PRE-TREATING ALUMINUM SURFACES WITH ZIRCONIUM-AND MOLYBDENUM-CONTAINING COMPOSITIONS

Abstract

Provided herein is a process for the pretreatment of workpieces having a surface of aluminum or aluminum alloys for noncutting forming and/or joining by welding or adhesive bonding to similarly pretreated, precoated workpieces, or to precoated parts. The parts are composed of steel, galvanized steel, and/or alloy-galvanized steel. Further provided herein is a subsequent permanent corrosion-protecting treatment by phosphating, by means of a chromium-free conversion treatment, by application of primer, or by surface coating, wherein the workpieces are a) pickled by means of an aqueous, acidic solution comprising mineral acid by dipping or spraying, b) rinsed with water, and c) brought into contact with an aqueous, acidic, chromium-free solution comprising Zr as complex fluoride and Mo as molybdate by application by dipping or spraying. After subsequent drying, a layer weight from 2 to 15 mg/m.sup.2 of Zr and Mo is formed.

Claims

1. A process for the pretreatment of workpieces having a surface of aluminum or aluminum alloys for noncutting forming and/or joining by welding or adhesive bonding to pretreated workpieces, precoated workpieces, or precoated parts composed of steel and/or galvanized steel and/or alloy-galvanized steel, and also for a subsequent permanent corrosion-protecting treatment by phosphating, by means of a chromium-free conversion treatment, by application of primer or by surface coating, wherein the process comprises: a) pickling the workpieces using an aqueous, acidic solution comprising mineral acid by one of dipping and spraying; b) rinsing the workpieces with water; c) applying an aqueous, acidic, chromium-free solution to the workpieces by one of dipping and spraying, the solution comprising: Zr as complex fluoride, and Mo as molybdate, wherein a weight ratio, calculated as Zr/Mo metal, of Zr:Mo is from 15:1 to 3.5:1, and wherein the solution includes from 100 to 800 mg/l of Zr and from 30 to 100 mg/l of Mo, calculated as Zr/Mo metal, and has a pH from 2.5 to 4.5; and d) drying the workpieces, wherein, after the drying, a layer weight is provided on each workpiece of from 2 to 15 mg/m.sup.2 of Zr and Mo.

2. The process according to claim 1, wherein the aqueous, acidic, chromium-free solution in step c) comprises Zr as complex fluoride and Mo as molybdate in the weight ratio, calculated as Zr/Mo metal, of Zr:Mo of from 13:1 to 7:1.

3. A process for the pretreatment of workpieces having a surface of aluminum or aluminum alloys for noncutting forming and/or joining by welding or adhesive bonding to pretreated workpieces, precoated workpieces, or precoated parts composed of steel, galvanized steel, and/or alloy-galvanized steel, and also for a subsequent permanent corrosion-protecting treatment by phosphating, by means of a chromium-free conversion treatment, by application of primer or by surface coating, wherein the process comprises: a) pickling the workpieces using an aqueous, acidic solution comprising mineral acid by dipping or spraying; b) rinsing the workpieces with water; c) applying an aqueous, acidic, chromium-free solution to the workpieces dipping or spraying, the solution comprising: Zr as complex fluoride, and Mo as molybdate, wherein a weight ratio, calculated as Zr/Mo metal, of Zr:Mo is from 2:1 to 1:2, and wherein the solution includes from 0.4 to 7.5 g/l of Zr and from 0.4 to 7.5 g/l of Mo, calculated as Zr/Mo metal, and has a pH from 1.0 to 3.0; and d) drying the workpieces, wherein, after the drying, a layer weight is provided on each workpiece of from 2 to 15 mg/m.sup.2 of Zr and Mo.

4. The process according to claim 3, wherein the aqueous, acidic, chromium-free solution in step c) comprises Zr as complex fluoride and Mo as molybdate in the weight ratio, calculated as Zr/Mo metal of Zr:Mo of from 1.4:1 to 1:1.4.

5. The process according to claim 1, wherein the aqueous, acidic, chromium-free solution in step c) further comprises at least one polymer selected from the group consisting of poly(meth)acrylic acid, (meth)acrylic acid copolymers, polyvinylphosphonic acid, vinylphosphonic acid copolymers, and maleic acid copolymers.

6. The process according to claim 5, wherein the at least one polymer is polyacrylic acid and/or an acrylic acid-maleic acid copolymer.

