EP3000918A1 - Method and device for the galvanic application of a surface coating - Google Patents

Abstract

The present invention relates to a method for galvanically applying a surface coating, in particular a chromium coating, to a body, for example a machine component, characterized in that a layer of a compound that can be oxidized by an electrolyte solution used, preferably a Polyhydroxy compound is applied with a viscosity of at least 1000 mPas at 25 ° C. The present invention also relates to a method for galvanically applying a surface coating, in particular a chromium coating, to a body, for example a machine component, the surface coating being carried out in a closed reactor in an at least two-stage, preferably three-stage process, characterized in that a reactor containing electrolytic solution with a temperature T1 is replaced by an electrolytic solution with a temperature T2 ‰  T1 for the implementation of a subsequent process step. The present invention further relates to a device for carrying out this method.

Description

The present invention relates to a method and a device for electroplating a surface coating, in particular a chromium coating.

For various technical applications, it is desirable or even necessary to use machine components with special surface properties. Examples include filament-leading components in the textile and carbon fiber area, rollers and rollers in the printing area, rollers in feed machines in the sheet metal industry, as well as temper rolling for texturing of sheets for example, the automotive industry called.

A suitable method for providing such surface coatings is a chrome plating of a corresponding component.

In the EP-0 565 070 B1 and the EP-0 722 515 B1 describes a method of galvanic surface coating, with which a chromium coating is applied under conditions of a specific current conduction galvanically applied to the surface of a substrate. This process has meanwhile been established in the market as a TOPOCROM ^® process. With the TOPOCROM ^® process, a chromium coating in various variations can be easily applied without the need for mechanical or chemical post-treatments of the coated surface.

In an exemplary embodiment, the TOPOCROM ^® process is carried out in a galvanic bath containing a chromium electrolyte, such as a sulphate of chromium electrolyte. The component to be coated forms the Cathode. In addition, an anode (for example made of platinum-plated titanium) is immersed in the galvanic bath. By applying direct current, a chromium layer is deposited on the component acting as a cathode.

That in the EP-0 565 070 B1 and the EP-0 722 515 B1 TOPOCROM ^® process described works very successfully and reliably. However, it has been shown that litigation could be further optimized or adjusted by authorities as a result of changing requirements. Thus, in the European Union area, the use of chromic acid containing compositions is becoming increasingly critical due to the high toxicity of Cr (VI) compounds. A completely closed, emission and wastewater-free process management with the most efficient recycling of the electrolyte would therefore be desirable or could possibly be required in the future.

It was the object of the present invention to provide an improved method for electroplating a surface coating, in particular a chromium coating, on a machine component.

The above object is solved by the subject matter of the independent claims.

More particularly, the present invention relates to a method for electroplating a surface coating, in particular a chromium coating, on a body, for example a machine component, wherein a layer of a compound which can be oxidized by an inserted electrolyte solution is applied to the body before the surface coating is applied by electroplating, preferably a polyhydroxy compound having a viscosity of at least 1000 mPas at 25 ° C is applied.

Methods for electroplating a surface coating are well known. Basically, this is an electrochemical process in which electrodes are placed in an electrolyte bath. If direct current is applied to the electrodes, a redox reaction (electrolysis) occurs and an associated generation of chemical elements or compounds at the electrodes occurs.

In the case of a chrome plating a surface is used as the electrolyte, a chromic acid-containing solution. Chromic acid (H ₂ CrO ₄ ) forms in dilute aqueous solutions of CrO ₃ . The reduction of the Cr (VI) ions from the electrolyte to the element Cr succeeds in the presence of a catalyst. Usually, sulfuric acid (H ₂ SO ₄ ) is used alone or together with hydrofluoric acid, complex fluorides or an aliphatic sulfonic acid having one to three carbon atoms (preferably methanesulfonic acid). Common electrolyte solutions contain, for example, 250 g CrO ₃ and 2.5 g sulfuric acid in 1 l water, or 200-300 g CrO ₃ , 1.9-3.3 g H ₂ SO ₄ and 1.5-12 g methanesulfonic acid in 1 l Water.

When chromium plating an anode of lead or preferably of platinized titanium can be used as the anode.

