Pretreating zinc surfaces prior to a passivating process

Abstract

The invention relates to a wet-chemical pretreatment of zinc surfaces prior to applying a corrosion-protection coating, which deposits a thin inorganic coating of oxide and/or metallic iron. An iron layer structure which is applied according to the invention, hereinafter referred to as ferrization, improves the achievable corrosion protection of wet-chemical conversion coatings on zinc surfaces. Furthermore, the ferrization process causes both a reduction of the contact corrosion of joined metal components which have zinc and iron surfaces as well as a reduction of corrosive coating migration on cut edges of galvanized steel strips with coating layer structures. In particular, the invention relates to an alkaline composition containing an iron ion source, a reducing agent based on oxoacids of nitrogen and phosphorus, and water-soluble organic carboxylic acids with an amino group at the , , or position with respect to the acid group and/or the water-soluble salts thereof.

Claims

1. A method for pretreating galvanized steel surfaces, wherein the galvanized steel surfaces i) optionally are firstly cleaned with an alkaline cleaner and degreased, ii) are brought into contact with an alkaline composition comprising: a) at least 3.0 g/l iron ions, b) one or more water-soluble organic carboxylic acids that comprise a carboxyl group and at least one amino group in an , , or position with respect to the carboxyl group, as well as water-soluble salts thereof, c) a reducing agent based on one or more oxoacids of phosphorus or nitrogen as well as water-soluble salts thereof, wherein at least one phosphorus atom or nitrogen atom is present in a moderate oxidation state; wherein the alkaline aqueous composition has a pH of at least 8.5 and no higher than 10.0; treatment time and temperature being selected such that a covering layer made substantially of oxidized and/or metallic iron is generated on the galvanized steel surfaces; and iii) after step ii) are subjected to a passivating wet-chemical conversion treatment that contains no chromium(VI).

2. The method according to claim 1, wherein step ii) occurs in electroless fashion.

3. The method according to claim 1, further comprising selecting a contact temperature and contact time for step ii) such that surface coverage of iron on the galvanized steel surfaces is at least 20 mg/m.sup.2 and no more than 250 mg/m.sup.2, based on the element iron.

4. The method according to claim 1, wherein the passivating wet-chemical conversion treatment of step iii) comprises bringing the galvanized steel surfaces pretreated in step ii) into contact with an acidic aqueous composition that contains in total at least 5 ppm but in total no more than 1500ppm water-soluble inorganic compounds of elements selected from Zr, Ti, Si, Hf and mixtures thereof, based on said elements.

5. The method according to claim 1, wherein the passivating wet-chemical conversion treatment of step iii) comprises bringing the galvanized steel surfaces pretreated in step ii) into contact with an acidic aqueous composition that has a pH in the 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 in each case of ionic compounds of a metallic element selected from nickel and cobalt, based in each case on the metallic element.

6. The method according to claim 1, wherein the iron ions in the alkaline composition of step ii) are present in an amount of in total no more than 10 g/l.

7. The method according to claim 1, wherein the alkaline composition of step ii) has a molar ratio of the iron ions to component b) that is equal to at least 1:12, but is no greater than 2:1.

8. The method according to claim 1, wherein the one or more water-soluble organic carboxylic acids in accordance with component b) of the alkaline composition of step ii) are selected from -amino acids.

9. The method according to claim 8, wherein the -amino acids comprise, in addition to amino and carboxyl groups, exclusively hydroxyl groups.

10. The method according to claim 9, wherein the -amino acids are selected from lysine, serine, threonine, alanine, glycine, aspartic acid, glutamic acid and mixtures thereof.

11. The method according to claim 1, wherein the alkaline composition of step ii) has a molar ratio of the iron ions to component c) of at least 1:10, but no greater than 3:1.

12. The method according to claim 1, wherein the oxoacids of phosphorus or nitrogen in accordance with component c) of the alkaline composition of step ii) are selected from hyponitrous acid, hyponitric acid, nitrous acid, hypophosphoric acid, hypodiphosphonic acid, diphosphoric(III, V) acid, phosphonic acid, diphosphonic acid, phosphinic acid, water-soluble salts of said oxoacids and mixtures thereof.

13. The method according to claim 1, wherein the alkaline composition of step ii) further comprises component d) one or more water-soluble -hydroxycarboxylic acids that comprise at least one hydroxyl group and one carboxyl group and/or salts thereof, different from component b).

14. The method according to claim 13, wherein the alkaline composition of step ii) has a molar ratio of iron ions to component d) that is equal to at least 1:4, but is no greater than 2:1.

15. The method according to claim 13, wherein the water-soluble -hydroxycarboxylic acids in accordance with component d) of the alkaline composition of step ii) comprise no more than 8 carbon atoms.

