EP1543180B1 - Method for electrolytic coating of materials with aluminium, magnesium or aluminium and magnesium alloys - Google Patents

Abstract

The invention concerns a method for electrolytic coating of materials with aluminium, magnesium or aluminium and magnesium alloys. Said method is characterized in that the material is pretreated by being immersed in an electrolytic solution, wherein it is anodized, the electrolytic coating being performed immediately after in the same electrolytic solution.

Description

The present invention relates to a process for the electrolytic coating of materials with aluminum, magnesium or alloys of aluminum and magnesium, wherein the material is dipped into an electrolyte for pretreatment and is switched there anodically and immediately thereafter the electrolytic coating takes place in the same electrolyte. By the method according to the invention, the quality of the deposited aluminum, magnesium or aluminum / magnesium coating is improved.

The deposition of aluminum, magnesium or aluminum / magnesium alloys on materials that are made of base metals is a tried and tested means to protect these materials from corrosion. You will be provided at the same time with a decorative coating. The protective metal layer is deposited here predominantly galvanically on the material. In this case, it is advantageous if the aluminum, magnesium or aluminum / magnesium layer takes place on the material without the application of metallic intermediate layers between said metal layer and the material. If intermediate layers between the material and the surface layer of aluminum, magnesium or aluminum / magnesium alloy are applied, there is a risk of contact corrosion due to the applied intermediate layer. In addition, thermal problems may occur due to the different expansion coefficients of the surface layer and the intermediate layer.

Electrolytes which have been well-known in the art include melt flow electrolytes such as electrolytes containing aluminum halides or aluminum alkyl complexes. All these electrolyte systems have in common that the material must be cleaned on its surface before coating. This is especially true for materials made of base metals which form an oxide layer, the problem that this oxide layer must be completely removed before the coating. If the surface of the materials is not completely cleaned, contaminants or residues of the oxide layer of the metal of which the material is made adversely affect the adhesion of the subsequently electrolytically applied metal layer. Furthermore, it is possible that at the points where impurities on the surface are present, no metal layer is applied, since the impurities are usually not electrically conductive and thus an electrolytic deposition is prevented at this point. This then inevitably leads to corrosion problems of the finished coated material at the point where the metal layer was not completely applied.

DE-C3-22 60 191 describes a method for the preparation of materials made of electrically conductive materials. In this case, the last method step used for shaping the materials, in which a new blank surface is formed on the material, is carried out in the absence of atmospheric oxygen and moisture in a suitable inert gas or inert liquid medium. A disadvantage of this method is that, in particular when using inert liquid medium, which covers the surface of the material and thus can be brought into the coating electrolyte, this subsequently contaminates or hydrolyzes the electrolyte. The use of inert gas media shows in the industrial application the problem that an absolutely oxygen-free inert gas atmosphere is practically impossible to realize. Traces of oxygen that are present in the inert gas atmosphere immediately oxidize the bare metal surface of the material and thus lead to the already described loss of quality of the subsequently galvanically applied metal layer. If, as described in DE-C3-22 60 191, the bare surface by a mechanical method, such as. As milling, cutting, sawing or drilling, or by strong deformation of the material with z. Rolling or by wire drawing, extrusion or other methods is performed, these methods cause an increase in the manufacturing tolerance of finished material. This makes materials that are produced by this process, not suitable for applications where a high quality and manufacturing consistency is necessary.

In DE-AS-12 12 213, the pretreatment of a material in a protective gas atmosphere is described. Alternatively, the oxide layer on the surface of the material may be removed by anodizing the material prior to depositing the aluminum layer in the electrolyte made from sodium fluoride and aluminum triethyl. Subsequently, a reversal of the current takes place, as well as a deposition of aluminum on the material. The disadvantage is that the electrolyte can only be used to deposit aluminum on materials. The deposition of magnesium or aluminum / magnesium layers is not possible because the presence of halide ions in the electrolyte in the anodic polarization would produce directly insoluble magnesium halide compounds which prevent deposition of magnesium or aluminum / magnesium on the material. The resulting magnesium halides would immediately prevent the flow of current in the electrolyte by blocking the electrodes.