When chromium plating, the body to be coated with chromium is used as the cathode. Basically, any body can be used as the cathode, which can be coated with chromium. According to the invention, the body to be coated is preferably a machine component, for example conveyor rollers for the textile and carbon fiber sector, for rolls and rolls in the printing field, rolls in feed machines in the sheet metal industry, as well as temper rolling for texturing of sheets for, for example, the automotive industry.

Such bodies are usually made of iron or steel, but may also consist of other materials.

According to the invention, the body to be coated is preferably a rotationally symmetrical body which can be rotated during the galvanic process to achieve a uniform surface coating.

The chrome plating is usually carried out with a direct current of 10 to 200 A / dm ² , preferably 25 to 150 A / dm ² and more preferably 30 to 100 A / dm ² . Particular preference is given to using a current guide, as used in the EP-0 565 070 B1 and the EP-0 722 515 B1 is described, ie by means of a DC application method, wherein by means of at least one initial pulse of the electrical voltage and / or the electric current on the surface to be coated nucleations of the deposition material are achieved and that subsequently by means of at least one subsequent pulse growth of Abscheidematerialkeime by addition of further deposition material is brought about, wherein during the nucleation phase, the increase of the electrical voltage and / or the electric current takes place in several stages, the time between the increases between 0.1 and 30 seconds, whereby current density changes in steps of 1 to 6 mA / cm ² ,

The body functioning as a cathode usually undergoes several pretreatment stages prior to use in the described electrodeposition process. In particular, the chrome plating of surfaces is difficult and runs with low current yields in the range of only about 15-20%. For the chromium deposition, a high current density (overvoltage) is required, whereby the reduction to elemental chromium at the cathode in competition with the formation of hydrogen (from the H ₃ O ⁺ ions of the acidic aqueous electrolyte solution) as well as the formation of Cr ³⁺ ions from the chromic acid stands. The current density required for the chromium deposition depends, inter alia, on the cathode material and the nature of the surface of the cathode material. In order to reduce the current density required for the chromium deposition, cathode materials are usually mechanically pretreated, for example by grinding or sandblasting, in order to obtain the smoothest possible surface. Additional chemical and / or electrochemical pretreatment stages usually follow. The entire pre-treatment of the body to be coated requires several separate pretreatment baths, it waste water and it must be taken costly measures for protection at work.

With the present invention, a simple but very advantageous method for pretreatment of the body to be coated is provided. According to the invention, the surface of the body to be coated is provided with a layer of a compound oxidizable by an inserted electrolyte solution, preferably a polyhydroxy compound, with a viscosity of at least 1000 mPas at 25 ° C.

According to the invention, each compound can be used for the pretreatment, which is oxidizable on the one hand by an inserted electrolyte solution, but on the other hand is sufficiently viscous that it dwells sufficiently long on the surface of the body to be coated and does not flow off the surface too quickly, ie forms a surface film , Cr (VI) compounds are known to be strong oxidants and can, for example, oxidize alcohols. It has been found according to the invention that polyhydroxy compounds, ie chemical compounds having at least two hydroxyl groups, are very suitable for the pretreatment according to the invention, provided they have a sufficient viscosity. According to the invention, the polyhydroxy compound is preferably selected from the group consisting of glycerol, carbohydrates, such as glucose, fructose or sucrose, preferably glucose, and certain polyalkylene oxides, such as polyethylene glycol. According to the invention, polyalkylene oxides which are liquid at room temperature or solutions of polyalkylene oxides such as, for example, polyethylene glycol 1500 (from Merck) can be used. Glycerol or polyethylene glycol 1500 is preferred according to the invention.

The compound to be used for the pretreatment must be sufficiently viscous that it lingers sufficiently long on the surface of the body to be coated and does not flow off the surface too quickly. According to the invention, the compound to be used should have a viscosity of at least 1000 mPas at 25 ° C. According to the invention, this is a dynamic viscosity which is measured using a conventional rotary viscometer (Searle system) according to DIN 53 019-1; 2008-09 at 25 ° C is determined.

