EP1163378A2 - 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

 "Chemically passivated article made of magnesium or its alloys"

The present invention relates to an object made of magnesium or its alloys, which has a conversion layer produced by passivation of the surface, 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 are therefore very susceptible to corrosion. To counteract this disadvantageous property, magnesium and its alloys are treated in aqueous passivation electrolytes. As a result of the redox process (without an external power source) taking place, a conversion layer is formed which consists of oxides of the magnesium material and oxidic reaction products which result from the constituents of the aqueous passivation electrolyte.

The term “conversion layer” is understood here and below to mean a layer which is not formed by application to a surface, but rather by chemical conversion (conversion) of the metallic surface and various components of the aqueous passivation electrolyte (cf. H. Simon, M. Thoma "Applied 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 ner driving is described in particular in the 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, Νo. 22). The chemical passivation using chromium (VI) -containing aqueous passivation electrolytes is easy to carry out. But this has the serious Disadvantage that the chromate-containing substances that are also contained in the conversion layers formed are carcinogenic

In addition, the recyclability of chromated articles made of magnesium or its alloys is a considerable problem, since, owing to their heavy metal content, they can only be recycled to so-called “high-purity” materials with considerable effort

For reasons of environmental protection and occupational safety, manufacturers and processors of passivated articles made of magnesium or its alloys are striving to find a replacement for conventional chromating using chromate-free aqueous passivation electrolytes

Aqueous passivation electrolytes based on stannate, sold by Dow Chemical, for example, are known as chromate-free aqueous passivation electrolytes for the passivation of objects made of magnesium or its alloys. However, it has been shown that the corrosion protection effect of the conversion layer obtained is less compared to the chromated magnesium materials

US Pat. No. 5,743,971 describes a method 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 which contains an oxidizing agent, a silicate and at least contains a 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. Kahumpermanganate is not mentioned as an oxidizing agent. This document also does not indicate which actual improvements the process described there brings for magnesium or its alloys compared to conventional chromating

In addition, the phosphating of objects made of magnesium or its alloys is known (see Dow Chemical Treatment No 18) tion A phosphating with simultaneous use of potassium permanganate is in D Hawk, DL Albright, ^M A Phosphate-permanganates conversion coating for magnesium " Metal Finishingmg, October 1995, S 34 - 38, described here is also used This aqueous passivation electrolyte has much less corrosion protection 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 ^, H ₂ S0 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 (Mn0 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 article made of magnesium or its alloys, the conversion layer of which can be obtained by an electrolytic, current-free process which can be used in a simple manner and can be transferred to an industrial scale. The corrosion protection effect of such a conversion layer should also 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 0 and Mn0 ₂ and at least one oxide from the group of vanadium, molybdenum and tungsten having.

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 a Passivation is subjected to an aqueous passivation electrolyte, the aqueous passivation electrolyte containing potassium permanganate and at least one alkali or ammonium salt of an anion from the group of vanadate, molybdate and tungstenate.

The conversion layer according to the invention has a golden brown to gray-brown, iridescent color and contains MgO, Mn ₂ O, MnO 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.

Particularly 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 only by combining the permanganate ions with the corresponding vanadate, molybdate and / or tungsten ions does a synergistic effect occur is achieved, 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 the formation of very strong, so-called heteropolyacids in the form of their soluble ammonium or alkali salts.

A particular advantage of the method 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 that would render the aqueous passivation electrolyte unusable for the passivation of objects made of magnesium or its alloys.

Therefore, it is possible in the present method in a simple manner to replenish the used chemicals in a simple manner after a long period of use, without the aqueous passivation electrolyte itself having to be replaced. 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 (e.g. 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 polymer layer is formed by polymerization processes known per se and familiar to the person skilled in the art (e.g. air drying, heating or UV radiation):

The amount of alkoxysilane compound in the solution to be applied can vary within wide limits. The solution generally contains 5 to 45% by weight, in particular 10 to 30% by weight, of the alkoxysilane compound. Depending on the required viscosity, the solution may also contain a polar solvent, which should be selected so that it does not react with the alkoxysilane compound (e.g. ethanol).

According to a preferred embodiment, the alkoxysilane compound corresponds to the general formula

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 radicals, in particular the alkenyl radicals having 2 to 6 carbon atoms, preferably the vinyl, 1-propenyl, 2-propenyl or butenyl radicals; 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 glycidyloxy alkyl radicals; or group X described above; and

A and b, the same or different from each other, 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 bridged with the alkoxysilane compound and the conversion layer via complex bonding. The reaction between the alkoxysilane compound and titanium compound also gives 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) .., in which R represents an alkyl radical having 1 to 6 carbon atoms, which is preferably selected from the group consisting of methyl, ethyl, n-propyl, i-propyl and butyl radicals.

Very good results were achieved with tetraethoxytitanate Ti (OC ₂ H ₅ ) _{/ 1} .

The molar ratio between alkoxysilane compound and titanium compound is not critical and is generally between 1 and 20.

Solutions containing both an alkoxysilane and contain a group capable of forming a titanium complex compound, are described for example in DE 41 38 218 Al and are available from various companies (for example Deltacoll ^® 80 from the company. 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 metal complex dye. Such a metal complex dye is ICI available for example under the trade name of Neozapon "from Fa. BASF, Orasol ^® by the company. Ciba-Geigy, Savinyl" from Messrs. Sandoz or Lampronol ^H from the company.. 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 / 1; that of the alkali or ammonium salt of the vanadate, molybdate and / or tungsten ions, preferably 1 to 10 g / 1. In particular, the upper limit of the vanadate, molybdate and / or tungstate concentration is not critical. Thus, the method according to the invention can also be carried out with an electrolyte which contains a saturated solution of these salts, even with undissolved constituents.