7. The process according to claim 1, wherein the workpieces are pickled using a solution comprising surfactant, hydrofluoric acid, and sulfuric acid.

8. The process according to claim 7, wherein the workpieces are pickled using the solution further comprising from 3 to 8 g/l of the sulfuric acid, from 50 to 150 mg/l of free fluoride, and from 1 to 3 g/l of a nonionic surfactant.

9. The process according to claim 1, further comprising pretreating the workpieces for noncutting forming, the pretreating comprising applying a lubricant before the noncutting forming.

10. The process according to claim 1, wherein said cleaning and said rinsing are carried out before said corrosion-protecting.

11. An aqueous acidic solution for the pretreatment of workpieces having a surface of aluminum or aluminum alloys, wherein the solution is chromium-free and comprises Zr as complex fluoride and Mo as molybdate in a weight ratio calculated as Zr/Mo metal of Zr:Mo of from 15:1 to 3.5:1, and wherein the solution further comprises from 100 to 800 mg/l of Zr and from 30 to 100 mg/l of Mo calculated as Zr/Mo metal and has a pH of from 2.5 to 4.5.

12. A concentrate from which a solution according to claim 11 is obtainable by dilution with water or an aqueous solution.

13. An aqueous acidic solution for the pretreatment of workpieces having a surface of aluminum or aluminum alloys, wherein the solution is chromium-free and comprises Zr as complex fluoride and Mo as molybdate in a weight ratio calculated as Zr/Mo metal of Zr:Mo of from 2:1 to 1:2, and wherein the solution further comprises from 0.4 to 7.5 g/l of Zr and from 0.4 to 7.5 g/l of Mo calculated as Zr/Mo metal and has a pH of from 1.0 to 3.0.

14. A concentrate from which a solution according to claim 13 is obtainable by dilution with water or an aqueous solution.

15. A workpiece comprising a surface of aluminum or aluminum alloys, wherein the workpiece has been pretreated by means of a process according to claim 1.

16. The process according to claim 10, further comprising applying an activation treatment prior to said corrosion-protecting.

17. The concentrate according to claim 12, wherein the solution is further obtainable through the adjustment of pH.

18. The concentrate according to claim 14, wherein the solution is further obtainable through the adjustment of pH.

Description

EXAMPLES

[0064] With the exception of a metal sheet which was cleaned under alkaline conditions (comparative example CE2 in Tab. 1), sheets composed of aluminum alloys of the grades AA 6111 and AA 5754 were firstly subjected to degreasing pickling at a temperature of 50? C. by dipping or spraying. The pickling solution comprised 6 g/l of sulfuric acid (100% strength), 100 mg/l of hydrofluoric acid (100% strength) and 2 g/l of nonionic surfactant consisting of an ethoxylated fatty alcohol and ethoxylated abietic acid in a weight ratio of 1:1.

[0065] The pickling process was carried out in such a way that the removal of material during pickling was from 0.05 to 0.2 g/m.sup.2 in the case of the alloy AA 5754 and was from 0.05 to 0.4 g/m.sup.2 in the case of the alloy AA 6111. Treatment times in the range from 5 to 20 seconds were required for this purpose.

[0066] The workpieces were subsequently rinsed thoroughly with water, in the last stage with deionized water. The volume resistances measured on the single sheet were about 60 pohm in the case of the alloy AA 5754, and about 13 pohm in the case of the alloy AA 6111.

[0067] This was followed by a treatment with solutions of hexafluorozirconic acid and/or of molybdate which in each case optionally comprised a polymer or copolymer and whose data in respect of the concentration of zirconium and/or molybdenum and also of (co)polymer, the pH and the application temperature are shown in Tab. 1, for 6 seconds by spray application. If necessary, the pH was adjusted using ammonia solution. Excess treatment solution was removed by means of squeegee rollers and the treated surface was subsequently dried.

[0068] The polymers A to D in Tab. 1 were the following:

A: Polyacrylic acid, MW=about 60 000 g/mol in colloidal solution,
B: Acrylic acid-maleic acid copolymer, MW=about 70 000 g/mol,
C: Polyacrylic acid, MW=about 250 000 g/mol,
D: Vinylphosphonic acid-acrylic acid copolymer, MW=4000 to 70 000 g/mol.

