Method for zinc phosphating metal components in series in a sludge-free manner so as to form layers

Abstract

The invention relates to a method for zinc phosphating components so as to form layers, said components comprising surfaces made of steel with a high tolerance against aluminum dissolved in the zinc phosphating bath, wherein the precipitation of poorly soluble aluminum salts can be largely prevented. In the method, a process is used of activating the zinc surfaces by means of dispersions containing particulate hopeite, phosphophyllite, scholzite, and/or hureaulite, wherein the proportion of particulate phosphates in the activation process must be adapted to the quantity of free fluoride and dissolved aluminum in the zinc phosphation.

Claims

1. A method for an anti-corrosion treatment of a series of metal components, the series comprising components that have, at least in part, iron surfaces, in which method the metal components of the series successively undergo the following wet-chemical treatment steps: (I) activation by contacting the metal components with an alkaline aqueous dispersion that has a D50 value of less than 3 μm and an inorganic particulate constituent of which comprises phosphates being composed at least in part of hopeite, phosphophyllite, scholzite and/or hureaulite, the phosphates from the inorganic particulate constituent being present in the alkaline aqueous dispersion in an amount less than 0.8 g/kg calculated as PO.sub.4 and based on the dispersion; (II) zinc phosphating by contacting the metal components from step (I) with an acidic aqueous composition containing (a) 5-50 g/l of phosphate ions, (b) 0.3-3 g/l of zinc ions, (c) at least 15 mmol/kg of aluminum ions in dissolved form, and (d) at least one source of fluoride ions, wherein the concentration of free fluoride in the acidic aqueous composition is at least 0.5 mmol/kg, wherein the concentration of phosphates in the form of particulate phosphate in mmol/kg, calculated as PO.sub.4 in the alkaline aqueous dispersion, is greater than seven hundredths of the following term in mmol/kg: $\begin{array}{cc}\frac{{\left[\mathrm{AI}\right]}^2}{3\left[AI\right]+\left[F\right].Math.\left(1+{10}^{3.12-\mathrm{pH}}\right)}& \left(\mathrm{Eq}.1\right)\end{array}$ [AI]: concentration of aluminum ions in dissolved form in mmol/kg [F]: concentration of free fluoride in mmol/kg pH: pH of the acidic aqueous composition of the zinc phosphating.

2. The method according to claim 1, wherein the proportion of phosphates based on the inorganic particulate constituents of the alkaline aqueous dispersion of step (I), is at least 30 wt. %, calculated as PO.sub.4.

3. The method according to claim 1, wherein the proportion of zinc in the inorganic particulate constituent of the alkaline aqueous dispersion of step (I), is at least 20 wt. %.

4. The method according to claim 1, wherein the proportion of titanium in the inorganic particulate constituent of the alkaline aqueous dispersion of step (I), is less than 5 wt. %.

5. The method according to claim 1, wherein the amount of phosphates from the inorganic particulate constituent of the alkaline aqueous dispersion of step (I), is at least 40 mg/kg, calculated as PO.sub.4 and based on the dispersion.

6. The method according to claim 1, wherein the pH of the alkaline aqueous dispersion of step (I), is greater than 8, but less than 12.

7. The method according to claim 1, wherein, in the acidic aqueous composition of the zinc phosphating, the total concentration, in mmol/kg, of sodium and/or potassium ions in dissolved form is less than the number 40 divided by the third root of the concentration of aluminum ions in dissolved form.

8. The method according to claim 1, wherein the concentration of aluminum ions in dissolved form in the acidic aqueous composition of the zinc phosphating is greater than 30 mmol/kg, but less than 100 mmol/kg.

9. The method according to claim 1, wherein the concentration of free fluoride is at least 2 mmol/kg, but not greater than 50 mmol/kg.

10. The method according to claim 1, wherein the pH in the acidic aqueous composition of the zinc phosphating is greater than 2.5, but less than 3.5.

11. The method according to claim 1, wherein neither a rinsing nor a drying step takes place between the activation and the zinc phosphating.

12. The method according to claim 1, wherein within the series, components that have aluminum surfaces and/or components that have aluminum surfaces in addition to the iron surfaces are also treated.