According to the invention, the upper limit of the viscosity of the compound to be used for the pretreatment is not critical. According to the invention, a compound to be used for the pretreatment preferably has a viscosity of from 1000 mPas to 6000 mPas, preferably from 1200 to 4500 mPas, at 25 ° C.

The compound to be used for the pretreatment can be used manually or advantageously with a cleaning cloth impregnated with the substance be applied by machine to the surface of the body to be coated. The application is preferably carried out with the aid of an orbital sander, which is provided with the compound to be used for the pretreatment and moved uniformly over the surface of the body to be coated.

The pretreatment step according to the invention leads to various unexpected advantages.

By this pretreatment, the otherwise customary, previously described elaborate pretreatment becomes obsolete. The body to be coated can be subjected to the galvanic coating process after any mechanical pretreatment such as grinding or sandblasting without additional complex chemical and / or electrochemical pretreatment. According to the invention, the body to be coated is preferably cleaned with an alcohol, preferably ethanol, before the pretreatment process according to the invention. For example, wipes impregnated with alcohol may be provided and passed over the surface of the body to be coated manually or by means of a suitable machine. As a result, any residues on the surface due to mechanical pretreatment such as grinding or sandblasting are removed.

Due to the elimination of the usual complex chemical and / or electrochemical pretreatment eliminates a significant portion of wastewater to be disposed of, and no costly measures for protection at work must be taken because the inventive pretreatment can be carried out with safely manageable, non-hazardous chemical substances.

It has been found according to the invention that a very effective activation of the surface of the body to be coated is achieved by the pretreatment according to the invention. Without wishing to be bound by theory, presumably already in the de-energized state, ie before the beginning of the actual electrodeposition, a chemical reaction between the electrolyte, preferably a chromic acid electrolyte, and the oxidizable layer on the body to be coated occurs. In the case of a chromic acid electrolyte, this reaction is likely to result in the formation of a Cr ³⁺ ion-containing layer on the surface of the body to be coated.

This layer obviously supports the subsequent deposition of chromium during the galvanic process, which can be concluded from the fact that due to the pretreatment according to the invention, an otherwise usual polarity reversal of the electrodes for activating the surface of the body to be coated is no longer necessary. This represents a considerable advantage, since iron ions (in the case of a body to be coated made of iron) or other foreign ions are formed during a conventional polarity reversal of the electrodes and pass into the electrolyte. This leads to increasing contamination of the electrolyte and requires its relatively early replacement. With the method according to the invention, by contrast, this reversal step is eliminated, which lengthens the life of the electrolyte extraordinarily. Especially with regard to the expected regulatory tightening when dealing with Cr (VI) -containing compositions, this is of considerable importance.

In addition, it is possible because of the elimination of Umpolungsschrittes, cheaper Rectifier (not umpolbare rectifier) use.

Finally, it has been found that due to the inventive pretreatment better adhering chromium coatings can be produced. This is due to the fact that due to the initial chemical reaction in the de-energized state, a uniform, Cr ³⁺ -ions containing layer is formed on the surface of the body to be coated, which then leads to the formation of a current to form a uniform chromium layer. In contrast, chromium deposition under galvanic conditions has proven to be less effective and disadvantageous.

With conventional plating process, such as a chromium plating process, such as the TOPOCROM ^® process, several layers of metal, preferably chromium layers deposited one above the other. For example, a base layer is first applied in one embodiment of the TOPOCROM ^® process which is little cracking and has a thickness of preferably 25 to 40 .mu.m, in particular 30 microns. Subsequently, a so-called structural layer can be applied to this base layer. For example, when TOPOCROM ^® process the structured chromium layer formed therein comprises hemispherical dome. A cover layer for protecting the structure layer can subsequently also be applied to the structure layer, which preferably has a thickness of preferably 2 to 20 μm, more preferably 3 to 15 μm and in particular 4 to 10 μm. The preparation of such a three-layer structure of chromium is for example in the EP-0 565 070 B1 and the EP-0 722 515 B1 described.

