Two-stage pretreatment of aluminum, in particular aluminum casting alloys, comprising pickle and conversion treatment

Abstract

The present invention relates to a method for the anti-corrosion treatment of components produced from aluminum, in particular casting parts such as vehicle rims, comprising a pretreatment stage and subsequent coating, wherein the pretreatment stage in turn includes a pickle on the basis of sulfuric acid aqueous solutions containing water-soluble compounds of the element Ti and at least one α-hydroxycarboxylic acid which is carried out upstream of an acidic conversion treatment on the basis of an acidic aqueous solution containing water-soluble compounds of the elements Zr and/or Ti.

Claims

1. A method for an anti-corrosion treatment of components produced from aluminum, comprising a pre-treatment stage and subsequent coating, wherein the pre-treatment stage comprises steps of: a) contacting an aluminum or aluminum alloy component with an aqueous phosphate-free sulfuric acid pickling solution which has a pH of from 1 to 2.5, a free acid content of at least 5 points and contains at least one water-soluble compound of the element Ti, a source of fluoride ions and at least one polybasic α-hydroxycarboxylic acid present in the pickling solution in an amount of at least 0.1 g/kg but does not exceed 4 g/kg; and subsequently b) contacting the aluminum or aluminum alloy component pickled in a) with an aqueous conversion treatment solution which has a pH of from 1 to 3.5 and contains at least one water-soluble compound of the elements Zr and/or Ti.

2. The method according to claim 1, wherein the at least one polybasic α-hydroxycarboxylic acid in the pickling solution is selected from citric acid and/or tartaric acid.

3. The method according to claim 2, wherein the at least one polybasic α-hydroxycarboxylic acid in the pickling solution is tartaric acid.

4. The method according to claim 1, wherein the water-soluble compounds of the element Ti are present in the pickling solution in an amount of at least 0.04 g/kg, but not exceeding 0.6 g/kg, in each case based on the element Ti.

5. The method according to claim 4, wherein the at least one water-soluble compound of the element Ti is present in the pickling solution in an amount of no more than 0.3 g/kg based on the amount of Ti.

6. The method according to claim 5, wherein the at least one water-soluble compound of the element Ti is present in the pickling solution in an amount of at least 0.1 g/kg based on the element Ti.

7. The method according to claim 6, wherein the ratio of Zr and Ti to total fluoride content in the conversion treatment solution is at least 0.4.

8. The method according to claim 1, wherein the pickling solution has a pH of less than 2.

9. The method according to claim 1, wherein the pickling solution has a free acid content of at least 6 points, but no more than 10.

10. The method according to claim 1, wherein the pickling solution has a total acid content of at least 12 points, but no more than 18 points.

11. The method according to claim 1, wherein the at least one water-soluble compound of the elements Zr and/or Ti in the conversion treatment solution is present in an amount of at least 0.1 mmol/kg, and does not exceed 5 mmol/kg, in each case calculated as corresponding amounts of the elements Zr and/or Ti.

12. The method according to claim 1, wherein the water-soluble compounds of the elements Zr and/or Ti are selected from water-soluble compounds of the element Zr, which are in turn selected from hexafluorozirconic acid and the salts thereof.

13. The method according to claim 1, wherein the aluminum or aluminum alloy component is brought into contact with the pickling solution for long enough to pickle at least 2 mg of aluminum per square meter from contacted surfaces of the component and to optionally produce a layer of at least 4 mg titanium per square meter on the contacted surfaces of the component.

14. The method according to claim 1, wherein before contacting with the pickling solution, the component undergoes alkaline degreasing, by contacting the component with an alkaline aqueous composition which has a pH of greater than 9 but less than 12 and a free alkalinity of at least 3 points but less than 6 points and which optionally contains surface-active compounds which are selected from non-ionic surfactants.

15. The method according to claim 14, wherein after the alkaline degreasing and before contact with the pickling solution, the aluminum or aluminum alloy component is subjected to a rinsing step, but not a drying step.

16. The method according to claim 1, wherein after contact with the pickling solution and before contact with the conversion treatment solution, the aluminum or aluminum alloy component is subjected to a rinsing step, but not a drying step.

17. The method according to claim 1, wherein after contact with the conversion treatment solution and before the subsequent coating, a rinsing step takes place, and immediately before the subsequent coating, a drying step also takes place.

18. The method according to claim 1, wherein the component is coated with a powder coating during the subsequent coating.

19. The method according to claim 1, wherein a ratio of Zr and Ti to total fluoride content in the conversion treatment solution is at least 0.1.

