Multi-stage anti-corrosion treatment of metal components having zinc surfaces

Abstract

The present invention relates to the field of phosphating for corrosion-protective pretreatment of zinc surfaces, being directed toward the use of largely nickel- and cobalt-free zinc phosphating solutions. The present invention makes available an alternative to trication zinc phosphating, in which the zinc surfaces of a component are firstly, before zinc phosphating, passivated with an alkaline composition containing iron(III) ions, and thereby preconditioned for a largely nickel- and cobalt-free zinc phosphating operation. In a further aspect, the invention relates to a component, in particular an automobile body, that comprises at least in part surfaces made of zinc, the zinc surfaces being covered by a two-layer system made up of a first, inner passive layer containing iron and resting on the zinc surface, and a second, outer crystalline zinc phosphate layer resting on the inner layer.

Claims

1. A method for corrosion-protective treatment of metal surfaces of a component that comprises at least in part surfaces made of zinc or zinc alloys, wherein the component is firstly, in step i), brought into contact with an alkaline aqueous composition (A) that contains a) at least 50 mg/L iron(III) ions, and b) 0.1 to 4 q/L phosphate ions c) at least 100 mg/L of components c) complexing agents selected from: organic compounds c1) that comprise at least one functional group selected from COOX, OPO.sub.3X, and/or PO.sub.3X, where X represents either a hydrogen atom, an alkali-metal and/or alkaline-earth-metal atom, and/or condensed phosphates c2) calculated as PO.sub.4, and the alkaline aqueous composition (A) having a free alkalinity of at least 1 point but less than 6 points, and a pH in a range from 10.5 to 14, and then in step ii), with or without an interposed rinsing step and with or without previous activation, is brought into contact with an acidic aqueous composition (B) for zinc phosphating that has a pH in a range from 2.5 to 3.6 and contains a) 0.2 to 3.0 g/L zinc(II) ions, b) 5.0 to 30 g/L phosphate ions, calculated as P.sub.2O.sub.5, and c) less than 0.1 g/L each of ionic compounds of metallic elements nickel and cobalt, based in each case on amount of the metallic element; wherein no copper and no water-soluble nickel salts are added to the acidic aqueous composition (B) for zinc phosphating.

2. The method according to claim 1, wherein composition (A) has a pH of no more than 13.

3. The method according to claim 1, wherein a mass-based ratio of iron(III) ions to phosphate ions in composition (A) is in a range from 1:20 to 1:2.

4. The method according to claim 1, wherein a molar ratio of all components c) to iron(III) ions in composition (A) is from greater than 1:1 to 5:1.

5. The method according to claim 1, wherein condensed phosphates c2) is selected from pyrophosphates, tripolyphosphates, and/or polyphosphates are contained as components c) in composition (A).

6. The method according to claim 5, wherein in addition to component c2), organic compounds c1) that in a protonated state have an acid number of at least 250 are contained in composition (A).

7. The method according to claim 4, wherein the organic compounds c1) in composition (A) are selected from one or more of a-hydroxycarboxylic acid, b-hydroxycarboxylic acid, g-hydroxycarboxylic acid, hydroxyethane-1,1-diphosphonic acid, [(2-hydroxyethyl) (phosphonomethyl)amino]methylphosphonic acid, diethylenetriaminepentakis (methylenephosphonic acid), and/or amino-tris(methylenephosphonic acid), and salts thereof, a molar ratio of components c1) to iron(III) ions being less than 1:1.

8. The method according to claim 1, wherein composition (A) contains less than a total of 10 mg/L ionic compounds of metallic elements nickel, cobalt, manganese, molybdenum, chromium, and/or cerium, based in each case on amount of the metallic element.

9. The method according to claim 1, wherein composition (B) for zinc phosphating additionally contains one or more of: TABLE-US-00010 0.001 to 4 g/L manganese(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).

10. The method according to claim 1, wherein composition (B) for zinc phosphating contains less than 0.01 g/L each of the ionic compounds of the metallic elements nickel and cobalt.

11. The method according to claim 1, wherein composition (B) for zinc phosphating contains less than 0.001 g/L copper(II) ions.

12. The method according to claim 1, wherein composition (B) for zinc phosphating contains water-soluble inorganic compounds that represent a source of fluoride ions.

13. The method according to claim 12, wherein composition (B) for zinc phosphating contains silicon in the form of water-soluble inorganic compounds.

