EP0328128B2 - Process concerning the adhesion between metallic materials and galvanic aluminium layers and the non-aqueous electrolyte used therein - Google Patents

Abstract

Processes are disclosed for the metal coating of metallic material, in particular of low-alloyed high-strength steels which are characterised in that electrodeposited adhesion promotion layers composed of iron, iron and nickel, nickel, cobalt, copper or alloys of the abovementioned metals or zinc/nickel alloys are deposited on said metallic materials from non-aqueous electrolytes and aluminium is then electrodeposited thereon in a manner known per se.

Description

The invention relates to methods for the metal coating of metal materials, in particular of low-alloy, high-strength steels.

The invention also relates to the non-aqueous electrolytes used in the above process.

Certain metals, such as copper, can be directly electroplated with aluminum after appropriate mechanical and / or chemical pretreatments to remove fat and / or oxide layers from the surface of the workpieces. On other metals, such as, for example, iron materials and in particular special steels, generally no adhesive layers of galvanoaluminium can be obtained in the same way.

Therefore, various methods for the preparation of workpieces made of electrically conductive materials, in particular metals for an adhesive galvanic coating with aluminum, have been proposed.

DE-PS 22 60 191 (Siemens AG, priority: 08.12.1972) describes such a method, which is characterized in that at least the last process step, which is used to shape the workpieces, is carried out under an aprotic, water- and oxygen-free protective medium. In the examples, milling, sawing or sanding are mentioned as the last process step used for shaping.

In DE-OS 31 12 919 (Siemens AG, priority: March 31, 1981) it is proposed to apply thin layers of cobalt or cobalt alloys galvanically from aqueous solutions to impart adhesion to iron workpieces. Layer thicknesses of a maximum of 1 µm should be sufficient to impart adhesion with the aluminum that is subsequently to be electroplated.

Previously (H. Lehmkuhl, dissertation, Technical University of Aachen, 1954), it was recommended to use copper layers electroplated from aqueous electrolytes to promote adhesion between iron workpieces and electroplated aluminum layers.

A decisive disadvantage of the two last-mentioned procedures is that during the electrodeposition of the metals serving as adhesion promoters, co-separation of hydrogen cannot be avoided. Low-alloy high-strength steels, for example as shown in Table 1, are very sensitive to embrittlement to hydrogen. Aqueous electrolyte solutions are therefore unsuitable for coating such steels.

DE-A-2012846 discloses non-aqueous, DMSO-containing solutions of metal salts for electroplating without hydrogen fragility.

The invention consequently relates to methods for the metal coating of metal materials, in particular low-alloy high-strength steels, galvanic bonding layers being formed from non-aqueous electrolytes of anhydrous metal compounds Iron, iron and nickel, nickel, cobalt, copper or alloys of the above-mentioned metals or tin-nickel alloys are deposited on these metal materials and then subsequently galvanically aluminum is deposited in a manner known per se.

When using these metals as intermediate layers for a subsequent aluminizing by galvanic means, a layer thickness of 1 to 4 µm is generally sufficient to ensure adhesion between the material, intermediate layer and galvanoaluminium layer.

in der
R für C₁ bis C₆ und Phenyl, und
R¹ für H oder Methyl steht,
oder Mischungen dieser Lösungen unter Zusatz wasserfreier Leitsalze, insbesondere Lithiumchlorid, Lithiumbromid oder entsprechende Tetraorganylammoniumhalogenide verwendet.In order to avoid the deposition of hydrogen and the associated risk of embrittlement of the materials, electrolytes are solutions of anhydrous metal salts of Fe, Co, Ni, Cu or Sn, in particular their anhydrous halides and / or the complex compounds of these metal halides with ethers, such as for example tetrahydrofuran, or with alcohols, such as ethanol, in anhydrous alkyl half ethers of a C₂-C₃ alkylene glycol of the formula

in the
R for C₁ to C₆ and phenyl, and
R¹ represents H or methyl,
or mixtures of these solutions with the addition of anhydrous conductive salts, in particular lithium chloride, lithium bromide or corresponding tetraorganylammonium halides.

Furthermore, such soluble anodes are expediently those from the concerned metal or in the deposition of alloys used from the metals to be deposited or corresponding alloy anodes.

In the case of Fe, Co, Ni and Sn compounds, the metal (II) compounds are expediently used; in the deposition of Cu, Cu (I) compounds are generally used.