TABLE-US-00001 TABLE 1 Concentration in mg/l (Comp.) ZrF.sub.6.sup.2? + Temp. Ex. Zr Mo ZrF.sub.6.sup.2? MoO.sub.4.sup.2? MoO.sub.4.sup.2? Polymer pH in ? C. CE1 500 50 1125 83 1208 A: 250 3.6 50 E1 500 50 1125 83 1208 A: 250 3.6 30 E2 500 50 1125 83 1208 B: 500 3.6 30 E3 500 50 1125 83 1208 C: 175 3.6 23 E4 500 50 1125 83 1208 D: 250 3.6 23 E5 300 80 675 132.8 807.8 C: 175 3.9 23 E6 600 40 1350 66.4 1416.4 C: 350 3.6 23 CE2 500 50 1125 83 1208 C: 175 3.6 23 E7 500 50 1125 83 1208 0 3.6 23 CE3 500 50 1125 83 1208 C: 175 2.1 23 CE4 500 25 1128 41.5 1166.5 C: 175 3.6 23 CE5 100 50 225 83 308 C: 175 3.6 23

TABLE-US-00002 TABLE 2 Layer weight (mg/m.sup.2) Volume resistance (?ohm) (Comp.) AA 6111 AA 5754 AA 6111 AA 5754 Ex. Zr Mo Zr Mo 0 d 30 d 0 d 30 d CE1 26 4 37 7 25 38 62 100 E1 9 5 14 6 8 10 9 11 E2 2 6 5 7 9 11 13 15 E3 4 8 8 7 12 14 13 15 E4 3 5 10 6 7 8 10 12 E5 3 5 8 8 10 14 4 16 E6 2 1 4 2 13 14 5 7 CE2 7 12 8 3 26 25 18 33 E7 7 4 9 6 15 15 6 15 CE3 18 8 24 12 67 73 40 79 CE4 6 4 8 6 26 44 9 21 CE5 4 3 8 8 19 22 9 18

TABLE-US-00003 TABLE 3 Phosphatability Modified APGE test (Comp.) (+/?/?) (cycles survived) Ex. AA 6111 AA 5754 AA 6111 AA 5754 CE1 n.m. n.m. 10-30 10-30 E1 + + ?45 ?45 E2 + + ?45 ?45 E3 + + ?45 ?45 E4 + + ?45 ?45 E5 + + ?45 ?45 E6 + + ?45 ?45 CE2 + n.m. n.m. E7 + ? ?45 ?45 CE3 n.m. n.m. CE4 ? n.m. n.m. CE5 ? n.m. n.m. n.m. = not measured

[0069] Column 2 in Tab. 2 shows the layer weights of Zr and of Mo achieved in this way in mg/m.sup.2. The measurement of the individual layer weights was carried out by means of X-ray fluorescence (XRF) analysis.

[0070] In addition, the volume resistances in pohm obtained in the individual measurements of the metal sheets are shown in column 3 of Tab. 2. Measurement of the volume resistances was carried out immediately after drying/evaporation (in each case first row 0 d) and after storage for 30 days (in each case second row 30 d). It was carried out in accordance with leaflet 2929 (of September 2001) of the Deutscher Verbandes f?r Schwei?en and verwandte Verfahren e.V. (DSV) using copper electrodes having a diameter of 20 mm.

[0071] The corresponding phosphatabilities and the results of the modified APGE test described below in cycles survived are given in columns 2 and 3 of Tab. 3.

[0072] The adhesion was determined by means of a modified APGE (Arizona Proving Ground Equivalent) test. For this purpose, two test plates (each 56.25?25?0.25 mm) in each case were coated with an industrial dry lubricant and adhesively bonded by means of a suitable industrial adhesive. Six such pairs of test plates were then screwed together at their respective ends to form a chain which was subjected to a tensile stress of 2400 N. For each week of the test, the following program of conditions was used: [0073] 1. Dipping for 15 minutes into a 5% strength by weight solution of NaCl in distilled water, [0074] 2. Drying for 105 minutes in a dry environment, [0075] 3. 22 hours at controlled temperature and atmospheric humidity: 50? C. and 90% relative atmospheric humidity, [0076] 4. 4 repetitions of steps 1 to 3, and [0077] 5. 48 hours at 50? C. and 90% relative atmospheric humidity.

[0078] One sequence of the steps 1 to 3 represents, by definition, one cycle. A cycle is in each case considered to have been survived when the adhesive bonding between all test plates of the chain stands fast. The test overall is counted as having been passed when at least 45 cycles have been survived.