For deposition of the various chromium layers, it is necessary to vary the temperature of the electrolyte as a function of the layer to be deposited. Usually, the heating of the electrolyte takes place directly in the electrolysis reactor, for example by external heating elements. However, this is disadvantageous for a completely closed process management which is desirable for environmental reasons and due to regulatory requirements. For the adjustment of the temperature of the electrolyte to the desired process temperature, a relatively high effort and time is required. External heating can lead to undesirable side reactions in the electrolysis reactor and the electrolyte used has a shorter service life. These disadvantages are also overcome by the present invention.

The present invention furthermore relates to a process for the electroplating of a surface coating, in particular a chromium coating, onto a body, for example a machine component, wherein the surface coating is carried out in a preferably closed reactor in an at least two-stage, preferably three-stage process, characterized in that an electrolyte solution containing in the reactor with a temperature T1 for the implementation of a subsequent process stage is replaced by an electrolyte solution with a temperature T2 ≠ T1.

With the method according to the invention it is possible to design the entire galvanic process in a closed reactor, wherein the galvanic process can be used to form a multi-layer structure. A multilayer structure here means the production of at least two, preferably three, but optionally also more layers one above the other on the surface of a body to be coated.

With the method according to the invention, an error-free multilayer coating is realized on the surface of a body to be coated, without the body being removed from the reactor must be removed. The process can be operated in compliance with the currently expected tightening of regulatory requirements without waste water and emissions (ie without pollution at the workplace, the exhaust air from the reactor is discharged through a closed system, cleaned and then easily discharged). The process is very gentle with respect to the electrolytes used. The electrolytes used have a very long lifetime, which is of considerable importance, in particular with regard to the expected regulatory tightening when handling Cr (VI) -containing compositions.

According to the present invention, the individual process stages are not realized by heating or cooling a single electrolyte contained in the reactor. Rather, according to the invention, an electrolyte solution having a temperature T1 for the next process stage is replaced by an electrolyte solution having a temperature T2 ≠ T1, i. There is an exchange of electrolyte solutions instead.

According to the invention, the replacement is preferably carried out by replacing the electrolyte solution with a temperature T1 by an electrolyte solution having a temperature T2≠T1 by introducing the electrolyte solution having a temperature T2≠T1 into the reactor and causing the electrolyte solution to extrude at a temperature T1 ,

This can be achieved, for example, by arranging at least one inlet for an electrolyte solution having a temperature T2≠T1 in the bottom area of the reactor or in the lower area, preferably in the lower third, particularly preferably in the lower quarter of the reactor for carrying out the galvanic process. Through this inlet can electrolyte solution be introduced at a temperature T2 ≠ T1 from a reservoir into the reactor, for example by means of a pump. The inlet is preferably equipped with a locking device, for example a valve or a door. At the same time, at least one outlet opening is arranged in the upper region, preferably in the upper third and particularly preferably in the upper fourth of the reactor. If now the inlet is opened into the reactor and electrolyte solution with a temperature T2 ≠ T1 introduced into the reactor, this electrolyte displaces the existing electrolyte in the reactor with a temperature T1, wherein the electrolyte is led out with the temperature T1 through the outlet from the reactor. The outlet may be equipped with a locking device, such as a valve or a door. Alternatively, the outlet can also be designed as an overflow system, ie at normal electrolyte level in the reactor, the outlet is above the electrolyte. Only by supplying electrolyte solution with a temperature T2 ≠ T1 in the reactor, the electrolyte level in the reactor is raised so that it reaches the outlet and can flow through it from the reactor.

According to the invention, the various electrolyte solutions are preferably stored in separate containers and adjusted to the desired temperature outside the reactor for carrying out the galvanic process. It may be in the containers and conventional liquid tanks, which are resistant to the electrolyte used. The adjustment of the temperature of the electrolyte can be carried out in a known manner, for example by heating elements.

The electrolyte containers are connected via connecting lines, preferably pipes, to the reactor for carrying out the galvanic process. The from the different electrolyte containers coming pipes can be fed via separate inlets into the reactor. However, it is also possible to combine the tubes coming from the various electrolyte containers in front of the reactor and to lead them via a single inlet into the reactor. In the latter case, prior to the point of convergence of the tubes in the individual tubes, blocking devices should be provided, for example a valve or a door, to allow selective introduction of a given electrolyte solution into the reactor.

