Method for the anti-corrosion and cleaning pretreatment of metal components

Abstract

The invention relates to a multiple-step method for the corrosion-protective pretreatment of components, said pretreatment being at least partially produced from a metal material predominantly consisting of at least one of the elements iron zinc and/or aluminium, according to which the components are first brought into contact with an acid aqueous composition (A) containing water-soluble compounds of the elements Zr and/or Ti and then with an acid aqueous composition (B) containing phosphate ions and an accelerator. The method is particularly suitable for the pretreatment before an electrocoating.

Claims

1. A method for the anti-corrosion pretreatment of metal surfaces, comprising the following successive method steps: I) contacting metal surfaces of a component, which-comprises a metal material composed of one or more of iron, zinc and/or aluminum, with an acidic aqueous composition (A) containing at least one water-soluble compound of the elements Zr and/or Ti and at least one aliphatic saturated polyhydroxy compound selected from the group consisting of erythritol, threitol, xylitol, arabitol, ribitol, mannitol, sorbitol and combinations thereof; II) contacting the component with an acidic aqueous composition (B) containing phosphate ions and an accelerator, which composition has a total content of less than 100 ppm of dissolved compounds of the element Ni, wherein no phosphate coating is achieved on any of the metal surfaces of the component that results in a layer weight of more than 1 g/m.sup.2, calculated as PO.sub.4.

2. The method according to claim 1, wherein the composition (A) contains at least 0.05 mmol/kg, but no more than 1.5 mmol/kg of water-soluble compounds of the elements Zr and/or Ti, based on these elements.

3. The method according to claim 1, wherein the composition (A) has a pH of less than 5.8, but no less than 3.9.

4. The method according to claim 1, wherein the composition (A) additionally contains a water-soluble source for fluoride ions in such an amount that free fluoride amount is at least 1 mg/kg, but no more than 100 mg/kg.

5. The method according to claim 1, wherein the composition (A) further comprises at least one aliphatic diol, which has at least 4 carbon atoms, but no more than 10 carbon atoms.

6. The method according to claim 1, wherein the composition (B) contains at least 0.5 g/kg, but no more than 10 g/kg of phosphate ions.

7. The method according to claim 1, wherein the accelerator in the composition (B) is selected from at least one water-soluble organic or inorganic compound of having a standard reduction potential greater than +0.2 V (SHE).

8. The method according to claim 1, wherein the accelerator in the composition (B) is selected from organic or inorganic compounds containing at least one non-metal atom selected from the elements nitrogen, phosphorus, oxygen, sulfur, chlorine and/or bromine in an oxidation stage which does not correspond to the lowest possible oxidation stage of the particular element; or at least one oxoanion of an element from subgroup VIB or VIIB of the periodic table.

9. The method according to claim 1, wherein the composition (B) contains a total of at least 0.1 mmol/kg, but no more than 5 mmol/kg, of accelerators.

10. The method according to claim 1, wherein the composition (B) has a pH of less than 6.0, but no less than 4.0.

11. The method according to claim 4, wherein the fluoride ions in the composition (B) are less than 10 mg/kg and total fluoride content is less than 100 mg/kg.

12. The method according to claim 1, wherein composition (B) comprises zinc ions in an amount that is less than 1 g/kg.

13. The method according to claim 1, further comprising coating-the metal surfaces with an organic film former.

14. The method according to claim 1, wherein the component comprises an iron material and is produced in a composite structure with a material which is composed of one or both of the elements zinc and aluminum.

15. The method according to claim 1, wherein the at least one aliphatic saturated polyhydroxy compound is selected from the group consisting of xylitol, arabitol, ribitol, mannitol, sorbitol and combinations thereof.

16. The method according to claim 15, wherein the at least one aliphatic saturated polyhydroxy compound is selected from the group consisting of xylitol, arabitol, ribitol and combinations thereof.

17. The method according to claim 1, wherein the at least one aliphatic saturated polyhydroxy compound is sorbitol.

18. The method of claim 13, wherein the coating step further comprises electrocoating.

Description

PRACTICAL EXAMPLES

(1) To illustrate the teaching according to the invention, cold-rolled steel sheets (ST 1405 Sidca®, ThyssenKrupp Steel AG) were subjected to a series of wet-chemical pretreatment steps and then provided with a cathodic electrocoat (binder GV 85-0030, pigment GV 86-0320; both BASF AG). The cathodic electrocoating was carried out at a deposition voltage of 220 V for 5 minutes. The pre-treatment steps comprised: A1. Alkaline cleaning in spraying at 50° C. for 60 seconds in a degreasing bath composed of equal parts of a 1 wt. % Bonderite® C-AK 6443 and a 0.2 wt. % Bonderite® C-AD 10004 (both Henkel AG & Co. KGaA) A2. Conversion treatment at 23° C. for 60 seconds (A2a) or 30 seconds (A2b) by spraying using an aqueous composition with a pH of 4.8, containing 16.2 g/kg hexafluorozirconic acid 32.0 g/kg magnesium nitrate hexahydrate 21.0 g/kg sorbitol 9.9 g/kg hexylene glycol A3. Phosphating at 50° C. for 60 seconds by spraying using an aqueous composition with a pH of 4.8, containing 3.26 g/kg phosphoric acid 0.26 g/kg m-nitrobenzenesulfonic acid 0.11 g/kg hydroxylamine sulfate 0.58 g/kg alkoxylated turpentine oil 0.20 g/kg ethoxylated-propoxylated C12 fatty alcohol B1. Immersion rinsing with city water at 23° C. for 60 seconds B2. Flushing with deionized water (κ<1 μScm.sup.−1) at 23° C. for 25 seconds

(2) A method sequence according to the invention for pretreatment (A2a-B1-B2-A3) comprising the cleaning conversion treatment A2 followed by the phosphating A3 was compared with a conventional method sequence (A1-B1-B2-A2b) comprising alkaline cleaning and conversion treatment. The conventional pretreatment produces a layer thickness of approximately 20 μpm in the dip coating, whereas with the pretreatment according to the invention, a dip coating layer thickness of 8 μm resulted in homogeneous coverage. Conventional phosphating according to method step A3 without preceding conversion treatment requires a comparatively longer phosphating time to achieve similarly low layer thicknesses and causes significantly higher contamination with phosphate sludges in the technical implementation during series production.