The use of 2-ethoxyethanol as a solvent of electrolytes for the deposition of Cu, Ni, Co has been described by AL Chaney, CA Mann, J. Phys. Chem 35 (1931) 2289. In contrast to the process according to the invention, however, only the water-containing compounds (Cu (ClO₄) ₂ · 2H₂O, Ni (ClO₄) ₂ · 2H₂O and Co (ClO₄) ₂ · 2H₂O have been described. The authors describe the type of metal deposition for Cu as good , Ni is less good because it is brittle, and Co less good because it is black, spongy, and it is not known whether these layers are suitable as adhesion-promoting layers for galvanoaluminium, but this must be the case, particularly because of the properties of the Ni or Co-layers such as brittleness or spongy character are doubted In any case, with the water components brought in by the metal salts, the co-separation of hydrogen remains unavoidable and the associated risk of embrittlement of the materials by hydrogen is maintained.

   Beide Effekte verstärken die Gefahr der Wasserstoffabspaltung.The deposition of nickel from solutions of NiCl₂ · 6H₂O in ethylene glycol (1,2-ethanediol) also described by AJ Dill (Plating 1972 , 59 (11), 1048-1052, Galvano-Organo 1974 , 43, 151-156) a water-containing metal salt. The co-separation of hydrogen cannot therefore be avoided. The same applies to the nickel deposition from 0.2 M solutions by AA Sarabi, VB Singh, Indian J. of Technology 25 (1987) 119 NiCl₂ undefined water content or NiSO₄ · 7 H₂O in 1,2-ethanediol or 2-methoxyethanol with the addition of boric acid (0.2 M). In 2-methoxyethanol-NiCl₂-H₃BO₃-x H₂O electrolytes, the nickel deposits are uniform, shiny gray and sticky at current densities of 0.1-0.3 A / dm², at higher current densities they show a tendency to peel off. Since the cathodic current yields are only 90-98%, it must be assumed that hydrogen is also deposited. It is generally known (FA Cotton, G. Wilkinson, Inorganic Chemistry, Verlag Chemie, Weinheim 1967, p. 245) that boric acid forms very easily boric acid esters with the elimination of water with alcohols. With 1,2-alkanediols, such as 1,2-ethanediol, strongly acidic chelate complexes of the type are formed

Both effects increase the risk of hydrogen elimination.

auf und scheiden kathodisch die gleichen Metallmengen entsprechend $${\text{M}}^{\text{2(+)}}{\text{+ 2 e}}^{\text{(-)}}\text{→ M}$$

ab. Bei CuCl enthaltenden Elektrolytlösungen werden pro 1 Faraday 63.54 g Cu entsprechend $${\text{M → M}}^{\text{(+)}}{\text{+ e}}^{\text{(-)}}$$

aufgelöst und die gleiche Metallmenge entsprechend $${\text{M}}^{\text{(+)}}{\text{+ e}}^{\text{(-)}}\text{→ M}$$

kathodisch abgeschieden. Als wasserfreie Metallsalze werden bevorzugt die wasserfreien Metalldichloride oder -dibromide bei Fe, Co und Ni bzw. Kupfer(I)chlorid oder -bromid oder deren Additionsverbindungen mit Alkoholen, wie beispielsweise Methanol oder Ethanol, oder mit Ethern, wie z.B. Diethylether, THF oder Dimethoxyethan, verwendet.This danger does not exist in the process according to the invention, since the anhydrous salts are used here and the solvent is also used anhydrous and without the addition of acids, in particular boric acid. The anodic and cathodic current yields are quantitative in relation to dissolved or deposited metal. Hydrogen is not separated out. 1 Faraday, ds 26.8 ampere hours, anodically dissolve 55.85 / 2 g iron, 58.94 / 2 g cobalt or 58.71 / 2 g nickel according to the electrolytic process $${\text{M → M}}^{\text{2 (+)}}{\text{+ 2 e}}^{\text{(-)}}$$

and cathodically separate the same amounts of metal accordingly $${\text{M}}^{\text{2 (+)}}{\text{+ 2 e}}^{\text{(-)}}\text{→ M}$$

from. In the case of electrolyte solutions containing CuCl, 63.54 g of Cu per 1 Faraday are used $${\text{M → M}}^{\text{(+)}}{\text{+ e}}^{\text{(-)}}$$

dissolved and the same amount of metal accordingly $${\text{M}}^{\text{(+)}}{\text{+ e}}^{\text{(-)}}\text{→ M}$$

deposited cathodically. The anhydrous metal salts are preferably the anhydrous metal dichlorides or dibromides in the case of Fe, Co and Ni or copper (I) chloride or bromide or their addition compounds with alcohols, such as, for example, methanol or ethanol, or with ethers, such as, for example, diethyl ether, THF or dimethoxyethane , used.

