EP1163378B1 - Chemically passivated object made of magnesium or alloys thereof - Google Patents

Abstract

An article made of magnesium or its alloys, some or all of whose surface has a conversion coating, the conversion coating comprising MgO, Mn2O3 and MnO2 plus at least one oxide from the group consisting of vanadium, molybdenum and tungsten; and also a process for producing such an article, and its use.

Description

The present invention relates to an article made of magnesium or its alloys, the conversion layer created by passivation of the surface has, and a method for producing such an object and its Use.

Magnesium and its alloys are the lightest, but also the least noble metallic Construction materials (normal potential of Mg: -2.34 volts) and therefore tend very strongly to corrosion. To counteract this disadvantageous property, magnesium and its alloys are treated in aqueous passivation electrolytes. Due to the redox process taking place (without external power source) a conversion layer, which consists of oxides of magnesium material and oxidic Reaction products consisting of the components of the aqueous passivation electrolyte originate, exists.

The term "conversion layer" here and below means a layer not by applying it to a surface, but by chemical transformation (Conversion) of the metallic surface and various components of the aqueous passivation electrolyte is formed (cf. H. Simon, M. Thoma "Angewandte Surface technology for metallic materials ", Carl Hanser Verlag, Munich (1985) p. 4).

For example, the chromating of objects made of magnesium or its alloys is known. The corresponding procedures are described in particular in MIL specifications M3171 Type I to Type III. Chromic acid or its salts are used for the passivation. The use of sodium dichromate in combination with potassium permanganate has also been described (Dow Chemical Treatment, No. 22). The chemical passivation using chromium (VI) -containing aqueous passivation electrolytes is easy to carry out. However, this has the serious disadvantage that the chromate-containing substances which are also contained in the conversion layers formed are carcinogenic.
In addition, the reusability of chromated articles made of magnesium or its alloys is a considerable problem, since they can only be recycled to so-called "high-purity" materials with considerable effort because of their heavy metal content.

For reasons of environmental protection and occupational safety, it is the aim of Manufacturers and processors of passivated objects made of magnesium or its Alloys, using a substitute for conventional chromating of chromate-free aqueous passivation electrolytes.

As a chromate-free aqueous passivation electrolyte for the passivation of objects made of magnesium or its alloys are, for example, from Dow Chemically marketed aqueous passivation electrolytes based on stannate are known. It However, it has been shown that the corrosion protection effect of the conversion layer obtained in this way is lower compared to the chromated magnesium materials.

US 5 743 971 describes a process for the formation of corrosion protection coatings on metals such as Zn, Ni, Ag, Fe, Cd, Al, Mg and their alloys.
These metals are immersed in a solution that contains an oxidizing agent, a silicate and at least one cation from the group of Ti, Zr, Ce, Sr, V, W and Mo. The pH of this solution is in particular in a range between 1.5 and 3.0.
The oxidizing agent is selected exclusively from the group of peroxo compounds. Potassium permanganate is not mentioned as an oxidizing agent. This document also does not show what actual improvements the process described there brings for magnesium or its alloys compared to conventional chromating.

The phosphating of objects made of magnesium or its alloys is also known (cf. Dow Chemical Treatment No. 18). Phosphating with simultaneous use of potassium permanganate is described in D. Hawk, DL Albright, "A Phosphate-Permanganate Conversion Coating for Magnesium", Metal Finishing, October 1995, pp. 34-38. Here, too, the corrosion protection obtained using these aqueous passivation electrolytes is significantly lower compared to a chromated layer.
Another possibility for chemical passivation is described by the CHIBA Institute of Technology, Japan (published in the conference material INTERFINISHING 96 World Congress, Birmingham, England, September 10-12, 1996, pp. 425-432), according to which a solution of potassium permanganate alone or in combination with small amounts of acids (HNO ₃ , H ₂ SO ₄ , HF) is contained in an aqueous passivation electrolyte. The temperature of the aqueous passivation electrolyte required for chemical passivation is between 40 and 84 ° C.
The conversion layer obtainable in this way shows a good protective effect, but the stability of the aqueous passivation electrolyte is not sufficient for an industrial application of this method. After a short time, brown stone (MnO ₂ ) precipitates, which renders the aqueous passivation electrolyte unusable for the further passivation of magnesium materials.