Analogously, the outlet or the outlets from the reactor are connected via connecting lines, preferably tubes, to the respective electrolyte containers. The tubes leading into the various electrolyte containers can be connected to the interior of the reactor via separate outlets in the upper region of the reactor. However, it is also possible to combine the tubes leading into the different electrolyte containers outside the reactor and to connect them to the interior of the reactor via a single outlet. In the latter case, locking means, such as a valve or door, are provided in the individual tubes prior to the point of fusion of the tubes, to allow selective transfer of a particular electrolyte solution from the reactor to the container intended for that electrolyte solution.

It is further preferred according to the invention that the electrolyte solution contained in the reactor during a process stage is circulated continuously by discharging out of the reactor and replacing it with the same electrolyte solution. This can be done, for example, in that for this electrolyte solution, an inlet into the reactor and an outlet from the reactor can be flowed through (preferably by opening corresponding blocking devices) and this electrolyte solution, for example is continuously circulated by operating a circulation pump. This ensures a constant quality of the electrolyte solution in the reactor.

The present invention thus further relates to a device for electroplating a surface coating, in particular a chromium coating, in particular for carrying out a method described above, comprising a reactor for receiving a body to be coated, for example a machine component, an anode and at least two, preferably two, electrolyte containers. characterized in that the electrolyte containers (are connected via connecting lines through separate inlets and outlets to the interior of the reactor.

The inventive method is particularly preferably designed such that the surface coating is carried out in a three-stage process, wherein the first process stage is performed in the reactor with an electrolyte solution having a temperature T1, then the second process stage with an electrolyte solution having a temperature T2 ≠ T1 is performed , and the third process stage is performed with an electrolyte solution having a temperature T3 ≠ T2. It is particularly preferred here that the temperature T3 is equal to the temperature T1. According to a preferred embodiment, T2 <T1, and most preferably T2 <T1 and T1 = T3.

This embodiment of the method according to the invention can be used to successively apply a base chrome layer, a structural chrome layer and a top layer in a chromium coating in three successive process steps. The current flow in these process stages can be as in the EP-0 565 070 B1 and EP-0 722 515 B1 described carried out become. In the first process stage, the deposition of the chromium basecoat, an electrolyte is introduced into the reactor having a temperature in the range of 40 to 60 ° C, preferably 45 to 55 ° C. Once the formation of the base layer is completed, this electrolyte is replaced by a second electrolyte having a lower temperature in the range of 25 to 39 ° C, preferably 30 to 38 ° C. With the aid of this second electrolyte, the deposition of the structural chromium layer is carried out. Once the formation of the structural chromium layer is completed, this electrolyte is replaced by a third electrolyte again having a higher temperature in the range of 40 to 60 ° C, preferably 45 to 55 ° C. With the help of this third electrolyte, the deposition of the top layer of chromium is carried out. If the same temperature is to be set for the first and third electrolytes, the same electrolyte can also be used for the first and third process stages.

The reactor for carrying out the galvanic process can have any desired shape. A cylindrical shape is preferred. The height and base area of the reactor can be varied depending on the body to be coated.

According to the invention, the top surface of the reactor can be opened, i. For example, be designed in the form of a lid to introduce the body to be coated in the reactor.

As described above, the reactor is provided with one or more inlets and one or more outlets for the electrolyte solutions, which are connected via respective connecting lines to the containers for the electrolyte solutions.

Furthermore, the reactor is connected via power lines to a rectifier, from which the reactor is supplied with the power required for the galvanic process. Rectifiers are known and need not be explained here. As stated above, according to the invention, it is not necessary to use reversible rectifiers, since polarity reversal is not required for the method according to the invention. Advantageously according to the invention therefore cheaper, non-reversible rectifier can be used.

Within the reactor, an anode is fixed. As described above, an anode of platinized titanium is preferably used in the process according to the invention. Lead electrodes can indeed be used in many cases, but have some disadvantages.

In the operating state, the body functioning as the cathode to be coated is arranged in the reactor such that its surface is at a distance from the anode in the range from 5 to 80 cm, preferably 30 to 60 cm.