14. The method according to claim 13, wherein the component that comprises at least in part surfaces made of zinc or zinc alloys, further comprises surfaces made of aluminum, and composition (B) has a temperature in a range from 20 C. to 65 C. and comprises a quantity of free fluoride (measured in g/L) that is no greater than a quotient of the number 8 and said temperature in C. (8/T).

15. The method according to claim 13, wherein composition (B) contains at least 0.025 g/L, but less than 1 g/L, of said silicon calculated as SiF.sub.6, and a product (Si/mM) (F/mM) of a concentration of silicon [Si in mM] in the form of water-soluble inorganic compounds and a concentration of free fluoride [F in mM] divided by points of free acid is no greater than 5, the points of free acid in composition (B) being at least 0.4 points but not exceeding a value of 3.0 points.

16. The method according to claim 14, wherein the aluminum surfaces of the component comprise, after method step ii), a zinc phosphate layer having a layer weight of less than 0.5 g/m.sup.2.

17. The method according to claim 15, wherein the zinc surfaces of the component comprise, after method step ii), a crystalline zinc phosphate layer having a layer weight in a range from 0.5 to 3.5 g/m.sup.2.

18. A method for corrosion-protective treatment of metal surfaces of a component that comprises at least in part surfaces made of zinc or zinc alloys, comprising steps of: step i), first, contacting metal surfaces of a component that comprises at least in part zinc or zinc alloy surfaces, with an alkaline aqueous composition (A) comprising: a) at least 50 mg/L iron(III) ions, and b) 0.1 to 4 g/L phosphate ions; c) at least 100 mg/L complexing agents c) selected from: c1) organic compounds that comprise at least one functional group selected from COOX, OPO.sub.3X, and/or PO.sub.3X, where X represents an atom selected from a hydrogen atom, an alkali-metal atom, alkaline-earth-metal atom, and combinations thereof; and c2) condensed phosphates, calculated as PO.sub.4, and combinations of said complexing agents; the alkaline aqueous composition (A) having a free alkalinity of at least 1 point but less than 6 points, and a pH in a range from 10.5 to 14, thereby producing a first inner passive layer comprising iron deposited on the zinc or zinc alloy surfaces; and after step i), with or without an interposed rinsing step and with or without previous activation, step ii), contacting the component having the first inner passive layer comprising iron deposited on the zinc or zinc alloy surfaces with an acidic aqueous composition (B) for zinc phosphating that has a pH in a range from 2.5 to 3.6 and comprises: a) 0.2 to 3.0 g/L zinc(II) ions, b) 5.0 to 30 g/L phosphate ions, calculated as P.sub.2O.sub.5, and c) less than 0.1 g/L of ionic compounds of cobalt, based in each case on amount of cobalt wherein the acidic aqueous composition (B) is nickel-free and free from added copper and forms an outer layer of crystalline zinc phosphate over the inner passive layer.

19. The method according to claim 1, wherein composition (A) further comprises at least one nonionic surfactant present in an amount of from 10 mg/L to 10 g/L, the nonionic surfactant selected from the group consisting of ethoxylated C.sub.10 to C.sub.18 fatty alcohols having from 2 to 12 alkoxy groups, propoxylated C.sub.10 to C.sub.18 fatty alcohols having from 2 to 12 alkoxy groups, and mixtures thereof.

Description

EXEMPLIFYING EMBODIMENTS

(1) Individual method steps in a dipcoating facility for corrosion-protective treatment of galvanized steel panels (HDG: Gardobond EA; Chemetall Co.):

(2) A. Alkaline cleaning (pH 11): 3 wt % Ridoline 1574A (Henkel Co.); 0.4 wt % Ridosol 1270 (Henkel Co.) containing H.sub.3PO.sub.4, K.sub.4P.sub.2O.sub.7, sodium gluconate, sodium salt of hydroxyethane-1,1-diphosphonic acid, KOH Treatment time at 60 C.: 180 seconds.

(3) B. Rinse with deionized water (<1 S cm.sup.1)

(4) C1. Alkaline passivation in accordance with composition (A):

(5) TABLE-US-00003 2.80 wt % KOH 0.19 wt % H.sub.3PO.sub.4 0.22 wt % K.sub.4P.sub.2O.sub.7 0.06 wt % sodium gluconate 0.10 wt % sodium salt of hydroxyethane-1,1-diphosphonic acid 0.23 wt % Fe(NO.sub.3).sub.39H.sub.2O Remainder deionized water ( < 1 Scm.sup.1) Free alkalinity: 3 pH: 11 Treatment time 120 seconds at 60 C.:

(6) C2. Alkaline passivation in accordance with composition (A):

