METHOD FOR SEQUENTIALLY CONSTRUCTING A CONVERSION LAYER ON COMPONENTS COMPRISING STEEL SURFACES

Abstract

The invention relates to a method for the anti-corrosion pre-treatment of a plurality of components in series, in which the components of the series are at least partially formed of iron and/or steel, and in which the components of the series each initially undergo a first conversion stage, followed by a rinsing stage and a subsequent second conversion stage, wherein, in the conversion stages, respective acidic aqueous conversion solutions based on compounds of the elements Zr and/or Ti dissolved in water are brought into contact with the components, and, additionally, copper ions are contained in the conversion solution for the second stage.

Claims

1. A method for anti-corrosion pre-treatment of a plurality of components in series, in which the components of the series are at least partially formed of iron and/or steel, and in which the components of the series each undergo successive method steps i)-iii) and at least surfaces of the iron and/or steel of the components are successively brought into contact with respectively provided aqueous solutions (I)-(III): i) a first conversion stage providing an aqueous conversion solution (I) having a pH in a range of 2.5 to 5.0, comprising free fluoride and at least 0.10 mmol/kg of compounds of the elements Zr and/or Ti dissolved in water; ii) a rinsing stage providing an aqueous rinsing solution (II) having a pH in a range of 5.0 to 10.0, containing less than 0.25 mmol/kg of free fluoride and a concentration of compounds of the elements Zr and/or Ti dissolved in water that is reduced by at least a factor of 5 compared to the aqueous conversion solution (I); iii) a second conversion stage providing an aqueous conversion solution (III) having a pH in a range of 2.5 to 5.0, comprising at least 0.10 mmol/kg of compounds of the elements Zr and/or Ti dissolved in water and at least 15 ?mol/kg of copper ions dissolved in water.

2. The method according to claim 1, wherein the contacting with the conversion solution (I) in the first conversion stage in method step (i) takes place at least for a period of time during which a coating of at least 20 mg/m.sup.2 is produced on the surfaces of steel and/or iron, but not for so long that the coating is more than 150 mg/m.sup.2, in each case based on the elements Zr and/or Ti results.

3. The method according to claim 2, wherein the contacting with the conversion solution (III) in the second conversion stage in method step (iii) does not continue until there is an increase of more than 15 mg/m2 on the surfaces of steel and/or iron, but at least for such a period of time that the coating on said surfaces is increased by at least 2 mg/m2, in each case based on the elements Zr and/or Ti.

4. The method according to claim 1, wherein in the conversion solution (I) of the first conversion stage in method step i), the compounds of the elements Zr and/or Ti dissolved in water are present in an amount of at least 0.15 mmol/kg.

5. The method according to claim 1, wherein in the conversion solution (I) of the first conversion stage in method step i), the free fluoride is present in an amount of at least 0.5 mmol/kg, but less than 8.0 mmol/kg of free fluoride.

6. The method according to claim 1, wherein the quotient ? in the conversion solution (I) of the first conversion stage in method step i) is according to the Formula (1) $\begin{array}{cc}?=\frac{F/\mathrm{mM}}{\sqrt{\mathrm{Me}/\mathrm{mM}}},& \left(1\right)\end{array}$ where F/mM and Me/mM are the free fluoride (F) or reduced zirconium and/or titanium concentration (Me) reduced by the unit of the concentration in mmol/kg, and the quotient ? is greater than 0.80.

7. The method according to claim 1, wherein in the conversion solution (III) of the second conversion stage in method step iii), the compounds of the elements Zr and/or Ti dissolved in water are present in an amount of less than 1.00 mmol/kg.

8. The method according to claim 1, wherein in the conversion solution (III) of the second conversion stage in method step iii), further comprises free fluoride present in an amount of at least 0.1 mmol/kg, and less than 3.00 mmol/kg.

9. The method according to claim 1, wherein the conversion solution (III) of the second conversion stage in method step iii), comprises copper ions dissolved in water in an amount of more than 40 ?mol/kg.

10. The method according to claim 1, wherein the compounds of the elements Zr and/or Ti dissolved in water in the conversion solutions (I) and (III) of the respective conversion stages of method steps i) and iii) are selected from fluoro complexes of the elements Zr and/or Ti.

11. The method according to claim 1, wherein the aqueous rinsing solution (II) of the rinsing stage in method step (ii) additionally contains at least 0.1 mmol/kg, but no more than 10 mmol/kg of a depolarizer selected from nitrate ions, nitrite ions, nitroguanidine, N-methylmorpholine N-oxide, hydrogen peroxide in free or bound form, hydroxylamine in free or bound form, reducing sugars and combinations thereof.

12. The method according to claim 1, wherein the aqueous conversion solutions of the conversion stages in method steps (i) and (iii) each have a pH above 3.0 but below 4.5.

13. The method according to claim 1, wherein in the rinsing stage of method step (ii), the contacting with the provided rinsing solution is carried out by dipping and/or spraying.

14. The method according to claim 1, wherein after the method step (iii), with an intermediate rinsing step, further coating of the components is carried out using a paint system comprising an electrodeposition coating.

15. The method according to claim 1, wherein the components of the series also have surfaces of zinc and/or aluminum in addition to the surfaces of steel and/or iron.

16. The method according to claim 1, wherein in the conversion solution (I) of the first conversion stage in method step i), the compounds of the elements Zr and/or Ti dissolved in water are present in an amount of at least at least 0.30 mmol/kg; the free fluoride is present in an amount of at least 1.5 mmol/kg, but less than 6.0 mmol/kg; and the contacting of the surfaces of steel and/or iron takes place for a period of time during which the coating produced is not more than more than 80 mg/m2, in each case based on the elements Zr and/or Ti.