DE-AS-21 22 610 describes a process for the anodic pretreatment of light metals for the electrodeposition of aluminum. The components are cleaned by treating the light metal materials in a molten electrolyte, the materials being subjected to anodic loading. The light metal materials cleaned in this way are humidified, so that they are still contaminated with the molten electrolyte, and immersed in an aluminizing cell. In this case, it can not be ruled out that atmospheric oxygen still reaches the pretreated material and oxidizes it again on the surface. Further, contamination of the aluminizing electrolyte by the surface-treating electrolyte, which is a molten electrolyte, takes place. Only if the material consists of beryllium or aluminum, it is possible that the material in the molten electrolyte, which serves for the surface treatment by anodic oxidation of the material, also for the electrodeposition of aluminum on the Beryllium or aluminum material is used. The melt electrolyte described in DE-AS-21 22 610 is only suitable for pretreating beryllium or aluminum materials in order subsequently to coat them with aluminum in the same melt electrolyte. The molten electrolyte is not suitable for electroplating aluminum, magnesium or aluminum / magnesium layers on other materials.

DE-A1-198 55 666 describes an electrolyte suitable for depositing aluminum / magnesium alloy layers. The disclosed aluminum-organic electrolyte contains K [AlEt ₄ ] or Na [Et ₃ Al-H-AlEt ₃ ], as well as Na [AlEt ₄ ], as well as trialkylaluminum. The electrolyte can be present as a toluene solution. The electrolytic deposition of aluminum / magnesium alloy layers from the described electrolyte is carried out using a soluble aluminum and a likewise soluble magnesium anode or using an aluminum / magnesium alloy anode. In the described method, the electrolyte composition is adjusted by pre-electrolysis so that the deposited layer has the desired aluminum / magnesium ratio. Alternatively, Mg [AlEt ₄ ] ₂ can also be added to the electrolyte. DE-A1-198 55 666 thus teaches that the ratio of aluminum and magnesium in the deposited aluminum / magnesium layer is very much dependent on the concentration ratio of magnesium and aluminum in the electrolyte. As with all prior art processes, great care must be taken in the pretreatment of the materials to be coated, as contamination of the surface of the material by oxidation or other influences leads to reduced quality of the electrodeposited metal layer.

The technical object of the present invention is to provide a method in which aluminum, magnesium or aluminum / magnesium layers can be applied to materials, wherein the quality of the metal coating is increased by an improved pretreatment of the material. In particular, a method is provided be, in which the materials to be coated are reliably and inexpensively freed from adhering oxide layers or other impurities, after the pretreatment of the materials, a renewed contamination or oxidation of the materials to be prevented.

The technical object of the present invention is achieved by a process for the electrolytic coating of materials with aluminum, magnesium or alloys of aluminum and magnesium, wherein the material is immersed in the electrolyte for pretreatment, is anodically switched there and immediately thereafter the electrolytic coating in the same Electrolytes takes place, wherein the electrolyte bath organoaluminum compounds of the general formula M [(R ¹ ) ₃ Al- (H-Al (R ² ) ₂ ) _n -R ³ ] (I) and Al (R ⁴ ) ₃ (II) as the electrolyte and n is 0 or 1, M is sodium or potassium and R ¹ , R ² , R ³ , R ^{4 may be the} same or different, wherein R ¹ , R ² , R ³ , R ^{4 are} C ₁ - to C ₄ alkyl group and is used as a solvent for the electrolyte, a halogen-free, aprotic solvent. By means of the method according to the invention, it is possible to pretreat the material in the bath in which the electrolytic coating takes place later. Surprisingly, impurities which adhere to the non-pretreated material, as well as existing oxide layers on the material are replaced. The impurities thus introduced into the electrolyte bath surprisingly do not impede the deposition of magnesium, aluminum or alloys of aluminum and magnesium on the material. Insoluble impurities can be continuously removed from the electrolyte bath by means of suitable filtration systems.

It is therefore no longer necessary to transfer the materials after pretreatment from the pre-treatment bath in the electrolyte bath. This step, which always carries the risk of re-contamination of the surface of the material, can be avoided.

