and/or dispersion medium and optionally adjusting the pH.

14. A composite metal construction comprising at least a portion made of aluminum and at least a portion made of zinc and optionally a further portion made of iron, wherein it is obtainable by a method according to claim 1.

15. A method of using the composite metal construction according to claim 14, the method comprising incorporating the composite metal construction into a component used in an automotive supplier industry, in bodywork construction in automobile manufacture, in agricultural machine construction, in shipbuilding, in a construction sector or for production of white goods.

16. The method according to claim 2, wherein the aqueous zinc phosphation composition in step (I) has a free fluoride content in a range from 20 to 150 mg/l.

17. The method according to claim 2, wherein the aqueous zinc phosphation composition in step (I) has a free fluoride content in a range from 20 to 120 mg/l.

18. The method according to claim 2, wherein the aqueous zinc phosphation composition in step (I) has a free fluoride content in a range from 50 to 120 mg/l.

19. The method according to claim 2, wherein the aqueous zinc phosphation composition in step (I) has a free fluoride content in a range from 70 to 120 mg/l.

20. The method according to claim 3, wherein the aqueous zinc phosphation composition in step (I) has a concentration of boron in the form of water-soluble inorganic compounds calculated as BF.sub.4 in a range from 0.08 to 2 g/l.

Description

EXAMPLES

[0160] Aqueous zinc phosphation solutions were made up, each of which had a points number of free acid with added KCl of 1.1 points, a Fischer total acid of 19.5 points, an A value of 0.056 and a pH in the range from 2.5 to 3.5. Said phosphation solutions were nickel-free and each comprised 15 g/l of phosphate ions calculated as P.sub.2O.sub.5, 0.8 g/l of zinc ions, 1.0 g/l of manganese ions, different amounts of free fluoride (see tab. 1), 1.0 g/l of boron in the form of and calculated as BF.sub.4, and 34 mg/l of the hydrogen peroxide accelerator. The phosphation solutions were brought to a temperature of 45 C.

[0161] Cleaned test sheets of aluminum (AA 6016) were each sprayed uniformly with one phosphation solution, rinsed and dipped at room temperature into an acidic aqueous passivation composition comprising 150 mg/l of H.sub.2ZrF.sub.6 calculated as Zr for 30 s. Without prior oven drying, the sheets were provided with an electrocoat and finally with a wet coat (standard automobile paint structure).

[0162] The test sheets were subjected to a 144-hour CASS test to DIN EN ISO 9227, 2017-07 and to a 1008-hour filiform test to DIN EN 3665, 1997-08. The corrosion results are compiled in the table below (F.sub.free=free fluoride content).

TABLE-US-00004 TABLE 1 Filiform Maximum thread CASS Average in mm length in mm F.sub.free in mg/l Max. (vertical score) (vertical score) mm.sup.2/cm 50 1.0 1.0 5.7 23.1 100 2.0 1.8 7.7 38.3 150 2.8 2.6 13.2 51.2 200 3.5 3.1 11.0 72.5

[0163] As shown by tab. 1, lowering of the free fluoride content in the phosphation solution leads to a distinct reduction in corrosion on aluminum both in the CASS test and in the filiform test.

[0164] The applicant took samples over a period of 2.2 years from a customers operating zinc phosphation bath with varying free fluoride content and determined the content of free fluoride (F.sub.free) and of zirconium (Zr). For this purpose, a fluoride-selective potentiometric combination electrode was used, or an ICP (inductively coupled plasma) analysis was performed.

[0165] As apparent from tab. 2, a decrease in the free fluoride content albeit with a time delay in some cases (cf. 10 to 26 months) was accompanied by a decrease in the zirconium content in the bath. Conversely, an increase in the free fluoride content albeit with a time delay in some cases (cf. 4.5 to 14.5 months) was accompanied by an increase in the zirconium content in the bath. A low free fluoride content thus led to lower bath poisoning by zirconium.

TABLE-US-00005 TABLE 2 Time in months F.sub.free in mg/l Zr in mg/l 0 160 7.5 1 100 7.0 3 68 5.0 4.5 55 4.0 6 220 6.5 8.5 250 7.0 9 230 8.0 10 240 8.0 12 100 10.0 14.5 100 9.0 17 100 8.0 19 59 3.0 21 51 3.5 24 46 3.0 26 40 4.0

[0166] Aqueous zinc phosphation solutions (ZPS No. 1-8) were made up, each of which had a points number of free acid with added KCl of 1.5 points. Said phosphation solutions were nickel-free and each comprised 15 g/l of phosphate ions calculated as P.sub.2O.sub.5, 0.8 g/l of zinc ions, 1.0 g/l of manganese ions, 100 mg/l of free fluoride and different amounts of sodium and potassium ions, and also either tetrafluoroborate or hexafluorosilicate. The phosphation solutions were brought to a temperature of 45 C. and stirred in a beaker by means of a stir bar at moderate speed. By means of ICP analysis, the dissolved concentrations of aluminum, silicon and boron were determined at the start (0 h) and after 24 h. When the concentration of dissolved aluminum fell, this was attributable to precipitation in the form of cryolite. If, by contrast, there was a decrease in the concentration of dissolved silicone, this was based on precipitation in the form of K.sub.2SiF.sub.6.

[0167] As apparent from tab. 3, a content of 5.0 g/l of sodium ions led to a decrease in the concentration of dissolved aluminum, i.e. to precipitation of cryolite (ZPSs 2 and 6). Cryolite precipitation in the presence of tetrafluoroborate (ZPS 2) was somewhat less marked than in the case of hexafluorosilicate (ZPS 6). The same is true of a content of 2.5 g/l of sodium ions and 2.5 g/l potassium ions (ZPS 4 vs. ZPS 8). In the case of a content of 2.5 g/l of sodium ions (ZPSs 1 and 5) or of 5.0 g/l potassium ions (ZPSs 3 and 7), by contrast, there was no cryolite precipitation.

[0168] In the case of hexafluorosilicate, in the presence of potassium ions, there was a decrease in the concentration of dissolved silicon, i.e. precipitation of K.sub.2SiF.sub.6 (ZPSs 7 and 8). It was not possible to detect the formation of a corresponding precipitate for tetrafluoroborate. There was correspondingly no decrease here in the concentration of dissolved boron (ZPSs 3 and 4).

TABLE-US-00006 TABLE 3 ZPS Na K BF.sub.4 SiF.sub.6 Al (mg/l) Si (mg/l) B (mg/l) No. (g/l) (g/l) (g/l) (g/l) 0 h 24 h 0 h 24 h 0 h 24 h 1 2.5 0 1.0 0 47 47 0 0 145 145 2 5.0 0 1.0 0 46 23 0 0 150 150 3 0 5.0 1.0 0 46 46 0 0 148 148 4 2.5 2.5 1.0 0 46 25 0 0 150 150 5 2.5 0 0 1.5 46 46 300 300 0 0 6 5.0 0 0 1.5 47 22 290 290 0 0 7 0 5.0 0 1.5 47 46 200 9 0 0 8 2.5 2.5 0 1.5 46 22 300 80 0 0