17. The method according to claim 16, wherein wherein the aqueous rinsing solution (II) of the rinsing stage in method step (ii) contains a total of less than 50 ?mol/kg of metal ions of the elements copper, nickel and cobalt that are dissolved in water.

18. The method according to claim 16, wherein the conversion solution (III) of the second conversion stage in method step iii), comprises the compounds of the elements Zr and/or Ti dissolved in water present in an amount of less than 0.70 mmol/kg; and the free fluoride is present in an amount of less than 2.50 mmol/kg but at least 0.1 mmol/kg.

19. The method according to claim 1, wherein the conversion solution (III) of the second conversion stage in method step iii), comprises copper ions dissolved in water in an amount of more than 50 ?mol/kg, but no more than 300 ?mol/kg.

20. The method according to claim 6, wherein the quotient ? is greater than 1.60.

Description

EXAMPLES

[0044] The advantages of a method sequence according to the invention are illustrated below on the basis of the anti-corrosion pre-treatment and cathodic electrodeposition coating of individual sheets of steel (CRS). [0045] I. Alkaline degreasing for 90 seconds at 55? C. by spray application with a composition (pH 10.5) consisting of the following process chemicals from Henkel AG & KGaA: [0046] 20 g/L Bonderite? C-AK 2011 [0047] 1 g/L Bonderite? C-AD 1270 [0048] II. Alkaline purification for 120 seconds at 55? C. by dip application with a composition (pH 11.0) consisting of the following process chemicals from Henkel AG & KGaA: [0049] 20 g/L Bonderite? C-AK 2011 [0050] 1 g/L Bonderite? C-AD 1270 [0051] III. Rinsing with deionized water (?<1 ?Scm.sup.?1) by dip application [0052] IV. First conversion stage for 120 seconds at 35? C. by dip application with a composition (pH 4.0) consisting of the following process chemicals from Henkel AG & KGaA: [0053] Variant (A) with 6 g/L of Bonderite? M-NT 1800 gives: [0054] 30 mg/kg of Zr [0055] 4 mg/kg of copper [0056] 34 mg/kg of free fluoride [0057] Variant (B) with 16.6 g/L of Bonderite? M-AD 110, 15 g/L Bonderite? M-NT 12001 MU gives: [0058] 150 mg/kg of Zr [0059] 4 mg/kg of copper [0060] 33 mg/kg of free fluoride [0061] V. Rinsing with deionized water (?<1 ?Scm.sup.?1) by dip application [0062] VI. Second conversion stage for 30 seconds at 35? C. by dip application with a nitric acid composition (pH 4.0) consisting of the following process chemicals from Henkel AG & KGaA: [0063] Variant (A) with 6 g/L of Bonderite? M-NT 1800 gives: [0064] 30 mg/kg of Zr [0065] 4 mg/kg of copper [0066] 27 mg/kg of free fluoride [0067] Variant (B) with 30.0 g/L of Bonderite? M-NT 1800 gives: [0068] 150 mg/kg of Zr [0069] 4 mg/kg of copper [0070] 29 mg/kg of free fluoride [0071] Variant (C) with 2.5 g/L of Bonderite? M-PT 54 NC gives: [0072] 150 mg/kg of Zr [0073] 17 mg/kg of free fluoride [0074] VII. Rinsing with deionized water (?<1 ?Scm.sup.?1) in dip application [0075] VIII. Drying with compressed air [0076] IX. Cathodic dip coating with CathoGuard? 800 (BASF Coatings AG) in a dry film thickness of ?? g/m2

[0077] The proportion of free fluoride was adjusted by means of an aqueous solution of ammonium bifluoride and the adjustment of the pH with ammonium bicarbonate.

[0078] The pre-treated and electro-dip coated sheets were then aged for 6 weeks over 30 cycles according to the VW PV 1210 alternating climate test and the scribe delamination after aging was determined.

[0079] As a result, it is evident that the method according to the invention leads to a significant improvement in corrosion protection compared to a two-stage conversion treatment in which no intermediate rinsing step is carried out (Table 1: V3 vs. E1). The presence of copper ions in the second conversion stage is also crucial for sufficient corrosion protection (Table 1: V4 vs. E2). In addition, an increase in layer weight of less than 10 mg/m.sup.2 of Zr in the second conversion stage or a content of compounds of the elements Zr in the second conversion stage dissolved in water proves advantageous in preventing corrosive delamination after electrodeposition (Table 1: E3 vs. E4 and E1 vs. E2).

TABLE-US-00001 TABLE 1 Layer weight* (mg/m.sup.2) Process Alternating climate test after step . . . sequence Stone (I-II-III- . . . - U/2 .sup.1 impact .sup.2 (IV) (VI) Ex. VII-VIII-IX) (mm) (K) Zr Cu Zr Cu V1 IV(A) 1.20 3 28 8 V2 IV(B) 1.25 3 35 30 V3 IV(A)-VI(A) 1.25 3 28 8 2 1 V4 IV(A)-V-VI(C) 1.40 3.5 28 8 16 V5 IV(B)-V-VI(C) 1.45 3.5 35 30 18 E1 IV(A)-V-VI(A) 0.80 2.5-3 28 8 4 1 E2 IV(A)-V-VI(B) 1.10 3 28 8 10 4 E3 IV(B)-V-VI(A) 0.90 2.5 35 30 2 0 E4 IV(B)-V-VI(B) 1.20 3 35 30 11 5 .sup.1 Corrosion and delamination according to DIN EN ISO 4628-8 .sup.2 Stone impact test according to DIN EN ISO 20567-1 *Measured by means of X-ray fluorescence analyzer (Thermo Fisher Scientific, Niton? XL3t 900) after compressed air drying of the sheet metal sections immediately after the rinsing following the conversion stage