In a preferred embodiment, in the process according to the invention, an electrolyte is used as a mixture of the complexes K [AlEt ₄ ], Na [AlEt ₄ ] and AlEt ₃ are used. The molar ratio of the complexes to AlEt ₃ is 1: 0.5 to 1: 3, with the ratio of 1: 2 being preferred.

In a preferred embodiment, 0 to 25 mol%, preferably 5 to 20 mol% Na [AlEt ₄ ], based on the mixture of the complexes K [AlEt ₄ ] and Na [AlEt ₄ ] used.

Preferably, a mixture of 0.8 mol of K [AlEt ₄ ], 0.2 mol of Na [AlEt ₄ ], 2.0 mol of AlEt ₃ in 3.3 mol of toluene can be used as the electrolyte.

Alternatively, a mixture of Na [Et ₃ Al-H-AlEt ₃ ] and Na [AlEt ₄ ] and AlEt ₃ can be used as the electrolyte in the process according to the invention. Preferably, the molar ratio of Na [Et ₃ Al-H-AlEt ₃ ] to Na [AlEt ₄ ] is 4: 1 to 1: 1, with a ratio of 2: 1 being preferred. It is further preferred that the molar ratio of Na [AlEt ₄ ] to AlEt _{3 is} 1: 2.

In a further preferred embodiment, the electrolyte used is a mixture of 1 mol of Na [Et ₃ Al-H-AlEt ₃ ], 0.5 mol of Na [AlEt ₄ ] and 1 mol of AlEt ₃ in 3 mol of toluene.

The electrolytic coating of materials with magnesium, aluminum or aluminum / magnesium alloys is preferably carried out at a temperature of 80 to 105 ° C. A temperature of the plating bath of 91 to 100 ° C is preferred.

The electrolytic deposition of aluminum, magnesium, or aluminum / magnesium layers on the materials is carried out using a soluble aluminum and a likewise soluble magnesium anode or using an aluminum / magnesium alloy anode. However, it is also possible to use only one aluminum or one magnesium anode.

In the method according to the invention, the anodic switching of the material for pretreatment can be carried out for a period of 1 to 20 minutes, with 5 to 15 minutes being preferred.

The anodic loading of the materials necessary for the pretreatment is carried out with a current density of 0.2 to 2 A / dm ² , preferably 0.5 to 1.5 A / dm ² .

As a material, any material can be used which is suitable for electrodeposition. It is preferred that the material consists of a metal and / or of a metal alloy and / or is a metallized, electrolyte-resistant material, which can be dissolved in the electrolyte by anodic circuit. The materials to be coated are preferably rack goods, bulk goods or endless products such as wire, square plates, screws or nuts.

The method according to the invention is characterized in that impurities or oxide layers which adhere to the materials are safely removed. Surprisingly, no disadvantageous change of the electrolyte composition occurs here, which would prevent a high-quality deposition of aluminum, magnesium or aluminum / magnesium layers on the materials. Furthermore, the electrodeposited metal layers are firmly adhering and homogeneously applied to the material, since after cleaning a renewed contamination of the material is prevented. In addition to the quality advantages mentioned, cost optimization of the coating of molded parts with metal layers is additionally achieved by the above-mentioned method steps.

The process according to the invention for the electrolytic coating of materials comprising magnesium, aluminum or alloys of aluminum and magnesium is illustrated by, but not limited to, the following examples.

Examples example 1

Phase a) A stamped part of an AlMg ₃ alloy was first pickled alkaline for 2 minutes in a solution of 100 g / l NaOH at a temperature of 60 ° C. After subsequent rinsing in water, the part was decanted in 10% nitric acid, then rinsed in distilled water and dried.

Phase b) The dry part was introduced into an argon or nitrogen-flooded coating cell and immediately introduced into the coating electrolyte after a pre-rinse in toluene. The electrolyte used was a mixture of the complexes K [AlEt _4] , K [AlEt ₄ ] and AlEt ₃ dissolved in toluene. The counterelectrode used was an AlMg25 alloy plate. The product to be coated was first anodically poled and treated at a current density of 1 A / dm ^{2 for} 5 minutes at an electrolyte temperature of 95 ° C. Then reversed without removing the part from the electrolyte and immediately for 45 minutes at a current density of 1 5 A / dm ² coated. An AlMg alloy layer of about 14 μm thickness was deposited.