Alkyl half ethers of an alkylene glycol such as 1-alkoxy-2-hydroxyethane or 1-alkoxy-2-hydroxypropane, in particular the easily and inexpensively accessible half ethers of 1,2-ethanediol ROCH₂CH₂OH, preferably those with R = methyl, ethyl, propyl or isopropyl, are used as solvents or those of 1,2-propanediol, in particular CH₃CH (OH) CH₂OCH₃, used.

0.02 to 0.1 M, preferably 0.044 to 0.05 M solutions are recommended as the metal salt concentration in these solvents. The concentrations of conductive salt, especially lithium bromide, should be of approximately the same to twice the order of magnitude.

The electrolysis temperatures are between room temperature and approximately 120 ° C. Temperatures between 50 and 80 ° C. are preferred. Good, uniform and shiny metal layers made of Fe, Co, Ni or Cu can be achieved with current densities between 0.2 and 1.5 A / dm², preferably 0.5 to 1.0 A / dm², see Table 1.

In the case of alloy deposits, mixtures of solutions of the metal salts of the alloy components are generally used according to the invention. The anodes can then be either those made of appropriate alloys or a plurality of electrodes made of the metals of the individual alloy components. If there is a large electrolyte supply, it is possible to work only with the anode made from one of the alloy components. The concentration of the other alloy component must then be refreshed from time to time in the solution by adding the appropriate salt. If the deposition tendencies of the individual metals are very different, electrolytes containing only the salt of the more difficult to deposit metal can also be used when using alloy anodes.

• 1. durch Veränderung des Verhältnisses der Metallsalze im Elektrolyten zueinander und/oder
• 2. durch Verwendung von mehreren Anoden unterschiedlicher wirksamer Fläche aus den Metallen der einzelnen Legierungsbestandteile und/oder
• 3. bei Einsatz mehrerer Anoden aus den Metallen der Legierungsbestandteile durch unterschiedliche Stromkreise zwischen der Kathode und den einzelnen Anoden.

The composition of the alloy to be deposited can be varied within a wide range (see Table 2)

• 1. by changing the ratio of the metal salts in the electrolyte to one another and / or
• 2. by using several anodes of different effective area from the metals of the individual alloy components and / or
• 3. when using multiple anodes made of the metals of the alloy components through different circuits between the cathode and the individual anodes.

In order to avoid air oxidation of the metal salt solutions and / or the galvanically deposited metal layers, the electrolyses are carried out in closed vessels in an inert gas atmosphere of, for example, argon and / or nitrous oxide and / or nitrogen. After finishing the intermediate coating, the workpieces are first washed with the solvent of the electrolyte. After draining the Solvents and drying in an inert gas stream or in a vacuum, the workpieces are washed with dry toluene and then transferred to the aluminizing bath via an inert gas lock. A particular advantage of such a procedure is that no new oxide or water layer can be formed on the metal surface. Furthermore, there is no need for subsequent, complex drying processes before entering the aluminizing bath, such as treatment with fluorocarbons containing wetting agents.

The invention is explained using the exemplary embodiments described in the following two tables.

example 1

A cylindrical glass vessel with an edge ground flat at the top serves as the electrolysis cell and can be firmly closed with a lid made of insulating material. A cathode made of the material to be coated, for example WL-1.6359, is suspended from the lid between two anode plates made of the electrochemically dissolvable metal, for example nickel. The attachments of the electrodes also serve as a power supply. The dry cell is filled with inert gas, such as argon or nitrogen. To coat the cathode with nickel, a solution of 0.05 mol NiCl₂ · 0.63 THF and 0.05 mol LiBr in 1 liter CH₃OCH₂CH₂OH is used as the electrolyte. Electrolysis is carried out at 60 ° C. with a cathode current density of 0.5 A / dm² at about 3-4 volts and thorough mixing of the electrolyte until a 1 μm thick nickel layer has deposited on the cathode. The anodic and cathodic current yields are quantitative in relation to the amount of electricity.