The object of the invention is to provide a chemically passivated object Made of magnesium or its alloys, the conversion layer of which by an electrolytic, Electroless method is available that is easy to use and is transferable to an industrial scale. The corrosion protection effect of a Such a conversion layer should not be worse than that of the known, chromated objects made of magnesium or its alloys.

This object is achieved according to the invention by an object made of magnesium or its alloys, the surface of which has a conversion layer in whole or in part, characterized in that the conversion layer comprises MgO, Mn ₂ O ₃ and MnO ₂ and at least one oxide from the group of vanadium, molybdenum and Has tungsten.

The conversion layer according to the invention can be obtained by passivating the object by means of an aqueous passivation electrolyte, this aqueous passivation electrolyte containing potassium permanganate and at least one alkali metal or ammonium salt of an anion from the group of vanadate, molybdate and tungstate.
The object on which the invention is based is equally achieved by a method for producing a conversion layer on an object made of magnesium or its alloys, characterized in that the object is subjected to passivation by means of an aqueous passivation electrolyte, the aqueous passivation electrolyte being potassium permanganate and at least one alkali or ammonium salt contains an anion from the group of vanadate, molybdate and tungstate.

The conversion layer according to the invention has a golden brown to gray-brown, iridescent color and contains MgO, Mn ₂ O ₃ , MnO ₂ and at least one oxide from the group of vanadium, molybdenum and tungsten.
Investigations have shown that the corrosion protection effect of this conversion layer is no less than that of a conventional chromate layer.

Especially against the background that the anions used according to the invention have a lower oxidizing power than chromate ions when compared individually with the chromate ions, it becomes clear that a synergistic effect is only achieved by combining the permanganate ions with the corresponding vanadate, molybdate and / or tungsten ions , which leads to the formation of a corrosion-inhibiting conversion layer on objects made of magnesium or its alloys.
This is of particular importance since the aqueous passivation electrolytes of the prior art containing potassium permanganate can only achieve such an oxidizing power of the electrolyte solution by lowering the pH and / or increasing the temperature.

A possible explanation for this synergistic effect can be found in education strong, so-called heteropolyacids in the form of their soluble ammonium or alkali salts lie.

A particular advantage of the process according to the invention is the fact that the aqueous passivation electrolyte is still stable even after a long standing time, without brown stone precipitating in an amount which would render the aqueous passivation electrolyte unusable for the passivation of objects made of magnesium or its alloys.
Therefore, in the present method it is possible in a simple manner to replenish the chemicals used after a long period of use in a simple manner without having to replace the aqueous passivation electrolyte itself.

According to a preferred embodiment of the present invention, a polymer layer is additionally applied to the conversion layer and can be obtained by polymerizing and / or crosslinking a solution which contains at least one alkoxysilane compound.
In this way, the mechanical and chemical properties of the conversion layer (eg corrosion resistance or abrasion resistance) are significantly increased. The conversion layer according to the invention acts as a primer.
Thus, the conversion layer obtainable in accordance with the method according to the invention has pores with a size between 200 and 1,000 nm.
The choice of an alkoxysilane compound as the compound to be polymerized and / or crosslinked ensures that the polymer layer on the conversion layer is connected to the surface of the conversion layer on the one hand as a result of chemisorption via Si-O bonds, and on the other hand also via chemisorption inside the pores. The penetration of the alkoxysilane compound into the pores of the conversion layer increases the contact area and thus the chemisorption between the conversion layer and the polymer layer.

The formation of the polymer layer takes place by means of those known per se and familiar to the person skilled in the art Polymerization processes (e.g. air drying, heating or UV radiation):

The amount of alkoxysilane compound in the solution to be applied can vary widely Limits vary. In general, the solution contains 5 to 45 wt .-%, in particular 10 to 30% by weight of the alkoxysilane compound. Depending on the required viscosity the solution additionally contain a polar solvent, which is to be chosen so that it does not react with the alkoxysilane compound (e.g. ethanol).