The adhesion of the layer was tested by cross hatch test and heat shock test (1 h at 220 ° C and quenching in cold water). It was found that excellent adhesion of the deposited layer to the base material was present. No peeling or bubbles could be detected.

Comparative Example 1

A treated as a comparative sample was pretreated and coated as in Example 1, but without anodic polarity in advance. The layer could be peeled off in the crosshatch test as a film. In heat shock test, the layer showed bubbles.

Example 2

A magnesium die-cast part made of an AZ-91 alloy was blasted with corundum (grain size 0-50 μm) at 2 bar pressure. The part was then immediately placed in the inert gas atmosphere of the coating cell, pre-rinsed in toluene and immersed in the electrolyte bath as described in Example 1. First, the product to be coated was anodized for 10 minutes at a current density of 1 A / dm ² . In this case, a layer of about 2 microns was removed on the product surface. It was then reversed and the part switched cathodically for 1 hour at 1.5 A / dm ² . An AlMg layer with 23-25% Mg content and a layer thickness of approximately 18 μm was deposited.

Subsequent adhesion tests showed no delamination in both the crosshatch test and the thermal shock test.

Claims (12)

1. A method for electrolytic coating of materials with aluminum, magnesium or alloys of aluminum and magnesium, in which method the material is immersed in an electrolyte for pretreatment, being connected as anode therein, and electrolytic coating is performed in the same electrolyte immediately thereafter, the electrolytic bath including organoaluminum compounds of general formulas (I) and (II)
   
           M[(R¹)₃Al-(H-Al(R²)₂)_n-R³]     (I)
   
   
   
   
           Al(R⁴)₃     (II)
   
   as electrolyte, wherein n is equal to 0 or 1, M is sodium or potassium, and R¹, R², R³, R⁴ can be the same or different, R¹, R², R³, R⁴ being a C₁-C₄ alkyl group, and a halogen-free, aprotic solvent being used as solvent for the electrolyte.
2. The method according to claim 1, characterized in that a mixture of the complexes K[AlEt₄], Na[AlEt₄], and AlEt₃ is employed as electrolyte.
3. The method according to claim 2, characterized in that the molar ratio of said complexes to AlEt₃ is from 1:0.5 to 1:3, preferably 1:2.
4. The method according to claim 2 or 3, characterized in that 0 to 25 mole-%, preferably 5 to 20 mole-% Na[AlEt₄] is employed, relative to the mixture of the complexes K[AlEt_4] and Na[AlEt₄].
5. The method according to one or more of claims 1 to 4, characterized in that a mixture of 0.8 mol of K[AlEt₄], 0.2 mol of Na[AlEt₄], 2.0 mol of AlEt₃ in 3.3 mol of toluene is used as electrolyte.
6. The method according to claim 1, characterized in that a mixture of Na[Et₃Al-H-AlEt₃] and Na[AlEt₄] and AlEt₃ is used as electrolyte.
7. The method according to claim 6, characterized in that the molar ratio of Na[Et₃Al-H-AlEt₃] to Na[AlEt₄] is from 4:1 to 1:1, preferably 2:1.
8. The method according to claim 6 or 7, characterized in that the molar ratio of Na[AlEt₄] to AlEt₃ is 1:2.
9. The method according to one or more of claims 6 to 8, characterized in that a mixture of 1 mol of Na[Et₃Al-H-AlEt₃], 0.5 mol of Na[AlEt₄] and 1 mol of AlEt₃ in 3 mol of toluene is used as electrolyte.
10. The method according to one or more of claims 1 to 9, characterized in that electrolytic coating is effected at temperatures of from 80 to 105°C, preferably from 91 to 100°C.
11. The method according to one or more of claims 1 to 10, characterized in that pretreatment is performed for a period of from 1 to 20 minutes, preferably from 5 to 15 minutes.
12. The method according to one or more of claims 1 to 11, characterized in that pretreatment is performed at an anodic load of the materials with a current density of from 0.2 to 2 A/dm², preferably from 0.5 to 1.5 A/dm².