16. The method according to claim 13, wherein the water-soluble -hydroxycarboxylic acids in accordance with component d) of the alkaline composition of step ii) are selected from the group consisting of polyhydroxymonocarboxylic acids having at least 4 carbon atoms, polyhydroxydicarboxylic acids having at least 4 carbon atoms, tartronic acid, glycolic acid, lactic acid, -hydroxybutyric acid and mixtures thereof.

17. The method according to claim 1 wherein in the alkaline composition of step ii), zinc ions are not contained in a quantity that produces a ratio of total molar proportion of zinc ions and iron ions in terms of total molar proportion of component b) and component d), that is greater than 1 : 1.

18. The method according to claim 3, wherein the contact time for step ii) ranges from about 3 seconds to no more than about 4 minutes and the contact temperature for step ii) ranges from at least about 30 C. to no more than about 70 C.

19. A method for pretreating galvanized steel surfaces, wherein the galvanized steel surfaces are zinc surfaces consisting of metallic zinc and/or iron-alloyed zinc, and i) optionally are firstly cleaned with an alkaline cleaner and degreased, ii) are contacted with an alkaline aqueous composition comprising: a) at least 0.01 g/l iron ions, b) one or more water-soluble organic carboxylic acids that comprise a carboxyl group and an NH.sub.2 group in an position with respect to the carboxyl group, as well as water-soluble salts thereof; c) a reducing agent based on one or more oxoacids of phosphorus or nitrogen as well as water-soluble salts thereof, wherein at least one phosphorus atom or nitrogen atom is present in a moderate oxidation state; wherein the alkaline aqueous composition has a pH of at least 8.5; and iii) after step ii) are subjected to a passivating wet-chemical conversion treatment.

20. The method according to claim 19, wherein the one or more water-soluble organic carboxylic acids are -amino acids comprising, in addition to the carboxyl groups and the NH.sub.2 group in an position with respect to the carboxyl groups, exclusively hydroxyl groups.

21. The method according to claim 20, wherein the -amino acids are selected from lysine, serine, threonine, alanine, glycine, aspartic acid, glutamic acid and mixtures thereof.

22. A method for pretreating galvanized steel surfaces, comprising: i) optionally cleaning and degreasing the galvanized steel surfaces with an alkaline cleaner; ii) contacting the galvanized steel surfaces with an alkaline aqueous solution comprising: a) at least 0.01 g/l iron ions, b) one or more water-soluble organic carboxylic acids that comprise a carboxyl group and at least one amino group in an , , or position with respect to the carboxyl group, as well as water-soluble salts thereof, c) a reducing agent in the form of one or more oxoacids of phosphorus as well as water-soluble salts thereof, wherein at least one phosphorus atom is present in a moderate oxidation state; wherein the alkaline aqueous solution has a pH of at least 8.5; wherein the alkaline aqueous solution of step ii), comprises zinc ions, with the proviso that the zinc ions are not contained in the aqueous solution in a quantity that produces a ratio of total molar proportion of zinc ions and iron ions in terms of total molar proportion of component b) and component d), that is greater than 1:1; and iii) after step ii) are subjected to a passivating wet-chemical conversion treatment.

Description

EXEMPLIFYING EMBODIMENTS

(1) The influence of various -amino acids with regard to ferrization homogeneity, after compositions according to the present invention are brought into contact with electrolytically galvanized steel by immersion, is reproduced in Table 1.

(2) Firstly, with all compositions according to the present invention (C1 to C4) thin coatings of oxidized and/or metallic iron are obtained on the zinc surfaces (ferrization), although particularly homogeneous coatings are formed especially by compositions according to the present invention (C1; C5) containing glycine.

(3) TABLE-US-00001 TABLE 1 Alkaline compositions according to the present invention for ferrization Component: C1 C2 C3 C4 C5 a) Iron(II) gluconate 12.50 12.50 12.50 12.50 1.25 Iron(II) lactate 18.75 18.75 18.75 18.75 1.87 b) Glycine 45.00 4.50 L-Glutamine 87.61 L-Glutamic acid 88.20 L-Lysine 87.63 c) NaH.sub.2PO.sub.2 45.00 45.00 45.00 45.00 4.50 NaOH, 50 wt % 25.00 32.60 76.70 25.00 2.50 Water 853.75 803.54 758.85 811.12 985.38 pH 9.0 9.0 9.0 9.0 9.0 Method parameters: C1 C2 C3 C4 C5 Dip application .sup.1 10 s @ 10 s @ 10 s @ 10 s @ 60 s @ 50 C. 50 C. 50 C. 50 C. 50 C. Visual score .sup.2 ++ + + ++ .sup.1 on electrolytically galvanized steel panel (Gardobond MBZE7) .sup.2 in terms of ferrization homogeneity: ++ homogeneous dark gray coating + almost complete coverage with dark gray coating incomplete coverage with dark gray to brownish coating inhomogeneous coverage with predominantly light gray to brownish coating

(4) The concentration of active components in a composition according to the present invention has a direct effect on deposition rate, so that diluted compositions need to be brought into contact with the galvanized steel surface for a correspondingly longer time in order to obtain a homogeneously coated zinc surface (see C1 compared with C5).