[0079] The phosphatability was determined with the aid of scanning electron micrographs. Here, + in Table 3 means a closed, finely crystalline phosphate layer, o means a closed, coarsened phosphate layer (crystal having an edge length of >20 ?m) and ? means a phosphate layer which is not closed through to not present.

[0080] The measured values in Tab. 2 and 3 allow the following to be concluded: the treatment of the aluminum sheets with an alkaline cleaner carried out to determine the necessity of an acidic pickling treatment in step a) in comparative example CE2 leads to sheets having poor volume resistances (18 pohm for AA 5754 and 26 pohm for AA 6111).

[0081] It can be seen from comparative example CE1 that an excessively high Zr layer weight of the layer obtained in the treatment according to step c) (37 mg/m.sup.2 in the case of AA 5754 and 26 mg/m.sup.2 in the case of AA 6111) results in very high volume resistances being obtained, especially after storage for 30 days (100 pohm in the case of AA 5754 and 38 pohm in the case of AA 6111).

[0082] In the case of comparative examples CE4 and CE5, the layer weights obtained are in the desired range, but as a result of the zirconium/molybdenum ratio in the treatment solution for process step c) of 20:1 (see CE4) or 2:1 (see CE5) the volume resistances obtained are unacceptable, especially for AA 6111 (26 and 44 pohm in the case of CE4 and 19 and 22 pohm in the case of CE5).

[0083] In the case of comparative example CE3 having a pH which is too low, namely 2.1, undesirably high applied weights of Zr (24 mg/m.sup.2 for AA 5754 and 18 mg/m.sup.2 for AA 6111) and very much too high volume resistances (40 and 79 pohm for AA 5754 and 67 and 73 pohm for AA 6111) are observed.

[0084] In comparison, examples E1 to E7 show that when the conditions essential to the invention in respect of the type of pickling treatment, the Zr/Mo ratio, the layer weight produced, the respective concentration and the pH ranges of the treatment solutions are adhered to, layers having extremely good volume resistances combined with good adhesion properties are obtained.

[0085] In addition, it can be seen from Tab. 3 that all examples E1 to E7 have passed the modified APGE test, i.e. have in each case survived at least 45 cycles, both in the case of AA 5754 and also in the case of AA 6111, while this clearly does not apply in the case of comparative example CE1 with in each case only from 10 to 30 cycles survived. The other comparative examples CE2 to CE5 were no longer subjected to the modified APGE test because of the poor results already obtained for the layer weight or volume resistance (Tab. 2, see above).

[0086] With regard to the phosphatability (see Tab. 3), all examples E1 to E7 (with the exception of E7 for AA 5754) always display a closed, finely crystalline phosphate layer (+), while comparative examples CE2 to CE5 in the case of AA 5754 merely have a phosphate layer which is not closed through to not present (?) and (with the exception of CE2) also show significantly poorer results in the case of AA 6611.

[0087] The test plates of examples E6 and E7 which have been pretreated according to the invention and also an unpretreated bare test plate CE6 were also subjected to a multistage anticorrosion treatment consisting of the following steps:

i) Alkaline cleaning (60? C.; 180 s)
ii) Rinsing (mains water; RT, 60 s)
iii) Activation (titanium phosphate; RT, 30 s)
iv) Phosphating (trications; 53? C., 180 s)
v) Rinsing (mains water; RT, 30 s)
vi) Passivation (zirconium fluoride; RT, 45 s)
vii) After-rinsing (demineralized water; RT, 30 s)
viii) Drying (convection oven; 100? C.; 7 min)

[0088] After subsequent electrophoretic coating and topcoating, a filiform corrosion test in accordance with DIN EN 3665 (average values in accordance with DIN EN ISO 4628-8) and a cyclic corrosion test in accordance with VDA 621-415 (average values in accordance with DIN EN ISO 4628-8) were each carried out. The smaller the measured migration under the coating in mm, the better was the anticorrosion. The results are shown in the following table.

TABLE-US-00004 TABLE 4 Filiform VDA (Comp.) (mm) (mm) Ex. AA 6111 AA 5754 AA 6111 AA 5754 CE6 0.8 0.1 0.1 0.1 E6 0.8 0.1 0.1 0.1 E7 1.0 0.1 0.1 0.1

[0089] The anticorrosion in the case of E6 and E7 is in each case comparable to that in the case of CE6. The pretreatment according to the invention thus does not have an adverse effect on the anticorrosion achieved subsequently by means of an anticorrosion treatment.