The synergistic effect between permanganate ions and vanadate, molybdate and / or tungsten ions becomes particularly clear when an attempt is made to passivate an object made of magnesium only with an aqueous potassium permanganate solution with a concentration of 1 to 10 g / 1 with the same working parameters. Because under these conditions it is not possible to obtain a conversion layer with sufficient 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, ZKόl and AZ31, AZ60, ZK30, ZK60, WE43 and WE54 (names according to ASTM).

The invention also relates to the use of a solution for producing an object according to the invention, this solution containing at least one of the alkoxysilane compounds described above. As a pretreatment for the chemical passivation according to the invention, the objects made of magnesium or its alloys are previously pickled in a manner known per se 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, such as those e.g. are available 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 condition (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 magnesium alloy AZ91HP with the dimensions 50 x 100 x 2 mm are pickled in 75% H PO _{/ (} for 30 seconds. Then rinsed with deionized water and the plates are neutralized in 10% NaOH at room temperature for 30 seconds; The plates are then rinsed again with deionized water, and the plates are immersed in an aqueous passivating electrolyte consisting of an aqueous solution of 3 g / 1 KMnO ₄ and 1 g / 1 NHNO ^ for 5 minutes at room temperature after removal the The gray-brown-looking conversion layer is rinsed with deionized water in plates from the passivation bath 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 the 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.

Table I

The smaller value corresponds to the time at which the first of the three plates shows insufficient corrosion protection; the larger value indicates the time at which the last of the three plates shows insufficient corrosion protection.

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 / 1 KMn0 ₄ and 1.5 g / 1 Na ₂ W0 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 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.

Table π

The smaller value corresponds to the time at which the first of the three plates shows insufficient corrosion protection; the larger value indicates the time at which the last of the three plates shows insufficient corrosion protection.

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 P0 _{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 / 1 KMn0 ₄ and 1 g / 1 NaV0 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 DI

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

Claims

Patent claims 1. 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 0 ₃ and Mn0 ₂ and at least one oxide from the group of vanadium, molybdenum and tungsten. 2. Article according to claim 1, characterized in that the conversion layer is obtainable by passivating the article 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 tungsten. 3. Article according to claim 1 or 2, characterized in that a polymer layer is additionally applied to the conversion layer, obtainable by polymerizing and / or crosslinking a solution which contains at least one alkoxysilane compound. 4. Article according to claim 3, characterized in that the alkoxysilane compound of the general formula corresponds in which 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 glycidyl oxyalkyl radicals; or group X described above; and • a and b, the same or different from each other, represent the value 0, 1, 2 or 3, the sum of a and b not exceeding 3. 5. Article according to claim 4, characterized in that the alkoxysilane compound is a tetraalkoxysilane, epoxyalkoxysilane or aminoalkoxysilane. 6. Article according to claim 5, characterized in that the alkoxysilane compound is selected from the group of tetraethoxysilane, 3-glycidyl oxypropyl trimethoxysilane, 3-aminopropyl trimethoxysilane and 3- (aminoethyl amine) propyl trimethoxysilane. 7. Article according to one of claims 3 to 6, characterized in that the solution additionally contains a compound capable of forming a titanium complex. 8. Article according to claim 7, characterized in that the compound capable of forming a titanium complex is an alkoxytitanium compound, a titanium ester or a titanium chelate and in particular corresponds to 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 residues. 9- Article according to claim 8, characterized in that the compound capable of forming a titanium complex is tetraethoxytitanate Ti (OC ₂ H ₅ ) ₄ . 10. Object according to one of claims 3 to 9, characterized in that the solution additionally contains at least one dye which is soluble in a polar solvent, in particular a metal complex dye. 11. A method for producing a conversion layer on an article made of magnesium or its alloys, characterized in that the article is subjected to passivation by means of an aqueous passivation electrolyte, the aqueous passivation electrolyte being potassium permanganate and at least one alkali metal or ammonium salt of an anion from the group of vanadate , Contains molybdate and tungstate. 12. The method according to claim 11, characterized in that the passivation is carried out in a pH range of the aqueous passivation electrolyte from 7.0 to 8.0. 13. The method according to claim 11 or 12, characterized in that the passivation is carried out at a temperature of the aqueous passivation electrolyte from 15 to 50 ° C, in particular from 20 to 30 ° C. 14. The method according to any one of claims 11 to 13, characterized in that the passivation is carried out for a period of 2 to 10 minutes. 15. The method according to any one of claims 11 to 14, characterized in that the concentration of potassium permanganate in the aqueous passivation electrolyte is 1 to 10 g / 1. 16. The method according to any one of claims 11 to 15, characterized in that the concentration of the alkali metal or ammonium salt from the group of vanadate, molybdate and tungstate in the aqueous passivation electrolyte is 1 to 10 g / 1. 17. The method according to any one of claims 11 to 16, characterized in that a varnish or a color is or is applied to the conversion layer. 8. Use of a solution for producing an article according to one of claims 3 to 10, characterized in that the solution contains at least one alkoxysilane compound according to one of claims 3 to 9. 9- Use of an object according to one of claims 1 to 10 and an object obtainable by a method according to one of claims 11 to 17 in the motor vehicle industry, electrical and electronics industry, mechanical engineering industry, aerospace.