(7) TABLE-US-00004 1.09 wt % KOH 0.19 wt % H.sub.3PO.sub.4 0.22 wt % K.sub.4P.sub.2O.sub.7 0.06 wt % sodium gluconate 0.10 wt % sodium salt of hydroxyethane-1,1-diphosphonic acid 0.23 wt % Fe(NO.sub.3).sub.39H.sub.2O 1.30 wt % NaHCO.sub.3 Remainder deionized water ( < 1 Scm1) Free alkalinity: 10 pH: 13 Treatment time 120 seconds at 60 C.:

(8) D. Activation: 0.1 wt % Fixodine 50CF (Henkel Co.) Remainder deionized water (<1 Scm.sup.1) Treatment time at 20 C.: 60 seconds

(9) E1. Nickel-free phosphating in accordance with composition (B):

(10) TABLE-US-00005 0.13 wt % zinc 0.09 wt % manganese 0.12 wt % nitrate 1.63 wt % phosphate 0.05 wt % N-methylmorpholine-N oxide 0.02 wt % ammonium bifluoride 0.03 wt % H.sub.2SiF.sub.6 Remainder deionized water ( < 1 Scm1) Free fluoride: 40 mg/L Free acid: 1.3 points (pH 3.6) Total acid: 24 points (pH 8.5) Hydrogen peroxide: 30 mg/L Treatment time at 51 C.: 180 seconds

(11) E2. Nickel-free, copper-containing phosphating in accordance with composition (B):

(12) TABLE-US-00006 0.13 wt % zinc 0.09 wt % manganese 0.001 wt % copper 0.12 wt % nitrate 1.63 wt % phosphate 0.05 wt % N-methylmorpholine-N oxide 0.02 wt % ammonium bifluoride 0.03 wt % H.sub.2SiF.sub.6 Remainder deionized water ( < 1 Scm1) Free fluoride: 40 mg/L Free acid: 1.3 points (pH 3.6) Total acid: 24 points (pH 8.5) Hydrogen peroxide: 30 mg/L Treatment time at 51 C.: 180 seconds

(13) E3. Nickel-containing phosphating (trication phosphating):

(14) TABLE-US-00007 0.13 wt % zinc 0.09 wt % manganese 0.09 wt % nickel 0.12 wt % nitrate 1.63 wt % phosphate 0.05 wt % N-methylmorpholine-N oxide 0.02 wt % ammonium bifluoride 0.03 wt % H.sub.2SiF.sub.6 Remainder deionized water ( < 1 Scm1) Free fluoride: 40 mg/L Free acid: 1.3 points (pH 3.6) Total acid: 25 points (pH 8.5) Hydrogen peroxide: 30 mg/L Treatment time at 51 C.: 180 seconds

(15) E4. Nickel-containing phosphating (trication phosphating): as E3, but 0.01 wt % nickel

(16) E5. Nickel-containing phosphating (trication phosphating): as E3, but 0.005 wt % nickel

(17) E6. Acid passivation:

(18) TABLE-US-00008 0.34 g/L H.sub.2ZrF.sub.6 0.12 g/L ammonium bifluoride 39 mg/L Cu(NO.sub.3).sub.23H.sub.2O Remainder deionized water ( < 1 Scm1) pH 4 Treatment time at 30 C.: 120 seconds

(19) F. Paint structure: Cathoguard 500 (BASF Co.): layer thickness 20 to 22 m

(20) The points of free acid in the exemplifying baths E1 to E5 in accordance with a composition (B) are determined by diluting 10 ml of bath sample to 50 ml and titrating with 0.1 N sodium hydroxide to a pH of 3.6. The sodium hydroxide consumed (in ml) indicates the points. The total acid content is determined correspondingly by titrating to a pH of 8.5.

(21) The free fluoride content in the exemplifying baths E1 to E3 in accordance with a composition (B) is sensed using a potentiometric electrode system (WTW Co., inoLab, pH/ion level 3). The electrode system contains a fluoride-sensitive glass electrode (WTW, F501) and a reference electrode (WTW, R503). For two-point calibration, the two electrodes are immersed together successively into calibration solutions having concentrations of 100 mg/L and 1000 mg/L free fluoride, produced from the Titrisol fluoride standard of the Merck company with no added buffer. The resulting measured values are correlated with the respective fluoride content (100 and 1000 respectively) and read into the instrument. The slope of the glass electrodes is then indicated on the instrument in mV per decade of the fluoride ion content in mg/L, and is typically between 55 and 60 mV. The fluoride content in mg/L is then determined directly by immersing the two electrodes into the exemplifying baths E1 to E5 at a temperature of 25 C.