20. The method according to claim 1, wherein the at least at least one polybasic α-hydroxycarboxylic acid is present in the pickling solution in an amount of at least 0.5 g/kg but does not exceed 2 g/kg.

Description

PRACTICAL EXAMPLES

(1) The anti-corrosion effect of a method sequence according to the invention is described below with reference to the pretreatment of aluminum sheets (EN AW-6014) and, in addition, the tendency toward sludge formation in sulfuric acid pickling solutions of the pretreatment stage containing fluoro complexes of the element titanium and 1 g/kg of dissolved aluminum is investigated.

(2) The method sequence for the treatment of aluminum sheets (EN AW-6014) involved the successive method steps I to IV, where each of the method steps I-III was followed by a rinsing step with deionized water (κ<1 μScm.sup.−1) and, after the rinsing step following method step III, the sheets were dried in an air stream before the coating was carried out in step IV:

(3) I. Cleaning:

(4) 30 g/L BONDERITE C-AK G 414 (Henkel AG & Co. KGaA) in city water Brought into contact by spraying for 160 seconds at 60° C. and 1 bar
II. Pickle: Sulfuric acid aqueous solution having a pH of 1.6, containing 4.4 g/kg Al.sub.2(SO.sub.4).sub.3.14H.sub.2O and in addition a. 0.5 g/kg H.sub.2TF.sub.6 b. 0.5 g/kg H.sub.2TF.sub.6/1.3 g/kg citric acid Brought into contact by spraying for 160 seconds at 50° C. and 1 bar
III. Conversion Treatment 30 g/L BONDERITE M-NT 4595 R5 MU (Henkel AG & Co. KGaA) in deionized water (κ<1 μScm.sup.−1) resulting in a treatment solution containing 100 mg/kg H.sub.2ZrF.sub.6, a pH of 2.8 having been set by means of NH.sub.4CO.sub.3 solution.
IV. Coating Powder coating Freiotherm P01857B plus clear coating Freiotherm KO1853KRA999 (both Emil Frei GmbH & Co. KG): Application amount of the powder coating was approximately 90 g/m.sup.2 and after baking for 10 minutes at 180° C. resulted in a dry film thickness of approximately 60 μm. Application amount of the clear coating was approximately 50 g/m.sup.2 and baking for 10 minutes at 150° C. resulted in a dry film thickness of approximately 20 μm.

(5) TABLE-US-00001 TABLE 1 Corrosion values on aluminum sheets (EN AW-6014) after coating layer build-up CASS test.sup.1 Method Disbonding at Delamination at No. sequence the scratch.sup.2/mm the scratch.sup.2/mm Sludge.sup.3 1 I-IIa-III-IV 0.2 0.2 Yes 2 I-IIb-III-IV 0.1 0.1 No .sup.1determined after 240 hours in the CASS test according to DIN EN ISO 9227 .sup.2U/2 value .sup.3visible sediment formation in a 3 L beaker having an outer diameter of 150 mm containing 2 liters of the pickling solution 24 hours after pickling solution has been applied (yes/no)

(6) The results of Table 1 demonstrate, with reference to the overall low disbonding values, the advantage of a procedure in which the pickling solution contains fluoro complexes of the element Ti.

(7) In addition to the very good anti-corrosion results in the CASS test, in particular in the presence of citric acid, sludge formation is effectively prevented in such pickling solutions by the addition of citric acid.

(8) TABLE-US-00002 TABLE 2 Sludge formation in the pickling solution in the presence of different inhibitors Inhibitor Sludge formation .sup.1 Polyacrylic acid Yes Xanthan gum Yes Maleic acid Yes Malonic acid Yes Succinic acid Yes Mandelic acid Yes Lactic acid Yes Citric acid No Tartaric acid No Gluconic acid Yes Methanesulfonic Yes Sulfamic acid Yes .sup.1 Formation of a sediment in a 3 L glass beaker having an outer diameter of 150 mm containing 2 liters of the pickling solution visible after 24 hours (yes/no)

(9) The sludge-inhibiting effect of the polybasic α-hydroxycarboxylic acids is illustrated in Table 2, which assesses the sludge formation in a pickling solution according to the above method step II, but to which solution a total of 1 g/kg of aluminum ions in the form of aluminum sulphate is added. It is clear from this table that inhibition of sludge formation, i.e. of the precipitation of aluminum and titanium salts, is achieved in the presence of citric acid or tartaric acid.