• X eine Alkoxy-, eine Aryloxy- oder eine Acyloxygruppe mit 1 bis 12 Kohlenstoffatomen, vorzugsweise mit 1 bis 4 Kohlenstoffatomen darstellt, und insbesondere ausgewählt ist aus der Gruppe der Methoxy-, Ethoxy-, n-Propoxy-, i-Propoxy-, Butoxy-, Phenoxy-, Acetoxy- und Propionyloxygruppen;
• R¹ und R², gleich oder verschieden voneinander, ausgewählt sind aus der Gruppe der
  • Amino-, Monoalkylamino- oder Dialkylaminoreste;
  • Alkylreste, insbesondere der Alkylreste mit 1 bis 6 Kohlenstoffatomen, vorzugsweise der Methyl-, Ethyl-, n-Propyl-, Isopropyl-, n-Butyl-, s-Butyl-, t-Butyl-, Pentyl-, Hexyl- oder Cyclohexylreste;
  • Alkenylreste, insbesondere der Alkenylreste mit 2 bis 6 Kohlenstoffatomen, vorzugsweise der Vinyl-, 1-Propenyl-, 2-Propenyl- oder Butenylreste;
  • Alkinylreste, insbesondere der Alkenylreste mit 2 bis 6 Kohlenstoffatomen, vorzugsweise der Acetylenyl- oder Propargylreste;
  • Arylreste, insbesondere der Arylreste mit 6 bis 10 Kohlenstoffatomen, vorzugsweise Phenyl- oder Naphtenylreste;
  • Epoxyreste, insbesondere der Epoxyreste mit 3 bis 16 Kohlenstoffatomen, vorzugsweise der Glycidyl-, Glycidylether-, Glycidylester- oder Glycidyloxyalkylreste; oder
  • zuvor beschriebenen Gruppe X; und
• a und b, gleich oder verschieden voneinander, den Wert 0, 1, 2 oder 3 darstellen, wobei die Summe von a und b den Wert 3 nicht überschreitet.

According to a preferred embodiment, the alkoxysilane compound corresponds to the general formula R ¹ _a R ² _b SiX _(4-ab) in the

• X represents an alkoxy, an aryloxy or an acyloxy group with 1 to 12 carbon atoms, preferably with 1 to 4 carbon atoms, and is particularly selected from the group of methoxy, ethoxy, n-propoxy, i-propoxy, butoxy -, phenoxy, acetoxy and propionyloxy groups;
• R ¹ and R ² , identical or different from one another, are selected from the group of
  • Amino, monoalkylamino or dialkylamino residues;
  • Alkyl radicals, in particular the alkyl radicals having 1 to 6 carbon atoms, preferably the methyl, ethyl, n-propyl, isopropyl, n-butyl, s-butyl, t-butyl, pentyl, hexyl or cyclohexyl radicals;
  • Alkenyl residues, in particular alkenyl residues with 2 to 6 carbon atoms, preferably vinyl, 1-propenyl, 2-propenyl or butenyl residues;
  • Alkynyl radicals, in particular the alkenyl radicals having 2 to 6 carbon atoms, preferably the acetylenyl or propargyl radicals;
  • Aryl radicals, in particular aryl radicals having 6 to 10 carbon atoms, preferably phenyl or naphtenyl radicals;
  • Epoxy radicals, in particular the epoxy radicals having 3 to 16 carbon atoms, preferably the glycidyl, glycidyl ether, glycidyl ester or glycidyloxyalkyl radicals; or
  • previously described group X; and
• a and b, identical or different from one another, represent the value 0, 1, 2 or 3, the sum of a and b not exceeding 3.

A corresponding alkoxysilane compound can be a tetraalkoxysilane, epoxyalkoxysilane or aminoalkoxysilane.
Very good results were obtained with tetraethoxysilane, 3-glycidyloxypropyl-trimethoxysilane, 3-aminopropyl-trimethoxysilane and 3- (aminoethylamine) propyl-trimethoxysilane as the alkoxysilane compound.

In order to further improve the adhesion between the conversion and polymer layers, it is advisable to add an additional compound capable of forming a titanium complex to the solution to be applied to the conversion layer. The term “compound capable of forming a titanium complex” denotes compounds which form TiO ₂ -SiO ₂ systems which are bridged with the alkoxysilane compound and the conversion layer via complex bonding. The reaction between the alkoxysilane compound and the titanium compound also results in a crosslinked polymer layer.
A particularly suitable compound is an alkoxytitanium compound, a titanium acid ester or a titanium chelate, in particular a compound of the formula Ti (OR) ₄ , in which R represents an alkyl radical having 1 to 6 carbon atoms, which is preferably selected from the group of methyl, ethyl , n-propyl, i-propyl and butyl radicals.
Very good results were achieved with tetraethoxytitanate Ti (OC ₂ H ₅ ) ₄ .
The molar ratio between alkoxysilane compound and titanium compound is not critical and is generally between 1 and 20.

