Fluoride-free zirconium-based metal pre-treatment for passivation

Abstract

A method for anti-corrosion pre-treatment of metal substrates using zirconium-based aqueous anti-corrosion agents comprising polycyclic hydrocarbons that have at least one anellated benzene ring, each having at least two ring-substituted hydroxyl groups in ortho position to each other. The aqueous anti-corrosion agent can be substantially free both of passivating chromium-containing compounds and of fluoride-containing compounds that pickle the metal substrate. Pre-treatment by drying (dry-in-place method) is especially advantageous. Accordingly, the method according to the invention is suitable in particular for the pre-treatment of metal strip, wherein excellent anti-corrosion results are achieved on surfaces of aluminum or steel. The invention further relates to a method for producing coated can lids from aluminum strip by using the previously mentioned zirconium-based anti-corrosion agent. A further aspect comprises an aqueous concentrate for providing the ready-to-use anti-corrosion agents.

Claims

1. A method for anti-corrosion pretreatment of a metal substrate, comprising steps of: 1) contacting at least one surface of a metal substrate with an aqueous anti-corrosion agent comprising: (A) at least one water-soluble compound of the element zirconium; and (B) at least one polycyclic hydrocarbon compound which comprises at least one anellated benzene ring, wherein the at least one anellated benzene ring has at least two hydroxyl functional groups on the ring in an ortho position relative to one another; wherein the aqueous anti-corrosion agent has a weight ratio of the water-soluble compounds (A) of the element zirconium, based on the element zirconium, to the polycyclic hydrocarbons (B) that is less than 0.2:1.

2. The method according to claim 1, wherein the at least one polycyclic hydrocarbon (B) has a solubility of less than 5 g/kg, at 20° C. in deionized water having a specific conductance of less than 1 μScm.sup.−1.

3. The method according to claim 1, wherein the at least one polycyclic hydrocarbon (B) comprises at least two of the anellated benzene rings having the at least two hydroxyl functional groups on the ring in the ortho position relative to one another; and the benzene rings being bridged in each case by being anellated on an acyclic hydrocarbon system.

4. The method according to claim 3, wherein the acyclic hydrocarbon system comprises at least one oxo group or hydroxyl group.

5. The method according to claim 1, wherein the at least one polycyclic hydrocarbon (B) is selected from anthraquinones which are substituted on the ring by at least two hydroxyl functional groups in the ortho position relative to one another.

6. The method according to claim 1, wherein the source of the at least one water-soluble compound (A) of the element zirconium is not a source of fluoride ions.

7. The method according to claim 1, wherein the at least one water-soluble compound (A) is selected from zirconyl nitrate, zirconium acetate, ammonium zirconium carbonate and combinations thereof.

8. The method according to claim 1, wherein the aqueous anti-corrosion agent comprises an aqueous phase having a total fluoride content of less than 50 g/kg.

9. The method according to claim 1, wherein the anti-corrosion agent has a pH of less than 2.0, but greater than 0.5.

10. The method according to claim 1, wherein the aqueous anti-corrosion agent further comprises organic compounds (C), different from (B), having a molar mass of greater than 5,000 g/mol, and present in an amount of at least 0.1 wt. % based in each case on the aqueous anti-corrosion agent.

11. The method according to claim 10, wherein the organic compounds (C) comprise at least in part functional groups selected from hydroxyl groups, carboxyl groups, phosphate groups, phosphonate groups and amino groups; and an acid number and a hydroxyl number which are in sum at least 100 milligrams KOH per gram, but no greater than 600 milligrams KOH per gram, of the organic compounds (C).

12. The method according to claim 11, wherein the organic compounds (C) are selected from at least one copolymer or copolymer mixture of alkenes and vinyl alcohol, having a hydroxyl number in a range of from 200 to 500 milligrams KOH per gram of copolymer or copolymer mixture.

13. The method according to claim 12, wherein the at least one copolymer or copolymer mixtures is present in an amount not exceeding 5 wt. %, based in each case on the aqueous anti-corrosion agent.

14. The method according to claim 1, wherein the metal substrates are selected from zinc and/or aluminum.

15. The method according to claim 1 wherein after contacting the metal substrate with the aqueous anti-corrosion agent, a wet film remains on the at least one surface of the metal substrate and the contacting step 1) is followed by a drying step 2), before a subsequent rinsing step or subsequent wet-chemical treatment.

16. The method according to claim 15, wherein the wet film has a film thickness which produces, after drying, a coating layer of greater than 5 mg/m.sup.2, but less than 150 mg/m.sup.2, measured as zirconium.

17. A method for producing coated can lids from strip aluminum, comprising steps of: a) in a first step, applying to strip aluminum a coating layer of greater than 5 mg/m.sup.2, measured as zirconium, according to the method of claim 1; b) optionally, applying and curing a primer; c) after step a) punching a lid material out of the strip aluminum; and d) shaping the lid material into can lids.

18. A method for anti-corrosion pretreatment of a metal substrate, comprising steps of: 1) contacting at least one surface of a metal substrate with an aqueous anti-corrosion agent comprising: (A) at least one water-soluble compound of the element zirconium; and (B) at least one polycyclic hydrocarbon compound which comprises at least one anellated benzene ring, wherein the at least one anellated benzene ring has at least two hydroxyl functional groups on the ring in an ortho position relative to one another; wherein the aqueous anti-corrosion agent comprises a homogenous aqueous phase having a molar ratio of the element zirconium to total fluoride that is greater than 1.