In principle, as described above, any body can be used as the cathode, which can be coated with the process according to the invention, preferably can be coated with chromium. According to the invention, the body to be coated is preferably a component of a machine, for example conveyor rollers for the textile and carbon fiber area, rollers and rollers in the printing area, rollers in drawing machines in the sheet metal industry, as well as temper rolling for texturing of metal sheets for example the automotive industry.

Such bodies are usually made of iron or steel, but may also consist of other materials.

According to the invention, the body to be coated is preferably a rotationally symmetrical body which can be rotated during the galvanic process to achieve a uniform surface coating.

For this purpose, the reactor is preferably equipped with a motor for rotating the body. According to the invention, the motor is preferably arranged on the top surface of the reactor and can be connected in a simple manner, for example by means of a plug connection, to the body to be coated.

According to the invention, the galvanic process is preferably carried out while rotating the rotationally symmetrical body to be coated.

It is particularly preferred according to the invention that both measures described here are combined with one another, ie. the galvanic process is carried out in an at least two-stage, preferably three-stage process, wherein an electrolyte solution containing in the reactor with a temperature T1 for the implementation of a subsequent process stage is replaced by an electrolyte solution having a temperature T2 ≠ T1, and wherein the body before the galvanic Applying the surface coating, a layer of a compound oxidizable by an inserted electrolyte solution, preferably a polyhydroxy compound, having a viscosity of at least 1000 mPas at 25 ° C is applied.

As described above, the polyhydroxy compound according to the invention is preferably selected from the group consisting of Glycerin, carbohydrates, and certain polyalkylene oxides such as polyethylene glycol, for example, polyethylene glycol 1500 (ex Merck). According to the invention, polyalkylene oxides which are liquid at room temperature or solutions of polyalkylene oxides can be used. Glycerol or polyethylene glycol 1500 is preferred according to the invention.

The pretreatment can be carried out as described above.

It is furthermore preferred according to the invention for the reactor to be operated during the surface coating by means of a ventilation system for removing emerging gases. During the execution of the galvanic process, hydrogen is formed at the cathode and oxygen at the anode. To avoid the formation of a blast gas mixture, the gaseous atmosphere in the reactor is preferably removed, for example by means of a suction pump, continuously or at certain times.

Once the preferably pretreated according to the invention pretreated body to be coated in the reactor and the reactor was closed, the entire inventive method can be carried out in a completely closed system. All process parameters and process steps, such as the current regulation, the supply and removal of the various electrolyte solutions, if appropriate, the suction of the reactor atmosphere, can be monitored and carried out with the aid of an electronic control unit.

After completion of the galvanic deposition process, the entire electrolyte solution is removed from the reactor and the coated body preferably with water or an aqueous Cleaning solution cleaned. Only then is the reactor opened to remove the coated body. There is no emission burden throughout the process. The electrolyte used is stored in sealed containers and has a very long shelf life.

The present invention is by way of non-limiting

Figures and examples explained in more detail.

Fig. 1

eine schematische Darstellung einer erfindungsgemässen Vorrichtung zur Durchführung des erfindungsgemässen Verfahrens

It shows:

Fig. 1

a schematic representation of an inventive device for carrying out the inventive method

example 1

Fig. 1 is a schematic representation of an inventive device for carrying out the inventive method. The device 1 comprises a reactor 2 for carrying out the galvanic process. The reactor 2 is closed by a removable cover 3.

In the reactor 2 is a body to be coated 4, preferably a rotationally symmetrical body, introduced as a cathode. Furthermore, an anode 5 is arranged in the reactor 2, which preferably consists of platinum-plated titanium. The body 4 to be coated is connected to the lid 3 via a rotatable rod 6.

Electrolyte solution can be introduced into the reactor 2 from the electrolyte containers 7, 8 via connection lines 7a, 8a. In the Fig. 1 are only two containers 7, 8 with respective connecting lines 7a, 8a shown; however, additional containers and connecting lines may be provided as needed. The connecting lines 7a, 8a can be opened and closed by means of blocking devices 7b, 8b, which are preferably valves, so that only a specific electrolyte selectively reaches the reactor 2.