Solutions containing both an alkoxysilane compound and one to form a Titanium complex capable compound are included, for example, in the DE 41 38 218 A1 and can be obtained from various companies (e.g. Deltacoll® 80 from Dörken).

If necessary, the polymer layer can also have a color. In this case, the solution to be polymerized and / or crosslinked additionally contains at least one dye which is soluble in a polar solvent, in particular a melall complex dye. Such a metal complex dye is available, for example, under the trade name Neozapon® from BASF, Orasol® from Ciba-Geigy, Savinyl® from Sandoz or Lampronol® from ICI.
Due to the solubility of the dye in a polar solvent, a homogeneous solution and thus a homogeneous structure of the polymer layer is achieved. There is therefore no accumulation of the dye in the polymer layer, which could act as a "predetermined breaking point" between the conversion and polymer layers.

In the process according to the invention for producing a conversion layer, the passivation is preferably carried out in a pH range of the aqueous passivation electrolyte from 7.0 to 8.0.
It is therefore not necessary to add acids. This means that it is not necessary to reduce the pH by adding acids in order to increase the oxidizing power of the permanganate anions.

Furthermore, with the method according to the invention it is possible for the first time to carry out an adequate passivation at a temperature of the aqueous passivation electrolyte of 15 to 50 ° C., in particular 20 to 30 ° C.
The passivation is usually carried out for a period of 2 to 10 minutes.

The concentration of potassium permanganate in the aqueous passivation electrolyte according to the invention is preferably 1 to 10 g / l; that of the alkali or ammonium salt the vanadate, molybdate and / or tungsten ions preferably 1 to 10 g / l. In particular is the upper limit of the vanadate, molybdate and / or tungstate concentration not critical. The method according to the invention is also the same with an electrolyte feasible, which is a saturated solution of these salts, even with undissolved components, contains.

The synergistic effect between permanganate ions and vanadate, molybdate and / or Tungsten is particularly evident when trying to find an object made of magnesium only with an aqueous potassium permanganate solution with a Passivating a concentration of 1 to 10 g / l with the same working parameters. Because under these conditions it is not possible to use a conversion layer with sufficient Preserve corrosion protection.

The objects passivated according to the invention are, for example, parts for the motor vehicle industry, electrical and electronics industry, mechanical engineering industry, aerospace technology and parts of sports equipment.
In particular, parts of engines and gearboxes, instrument panels, doors and individual parts thereof, steering gearboxes, wheel stars for motorcycles, throttle valve housings, mounting devices for milling cutters, rotors or displacement housings for compressors, sealing jaws for packaging machines, parts for plug strips and electrical connectors, lamp holders, lamp housings, Rotor housings for helicopters, housings for electrical devices and parts for sports arches.

Magnesium alloys that can be used are all common die casting, Cast and wrought alloys. Examples of these are in particular AZ91, AZ81, AZ61, AM60, AM50, AM20, AS41, AS21, AE42, QE22, ZE41, ZK61 and AZ31, AZ60, ZK30, ZK60, WE43 and WE54 (designations according to ASTM).

The articles are used as pretreatment for the chemical passivation according to the invention magnesium or its alloys previously in a manner known per se pickled with mineral acids such as phosphoric acid, hydrofluoric acid, nitric acid etc.

Furthermore, it is possible that a varnish or a paint is additionally applied to the conversion layer with or without an additional polymer layer.
All commercially available powder or epoxy-based paints and electro-dip paints are suitable as paints. Powder coatings based on high molecular weight epoxy resins of the bisphenol-A type are preferred, optionally combined with a carboxyl-containing polyester resin, as are available, for example, under the name Delta-S-NT powder coating from Dörken, Herdecke.

The following examples serve to illustrate the invention.