In an analogous manner, other metal deposits listed in Table 1 were carried out with the electrolytes shown in Table 1.

Example 2

This example describes Experiment 1 listed in Table 2 in detail. A 0.05 molar (M) solution of LiBr in CH₃OCH₂CH₂OH, which is also 0.029 M in NiCl₂ and 0.015 M in FeCl₂, is electrolyzed at 65 ° C in an inert gas atmosphere with a current density of 0.5 A / dm². Nickel and iron sheets are used as anodes; the area ratio of both metal anodes is 1.0: 0.5. A workpiece made of WL-1.7176 is used as the cathode. Electrolysis is carried out until an approximately 3 μ thick alloy layer has been deposited on the cathode. The alloy consists of 75% Fe and 25% Ni. Experiments 2-9 were carried out analogously with the electrolytes and anodes specified in Table 2.

After the intermediate coating according to Examples 1 and 2 and the tests summarized in Tables 1 and 2, the workpieces are washed with the solvent of the electrolyte and dried in a stream of inert gas. The workpieces are then washed with dry toluene and transferred to the aluminizing bath via an inert gas lock.

In order to obtain a quantitative measure of the adhesive strength of electroplated layers on materials, methods are used to measure the force that is required to tear off the precipitate from the substrate. The "tape test" is a quantitative comparative method that enables liability assessments to be carried out in a simple manner. For this purpose, an adhesive strip is first pressed firmly onto the galvanic layer and then quickly torn off. In the event of poor or moderate adhesion, the galvanic layer and the adhesive strip come off the material base. If the adhesion is good, only small areas of the electroplating layer will be removed and if the adhesion is very good, the bonding of the electroplating layer to the base will remain intact.

Claims (8)

1. A process for metal-plating of metallic materials, and more specifically low-alloy high-strength steels, wherein adhesion-bonding layers of iron, iron and nickel, nickel, cobalt, copper or alloys of said metals or tin-nickel alloys are electrodeposited on said metallic materials from non-aqueous electrolytes of anhydrous metal compounds, and then aluminium is electrodeposited thereon in a per se known manner, wherein, as the anhydrous compounds, there are employed metal salts of iron, cobalt, nickel, copper or tin, and more specifically of the anhydrous chlorides thereof and/or anhydrous bromides thereof and/or the complex compounds of the metal chlorides and/or metal bromides with ethers or with alcohols in water-free alkyl semi-ethers of a C₂- to C₃-alkylene glycol of the formula

   

   wherein

   R   represents C₁- to C₆-alkyl and phenyl, and

   R¹   represents a hydrogen atom or a methyl group,

   or mixtures of these solutions with the addition of anhydrous supporting electrolytes, and more specifically lithium chloride, lithium bromide or respective tetraorganylammonium halides.
2. The process according to claim 1, characterized in that as the anhydrous metal compounds there are employed metal(II) compounds of iron, cobalt, nickel or tin or metal(I) compounds of copper.
3. The process according to claims 1 or 2, characterized in that as the anode a metal anode is used which has the same alloy composition as the metal cations of the metal salts of the electrolyte.
4. The process according to claims 1 to 3, characterized in that as the anhydrous solvent there is used a C₁- to C₄-alkyl semi-ether, and preferably a C₁- to C₃-alkyl semi-ether, of an alkylene glycol, namely of 1,2-ethylene diol or of 1,2-propanediol.
5. The process according to claims 1 to 4, characterized in that in the solvent of the water-free electrolyte the metal salts are present in concentrations of from 0.02 to 0.1M, and preferably of from 0.044 to 0.05M, and the supporting electrolyte, more particularly the lithium bromide, is present in an equimolar to twice the equimolar concentration relative to said metal salts.
6. The process according to claims 1 to 5, characterized in that the bonding layers are electrodeposited at current densities at from 0.2 to 1.5 A/dm², and preferably at from 0.5 to 1.0 A/dm², at electrolysis temperatures between room temperature (20 °C) and about 120 °C, and preferably between 50 °C and 80 °C.
7. The process according to claims 1 to 6, characterized in that the electrodeposition is effected in an inert gas atmosphere.
8. Non-aqueous electrolytes having the compositions as set forth in claims 1, 2, 4 and 5 for metal-plating of metallic materials, and more specifically low-alloy high-strength steels, and for adhesion-bonding between said metallic materials and aluminium layers electrodeposited thereon.