19. A concentrate of an anti-corrosion agent comprising: at least 1 wt. % of a water-soluble compound of the element zirconium, based on the element zirconium; and at least 0.01 wt. % of at least one polycyclic hydrocarbon compound which comprises at least one anellated benzene ring, wherein the at least one anellated benzene ring has at least two hydroxyl functional groups on the ring in an ortho position relative to one another, wherein the at least one anellated benzene rings are bridged in each case by being anellated on an acyclic hydrocarbon system; wherein the aqueous anti-corrosion agent has a weight ratio of the water-soluble compounds (A) of the element zirconium, based on the element zirconium, to the polycyclic hydrocarbons (B) that is less than 0.2:1; and wherein the concentrate has a pH in a range of from 0.5 to 2.0.

Description

PRACTICAL EXAMPLES

(1) The effectiveness of the pretreatment according to the invention for forming a potential paint base can be verified after applying in drops a small amount (approx. 1 ml) of an acidic aqueous pretreatment solution (pH 1.5) according to the invention which contains 15 g/kg Zr in the form of zirconyl nitrate and 500 mg/kg of alizarin to an aluminum sheet (AI 3008; 0.2 mm thickness) and subsequently drying said solution out at 30° C., by comparison with a treatment involving a solution that does not contain alizarin. While the treatment according to the invention provides an iridescent coating that cannot be wiped off, the whiter coating based merely on the zirconyl nitrate-containing solution can easily be removed with a cloth.

(2) In order to demonstrate the suitability of the pretreatment according to the invention for providing an effective paint base, various coating systems for can lids were applied and the paint adhesion, in particular lifting of paint (“feathering”), and discoloration (“blushing”) after precipitation under sterilization conditions with which a coating of a material which is intended to be suitable for storing food and drink and is therefore in direct contact with food and drink must usually comply were evaluated.

(3) Table 1 lists the various pretreatments and primer coatings which were tested in this regard. The pretreatment was carried out on thin aluminum sheets (AI 3006) that had been cleaned with an alkaline (Bonderite® C-AK 1803 from Henkel AG & Co. KGaA, 15 g/L, 60° C., 10 s), had been rinsed with deionized water (κ<1 μScm.sup.−1) and had a thickness of 0.2 mm and, for this purpose, a wet film of the pretreatment solution of approx. 4-6 ml/m.sup.2 was applied and dried at 80° C. such that the coating layer of zirconium was in each case 12 mg/m.sup.2. The organic primer was applied by knife coating immediately after the drying step and dried out and cured at 249° C. PMT (Peak Metal Temperature), a dry film layer of primer of approx. 12 g/m.sup.2 being set.

(4) The aluminum sheets coated in this way were precipitated under sterilization conditions at 121° C. using tap water or in tap water containing 2 wt. % of citric acid for 30 minutes in each case in the autoclave. Paint adhesion at the cross-cutting according to DIN EN ISO 2409 and the “blushing”, i.e. the presence of whitish discoloration, were then evaluated. The results are summarized in Table 2.

(5) TABLE-US-00001 TABLE 1 Test no. Pretreatment Primer E1 zirconyl nitrate 6 g/kg epoxide alizarin 0.08 g/kg dispersion.sup.3 ethylene-vinyl 4.8 g/kg alcohol copolymer.sup.1 E2 zirconyl nitrate 6 g/kg acrylate alizarin 0.08 g/kg dispersion.sup.4 ethylene-vinyl 4.8 g/kg alcohol copolymer.sup.1 CE1 base: Bonderite ® epoxide MNT-802 N.sup.2 dispersion.sup.3 CE2 base: Bonderite ® acrylate MNT-802 N.sup.2 dispersion.sup.4 .sup.1degree of hydrolysis 93 mol. % .sup.210 wt. % of aqueous solution of the commercial product (Henkel AG & Co. KGaA) containing H.sub.2ZrF.sub.6 and polyacrylic acid in a weight ratio of 1.23:1 .sup.32489-814 (PPG) .sup.42466-810 (PPG)

(6) It is evident that the pretreatment according to the invention provides excellent paint adhesion values in particular for the coating based on the acrylate-based primer, by comparison with a conventional pretreatment based on fluorozirconate, whereas at least equally good results are obtained both in terms of paint adhesion and blushing for a coating based on the epoxide-based primer.

(7) TABLE-US-00002 TABLE 2 Test Cross cutting.sup.1 Blush.sup.2 no. Tap water Citric acid Tap water Citric acid E1 0 0 1 2 E2 0 2 0 2 CE1 0 0 1 2 CE2 0 5 3 4 .sup.1according to DIN EN ISO 2409 (0-5) .sup.20: no discoloration 1: less than 10% of the surface area is discolored; individual points 2: less than 20% of the surface area is discolored; individual points 3: at least 20% of the surface area is discolored; in strips 4: at least 40% of the surface area is discolored 5: at least 60% of the surface area is discolored