13. The method according to claim 12, wherein each component of the series is of the same composition and a pickling rate of aluminum based on the surface area of each component in the zinc phosphating is not greater than: $\begin{array}{cc}0.27.Math.\frac{A}{100}{\mathrm{gm}}^{-2}& \left(\mathrm{Eq}.2\right)\end{array}$ A: actual drag-out from the zinc phosphating bath indicated in milliliters of the acidic aqueous composition per component and per square meter of the component.

14. The method according to claim 12, wherein each component of the series is of the same composition and a pickling rate of aluminum based on the surface area of each component in the zinc phosphating is greater than: $\begin{array}{cc}0.27.Math.\frac{A}{100}{\mathrm{gm}}^{-2}& \left(\mathrm{Eq}.2\right)\end{array}$ A: actual drag-out from the zinc phosphating bath indicated in milliliters of the acidic aqueous composition per component and per square meter of the component; wherein a partial volume of the acidic aqueous composition is removed continuously or discontinuously from the zinc phosphating and an equally large partial volume is supplied continuously or discontinuously to the zinc phosphating by means of one or more aqueous compositions of this kind, which in each case, based on the partial volume, have a higher concentration in comparison to the concentration of the corresponding ions in the removed partial volume, with respect to the phosphate ions, zinc ions and/or the source of fluoride ions, but, with respect to the aluminum ions in dissolved form, have a lower concentration than in the removed partial volume.

15. The method according to claim 1, wherein after the zinc phosphating, the metal components are subjected to a rinsing step and no drying step, followed by electrocoating.

16. The method according to claim 1, wherein the amount of phosphates from the inorganic particulate constituent in the alkaline aqueous dispersion is less than 0.4 g/kg calculated as PO.sub.4 based on the dispersion.

17. The method of claim 1, wherein the acidic aqueous composition includes less than 10 ppm of cobalt.

Description

EXAMPLES

(1) Aluminum (AA6014) and steel sheets (CRS) were treated in zinc phosphating baths with different levels of free fluoride and dissolved aluminum after prior activation with dispersions of particulate zinc phosphate and the appearance of the coatings was evaluated immediately after the zinc phosphating. Table 1 contains an overview of the activation and zinc phosphating compositions and the results of the evaluation of the quality of the coatings. The sheets underwent the following method steps in the sequence indicated: A1) cleaning and degreasing by dipping at 55° C. for 180 seconds 15 g/L BONDERITE® C-AK 11566 (Henkel AG & Co. KGaA) 1.1 g/L BONDERITE® M-AD ZN-2 (Henkel AG & Co. KGaA) 5 g/L BONDERITE C-AD 1561 (Henkel AG & Co. KGaA) 2.2 g/L NaHCO.sub.3 preparing with deionized water (κ<1 μScm.sup.−1); adjusting the pH to 10.8 using potassium hydroxide solution. A2) cleaning and degreasing by spraying at 1 bar and 55° C. for 70 seconds using a composition as in A1) B) rinsing with deionized water (κ<1 μScm.sup.−1) at 20° C. for 60 seconds C) dip activation at 20° C. for 30 seconds 0.6-4 g/kg PREPALENE® X (Nihon Parkerizing Co., Ltd.) contains 8.4 wt. % of zinc in the form of Zn.sub.3(PO.sub.4)2*4H.sub.2O

(2) TABLE-US-00001 200 mg/kg K.sub.4P.sub.2O.sub.7 preparing with deionized water (κ<1 μScm.sup.−1); adjusting the pH to 10.3 using H.sub.3PO.sub.4. The D50 value of the dispersion for activation was 0.25 μm at 20° C., determined on the basis of the static scattered light analysis according to Mie theory in accordance with ISO 13320:2009 by means of particle analyzer HORIBA LA-950 (Horiba Ltd.) assuming a refractive index of the scattering particles of n=1.52−i.Math.0.1. D) zinc phosphating by immersion at 50° C. for 150 seconds:

(3) TABLE-US-00002 1.2 g/kg zinc 1.0 g/kg manganese 0.9 g/kg nickel 15.3 g/kg  phosphate 1.9 g/kg nitrate 2.0 g/kg N-methylmorpholine-N-oxide  20 mg/kg hydrogen peroxide An amount of a source of fluoride and an amount of aluminum were added according to table 1. Preparing with deionized water (κ<1 μScm.sup.−1); adjusting the pH to pH 3.0 using 10% NaOH Free acid: 1.1-1.3 points The free acid is determined from 10 ml sample volume diluted to 50 ml with deionized water and subsequent titration with 0.1 N NaOH to pH 3.6, the consumption of sodium hydroxide solution in milliliters corresponding to the amount of free acid in points. The zinc phosphating baths were formulated without adding sodium salts. The proportion of sodium was less than 1 mg/kg. E) Rinsing with deionized water (κ<1 μScm.sup.−1) at 20° C. for 60 seconds F) Drying at 50° C. in a drying cabinet after blowing off with compressed air

(4) It can be seen from Table 1 that satisfactory phosphate coatings which thus appear to the naked eye to be homogeneous and closed on the sheets, can be achieved by adapting the amount of particulate zinc phosphate in the activation to the amount of free fluoride and the amount of dissolved aluminum in the zinc phosphating (CRS-L-A1-h; CRS-H-A1-I; CRS-H-A2-I; CRS-H-A3-I). If the amount of particulate zinc phosphate in the activation falls below the value defined by the free fluoride amount and the concentration of dissolved aluminum, either non-homogeneous coatings are achieved (CRS-L-A2-I; CRS-L-A3-h) or the phosphate coatings are virtually closed, the substrate surface nevertheless remaining visible after phosphating (CRS-L-A1-I; CRS-L-A2-h). Even on aluminum, closed phosphate coatings are produced in the method variants according to the invention in accordance with table 1, such that the suitability of the method according to the invention for the corrosion-protective treatment of a series of components that comprise components having surfaces of iron and surfaces of aluminum is demonstrated.

(5) TABLE-US-00003 TABLE 1 Appearance 0: closed homogeneous layer Activation Zinc phosphating, pH: 3.0 1: almost closed, but PO.sub.4/ Coating weight/ shimmering substrate Example.sup.1 mmolkg.sup.−1 [F]*/mmolkg.sup.−1 [Al]**/mmolkg.sup.−1 0.07.Math.Term.sup.# gm.sup.−2 surface CRS-L-A1-l 0.63 4.9 29.7 0.62 1.5 1 CRS-L-A1- 0.63 14.8 29.7 0.50 1.8 0 AA-L-A1-l 0.63 4.9 29.7 0.62 1.2 1 AA-L-A1-h 0.63 14.8 29.7 0.50 1.4 0 CRS-H-A1-l 3.57 4.9 29.7 0.62 1.8 0 AA-H-A1-l 3.57 4.9 29.7 0.62 1.5 0 CRS-L-A2-l 0.63 6.2 37.0 0.76 — 2 CRS-L-A2- 0.63 18.5 37.0 0.62 1.5 1 AA-L-A2-l 0.63 6.2 37.0 0.76 — 2 AA-L-A2-h 0.63 18.5 37.0 0.62 1.2 1 CRS-H-A2-l 3.57 6.2 37.0 0.76 1.7 0 AA-H-A2-l 3.57 6.2 37.0 0.76 1.5 0 CRS-L-A3- 0.72 27.5 55.6 0.94 — 2 AA-L-A3-h 0.72 27.5 55.6 0.94 — 2 CRS-H-A3-l 3.57 9.3 55.6 1.15 1.6 0 AA-H-A3-l 3.57 9.3 55.6 1.15 1.2 0 .sup.1The first letters indicate the substrate; L (low) and H (high) the content of PO.sub.4 in the activation; A1 to A3 the increasing content of aluminum in the zinc phosphating; and the last letter l (low) and h (high) the content of free fluoride in the zinc phosphating *free fluoride measured with ion meter pMX 3000/Ion (Xylem Inc.); source: ammonium bifluoride **source: aluminum trichloride