(22) Table 1 shows the influence of alkaline passivation followed by nickel-free or rather low-nickel zinc phosphating (Examples 1 to 4 and 5, 6) on adhesion of the cathodic dipcoating paint to the zinc substrate after water aging and subsequent cross cut testing. As compared therewith, nickel-free zinc phosphating that is performed based on a composition (B) with or without the addition of copper ions, but without alkaline passivation using a composition (A), yields insufficient paint adhesion on the galvanized substrate (Examples 7, 8). Low-nickel phosphating (Examples 10, 11) performed without alkaline passivation already yields poorer results in the cross cut test as compared with nickel-containing trication phosphating (Example 9), while together with alkaline passivation (Examples 5, 6) outstanding paint adhesion can again be achieved.

(23) It can further be gathered from the table that nickel-containing trication phosphating (Example 9), as known in the existing art, produces outstanding adhesion of the paint structure to the substrate. In the method according to the present invention, adhesion that is entirely equivalent to nickel-containing trication phosphating is achieved when the surface coverage of iron after alkaline passivation is moderate, i.e. for example approx. 100 mg/m.sup.2 based on the element iron (Examples 1, 3). Greater surface coverages of iron (in the range of approx. 250 mg/m.sup.2), which are deposited in a method not in accordance with the invention according to Examples 2 and 4, result, together with nickel-free zinc phosphating, in poorer paint adhesion as compared with trication phosphating (Example 9).

(24) The method according to the present invention (see Examples 1, 3, 5, and 6) likewise produces an appreciable improvement in paint adhesion on the zinc surfaces as compared with alternative treatment methods that provide, instead of phosphating, for a conversion treatment based on fluorine complexes of zirconium (Examples 12, 13).

(25) TABLE-US-00009 TABLE 1 Various method sequences for corrosion-protective treatment of galvanized steel strip, and results after crosscut adhesion testing Ex- Surface Surface am- Crosscut* coverage** coverage*** ple Method sequence (0-5) ZnPO.sub.4 (g/m.sup.2) iron (mg/m.sup.2) 1 A-C1-B-D-E1-B-E-F 0 2.5 102 2 A-C2-B-D-E1-B-E-F 1-2 2.6 252 3 A-C1-B-D-E2-B-E-F 0 2.5 113 4 A-C2-B-D-E2-B-E-F 1-2 2.4 245 5 A-C1-B-D-E4-B-E-F 0 2.7 112 6 A-C1-B-D-E5-B-E-F 0 2.5 110 7 A-B-D-E1-B-E-F 5 1.7 8 A-B-D-E2-B-E-F 5 1.7 9 A-B-D-E3-B-E-F 0 3.5 10 A-B-D-E4-B-E-F 1 2.2 11 A-B-D-E5-B-E-F 2 2.1 12 A-C1-B-E6-B-E-F 3 114 13 A-C2-B-E6-B-E-F 4 260 *Panels aged in deionized water ( < 1 Scm.sup.1) at 80 C. for 30 minutes; panels cooled for 30 minutes at 20 C.; crosscut adhesion testing per DIN EN ISO 2009 and panels subsequently bent 180 at the crosscut; paint adhesion evaluated per DIN EN ISO 2009 (0 = no paint adhesion; 5 = complete paint adhesion). **Determined by dissolving off the zinc phosphate layer with aqueous 5-wt % CrO.sub.3 that was brought into contact with a defined area of the galvanized panel immediately after method step E at 25 C. for 5 minutes, and determining the phosphorus content in the same pickling solution using ICP-OES. The coating weight of zinc phosphate is determined by multiplying the quantity of phosphorus per unit area by a factor of 6.23. ***Quantitative determination of the quantity of iron(III) ions by UV photometry (PhotoFlex, WTW company) in 300 l sample volume of a 5-wt % nitric acid solution that was pipetted onto a defined area (1.33 cm.sup.2) of the galvanized panel immediately after method step C using a measurement cell ring (Helmut Fischer company) and taken up with the same pipette after 30 seconds of exposure time at a temperature of 25 C. and transferred into the UV measurement cuvette, in which 5 ml of a 1.0% sodium thiocyanate solution had been prepared, for determination of absorption at a wavelength of 517 nm and a temperature of 25 C. Calibration was effected using a two-point method, by determining absorption values of identical volumes (300 l) of two standard solutions of iron(III) nitrate in 5-wt % nitric acid, which were transferred into the measurement cuvette containing 5 ml of a 1.0% sodium thiocyanate solution for determination of absorption values at 25 C.