Comparative Example 1

12 plates made of the AZ91HP magnesium alloy with the dimensions 50 x 100 x 2 mm are chromated according to the MIL specification M3171 type I.
Three of the plates passivated in this way are subjected to a salt spray test in accordance with DIN 50021-SS in their original state (without sealing) and with special lacquer coatings.
A silane combination (DELTACOLL 80 from Dörken) and / or an epoxy polyester powder coating (Delta-S-NT powder coating from Dörken) is used as the sealer in accordance with the conditions specified in Table I.
The results of the salt spray tests are given in Table I.

example 1

12 plates made of the magnesium alloy AZ91HP with the dimensions 50 x 100 x 2 mm are pickled in 75% H ₃ PO ₄ for 30 seconds. It is then rinsed with deionized water and the plates are neutralized in 10% NaOH at room temperature for 30 seconds; then the plates are rinsed again with deionized water. The wet plates are immersed for 5 minutes at room temperature in an aqueous passivation electrolyte consisting of an aqueous solution of 3 g / l KMnO ₄ and 1 g / l NH ₄ VO ₃ . After removing the plates from the passivation bath, the gray-brown-looking conversion layer is rinsed with deionized water and then dried at 110 ° C. for 30 minutes.
Three of the plates passivated in this way are subjected to a salt spray test in accordance with DIN 50021-SS in their original state (without sealing) and with special lacquer coatings.
A silane combination (DELTACOLL 80 from Dörken) and / or an epoxy polyester powder coating (Delta-S-NT powder coating from Dörken) is used as the sealer in accordance with the conditions specified in Table I.
The results of the salt spray tests are given in Table I.

Comparative Example 2

6 plates made of the magnesium alloy AM50HP with the dimensions 50 x 100 x 2 mm are chromated according to the MIL specification M3171 type I.
Three of the plates passivated in this way are subjected to a salt spray test according to DIN 50021-SS in the original state (without sealing) and with a silane combination (DELTACOLL 80 from Dörken).
The results of the salt spray tests are given in Table II.

Example 2

6 plates made of the magnesium alloy AM50HP with the dimensions 50 x 100 x 2 mm are pickled in 40% HF for 60 seconds at room temperature. After rinsing with deionized water, the plates are immersed in an aqueous passivation electrolyte consisting of an aqueous solution with 4 g / l KMnO ₄ and 1.5 g / l Na ₂ WO ₄ for 10 minutes at room temperature. After removing the plates, the golden-brown iridescent conversion layer is rinsed with deionized water and dried at 110 ° C. for 60 minutes.
Three of the plates passivated in this way are sealed in their original condition (without sealing) and with a silane combination (DELTACOLL 80 from Dörken). subjected to a salt spray test according to DIN 50021-SS.
The results of the salt spray tests are given in Table II.

Comparative Example 3

6 plates made of the magnesium alloy AZ91HP with the dimensions 50 x 100 x 2 mm are chromated according to the MIL specification M3171 type I.
Three of the plates passivated in this way are sealed with a silane combination (DELTACOLL 80 from Dörken) and with an epoxy polyester powder coating (Delta-S-NT powder coating from Dörken) and then subjected to a salt spray test according to DIN 50021-SS ,
The number of corrosion points as a function of time was determined. The results are shown in Table III.

Example 3

6 plates of AZ91HP with the dimensions 50 x 100 x 2 mm are pickled in 75% H ₃ PO _{4 for} 30 seconds. It is then rinsed with deionized water and the plates are neutralized with a 10% aqueous NaOH for 45 seconds and then rinsed again with deionized water. The plates are then immersed in the wet state for 4 minutes at room temperature in an aqueous passivation electrolyte consisting of an aqueous solution of 3 g / l KMnO ₄ and 1 g / l NaVO ₃ . After removing the plates, the gray-brown-looking conversion layer is rinsed with deionized water and then dried at 110 ° C. for 45 minutes.
Three of the plates passivated in this way are sealed with a silane combination (DELTACOLL 80 from Dörken) and with an epoxy polyester powder coating (Delta-S-NT powder coating from Dörken), and then subjected to a salt spray test according to DIN 50021-SS ,
The number of corrosion points as a function of time was determined. The results are shown in Table III.

Table III clearly shows improved corrosion protection for the invention Conversion layer when using a silane combination.