(5) The effect of ferrization in the context of the use of compositions according to the present invention with reference to process chains for corrosion-protective pretreatment of zinc surfaces, will be presented below. Table 2 indicates the corrosive infiltration of a dipcoating paint on electrolytically galvanized steel after the respective process chain for corrosion-protective pretreatment, in the alternating climate test and stone impact test.

(6) The individual method steps of the process chains listed in Table 2 for corrosion-protective treatment of individual galvanized steel panels (Gardobond MBZE7) are shown below: A. Alkaline cleaning (pH 11): 3 wt % Ridoline 1574A (Henkel Co.); 0.4 wt % Ridosol 1270 (Henkel Co.) Treatment time at 60 C.: 180 seconds. B. Rinse with deionized water (<1 S cm.sup.1) C. Ferrization using a composition according to Table 1: Treatment time at 50 C.: 60 seconds D. Activation: 0.1 wt % Fixodine 50CF (Henkel Co.) Remainder deionized water (<1 S cm.sup.1) Treatment time at 20 C.: 60 seconds E1. Acidic passivation: 0.34 g/l H.sub.2ZrF.sub.6 0.12 g/L ammonium bifluoride 0.08 g/L Cu(NO.sub.3).sub.2.3H.sub.2O Remainder deionized water (<1 S cm.sup.1) pH: 4 Treatment time at 30 C.: 120 seconds E2. Nickel-free phosphating: 0.13 wt % zinc 0.09 wt % manganese 0.12 wt % nitrate 1.63 wt % phosphate 0.25 wt % hydroxylamine sulfate 0.02 wt % ammonium bifluoride 0.10 wt % H.sub.2SiF.sub.6 Remainder deionized water (<1 S cm.sup.1) Free fluoride: 40 mg/L Free acid: 1.3 points (pH 3.6) Total acid: 26 points (pH 8.5) Treatment time at 50 C.: 180 seconds E3. Nickel-containing phosphating (trication phosphating): 0.13 wt % zinc 0.09 wt % manganese 0.10 wt % nickel 0.32 wt % nitrate 1.63 wt % phosphate 0.25 wt % hydroxylamine sulfate 0.02 wt % ammonium bifluoride 0.10 wt % H.sub.2SiF.sub.6 Remainder deionized water (<1 S cm.sup.1) Free fluoride: 40 mg/L Free acid: 1.3 points (pH 3.6) Total acid: 26.5 points (pH 8.5) Treatment time at 50 C.: 180 seconds F Paint structure: EV2007 (PPG Co.): layer thickness 17 to 19 m

(7) It is clearly evident from Table 2 that in a process chain according to the present invention that wet-chemical conversion by means of aqueous zirconium-containing passivation solutions (B1), ferrization produces improved corrosion protection as compared with an analogous process chain in which ferrization is omitted (V1).

(8) The same can be noted for the improvement in corrosion protection of those galvanized steel panels which were subjected to nickel-free zinc phosphating. Here as well, prior ferrization (B2) results in substantially improved corrosion values as compared with zinc phosphating alone (B2). The corrosion results obtained with ferrization (B2) are even improved as compared with trication phosphating (V3), often used in the existing art for corrosion-protective pretreatment of components fabricated with mixed materials.

(9) TABLE-US-00002 TABLE 2 Various method sequences for corrosion-protective treatment of electrolytically galvanized strip steel (Gardobond MBZE7, Chemetall Co.), and results in terms of scratch infiltration and the stone impact test Surface Surface Scratch coverage.sup.2 coverage.sup.3 infiltration.sup.1 K of ZnPO.sub.4 of iron Method sequence (mm) value.sup.1 (g/m.sup.2) (mg/m.sup.2) B1 A-B-C5-B-E1-B-F 2.0 3.5 193 B2 A-B-C5-B-D- 1.9 2.5 2.6 202 E2-B-F V1 A-B-E1-B-F 4.0 4.5 V2 A-B-D-E2-B-F 3.9 5.0 2.9 V3 A-B-D-E3-B-F 2.3 3.5 3.0 .sup.1Stone impact and scratch infiltration per DIN EN ISO 20567-1 after exposure using VDA 621-415 alternating climate test (10 weeks) .sup.2Determined 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 E2 or E3 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. .sup.3Quantitative 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.