Claims (18)

1. Object made of magnesium or alloys thereof, the whole or part of the surface of which has a conversion layer, characterised in that the conversion layer contains MgO, Mn₂O₃ and MnO₂ as well as at least one oxide selected from vanadium, molybdenum and tungsten.
2. Object according to claim 1, characterised in that the conversion layer is obtainable by passivating the object by means of an aqueous passivating electrolyte, this aqueous passivating electrolyte containing potassium permanganate and at least one alkali metal salt or ammonium salt of an anion selected from vanadate, molybdate and tungstate.
3. Object according to claim 1 or 2, characterised in that in addition a polymer layer, obtainable by polymerisation and/or cross-linking of a solution which contains at least one alkoxysilane compound, is applied to the conversion layer.
4. Object according to claim 3, characterised in that the alkoxysilane compound corresponds to the general formula R¹ _aR² _bSiX_(4-a-b) wherein

   X denotes an alkoxy, an aryloxy or an acyloxy group having 1 to 12 carbon atoms, preferably having 1 to 4 carbon atoms, and in particular is selected from the methoxy, ethoxy, n-propoxy, i-propoxy, butoxy, phenoxy, acetoxy and propionyloxy groups;

   R¹ and R², identical to or different from one another, are selected from the

   amino, monoalkylamino or dialkylamino groups;

   alkyl groups, in particular the alkyl groups having 1 to 6 carbon atoms, preferably the methyl, ethyl, n-propyl, isopropyl, n-butyl, s-butyl, t-butyl, pentyl, hexyl or cyclohexyl groups;

   alkenyl groups, in particular the alkenyl groups having 2 to 6 carbon atoms, preferably the vinyl, 1-propenyl, 2-propenyl or butenyl groups;

   alkinyl groups, in particular the alkenyl groups having 2 to 6 carbon atoms, preferably the acetylenyl or propargyl groups;

   aryl groups, in particular the aryl groups having 6 to 10 carbon atoms, preferably phenyl or naphthenyl groups;

   epoxy groups, in particular the epoxy groups having 3 to 16 carbon atoms, preferably the glycidyl, glycidyl ether, glycidyl ester or glycidyl oxyalkyl groups; or

   the previously described group X; and

   a and b, identical to or different from one another, represent the value 0, 1, 2 or 3, the sum of a and b not exceeding the value 3.
5. Object according to claim 4, characterised in that the alkoxysilane compound is a tetraalkoxysilane, epoxyalkoxysilane or aminoalkoxysilane.
6. Object according to claim 5, characterised in that the alkoxysilane compound is selected from tetraethoxysilane, 3-glycidyloxypropyltrimethoxysilane, 3-aminopropyltrimethoxysilane and 3 - (aminoethylamine)propyltrimethoxysilane.
7. Object according to one of claims 3 to 6, characterised in that the solution contains in addition a compound which is capable of forming a titanium complex.
8. Object according to claim 7, characterised in that the compound which is capable of forming a titanium complex is an alkoxytitanium compound, a titanate ester or a titanium chelate and in particular corresponds to the formula Ti(OR)₄, wherein R denotes an alkyl group having 1 to 6 carbon atoms, which is preferably selected from the methyl, ethyl, n-propyl, i-propyl and butyl groups.
9. Object according to claim 8, characterised in that the compound which is capable of forming a titanium complex is tetraethoxytitanate Ti(OC₂H₅)₄.
10. Object according to one of claims 3 to 9, characterised in that the solution contains in addition at least one dye which is soluble in a polar solvent, in particular a metal-complex dye.
11. Process for producing a conversion layer on an object made of magnesium or alloys thereof, characterised in that the object is subjected to a passivation by means of an aqueous passivating electrolyte, the aqueous passivating electrolyte containing potassium permanganate and at least one alkali metal salt or ammonium salt of an anion selected from vanadate, molybdate and tungstate.
12. Process according to claim 11, characterised in that the passivation is carried out using an aqueous passivating electrolyte in a pH range of 7.0 to 8.0.
13. Process according to claim 11 or 12, characterised in that the passivation is carried out using an aqueous passivating electrolyte at a temperature of 15 to 50°C, in particular of 20 to 30°C.
14. Process according to one of claims 11 to 13, characterised in that the passivation is carried out for a period of 2 to 10 minutes.
15. Process according to one of claims 11 to 14, characterised in that the concentration of potassium permanganate in the aqueous passivating electrolyte is 1 to 10 g/l.
16. Process according to one of claims 11 to 15, characterised in that the concentration of the alkali metal salt or ammonium salt selected from vanadate, molybdate and tungstate in the aqueous passivating electrolyte is 1 to 10 g/l.
17. Process according to one of claims 11 to 16, characterised in that a coating or a paint has been or is applied to the conversion layer.
18. Use of an object according to one of claims 1 to 10 and of an object obtainable by a process according to one of claims 11 to 17 in the automobile industry, electrical and electronic industry, engineering industry, aeronautical engineering and space technology.