The connecting lines 7 a, 8 a terminate in inlets arranged in the bottom surface of the reactor 2. In the upper third of the reactor 2 outlets are arranged, via which drain electrolyte and can flow back via connecting lines 7c, 8c in the electrolyte container 7,8. The connection lines 7c, 8c can be opened and closed by means of blocking devices 7d, 8d, which are preferably valves, so that only a specific electrolyte from the reactor 2 reaches the intended electrolyte container 7, 8 in a targeted manner.

For conveying the electrolyte through the conduits 7a, 7c, 8a, 8c, pumps (not shown) are provided.

A rectifier 9 operated with alternating voltage supplies the cathode 4 and anode 5 with the direct current required for the process via power lines 9a, 9b.

The device 1 is controlled by means of an electronic processing unit (not shown).

According to the invention, the rotationally symmetrical body is preferably pretreated before it is introduced into the reactor 2. After a mechanical surface treatment, for example by grinding or sandblasting, the surface of the body 4 is first cleaned with a soaked in ethanol cleaning cloth. Subsequently, by means of an orbital sander a film of polyethylene glycol 1500 (from Merck) is applied to the surface of the body 4.

The thus pretreated body 4, for example a steel cylinder, is brought into the reactor 2, and the reactor 2 is closed with the lid 3. Now, a mixture of 250 g of CrO ₃ and 2.5 g of sulfuric acid in 1 l of water is pumped from the container 7 as an electrolyte into the reactor 2. The electrolyte is previously heated to 50 ° C. The body 4 is rotated, current is applied, and a first chromium layer is formed. During this first process stage, the blocking devices 7b and 7d are opened and the blocking devices 8b, 8d are closed, and the electrolyte from the container 7 is continuously circulated.

After completion of the first process stage, the locking device 7b is closed and for the locking device 8b opened. The locking device 7d remains open while the locking device 8d is closed. Now a mixture of 250 g of CrO ₃ and 2.5 g of sulfuric acid in 1 l of water is pumped from the container 8 as an electrolyte into the reactor 2. The electrolyte is previously heated to 37 ° C. The electrolyte from the container 8 forces the warmer electrolyte originating from the container 7 back into the container 7 via the line 7c. As soon as the electrolyte from the container 7 has been completely displaced from the reactor 2, the blocking device 7d is closed and the blocking device 8d is opened. In the reactor 2 is now the electrolyte from the container 8. The body 4 is rotated, it is applied current, and it forms a second chromium layer (structure layer). During this second process stage, the blocking devices 8b and 8d are opened and the electrolyte from the container 8 is continuously circulated.

After completion of the second process stage, the locking device 8b is closed and for the locking device 7b opened. The locking device 8d remains open while the locking device 7d is closed. Now, a mixture of 250 g of CrO ₃ and 2.5 g of sulfuric acid in 1 l of water is pumped from the container 7 as an electrolyte into the reactor 2. The electrolyte is previously heated to 50 ° C. The electrolyte from the container 7 forces the warmer electrolyte originating from the container 8 back into the container 8 via the line 8c. As soon as the electrolyte from the container 8 has been completely displaced from the reactor 2, the blocking device 8d is closed and the blocking device 7d is opened. In the reactor 2 is now the electrolyte from the container 7. The body 4 is rotated, it is applied current, and it forms a third chromium layer (cover layer). During this third process stage, the blocking devices 7b and 7d are opened, and the electrolyte from the container 7 is continuously circulated.

During all process stages, the gas atmosphere in the reactor 2 can be sucked off by means of a pump (not shown) in order to prevent the formation of a blast gas mixture.

After completion of the third process stage, the locking device 7b is closed while the locking device 7d remains open. The entire electrolyte is removed from the reactor 2. The coated body 4 is cleaned with water or an aqueous solution which is introduced from a line (not shown) into the reactor 2. The cleaning water is then discharged from the reactor 2 and cleaned. The reactor 2 is now opened and the coated body 4 is removed.

Claims (15)

Method for the galvanic application of a surface coating, in particular a chromium coating, to a body (4), for example a machine component, characterized in that a layer of a compound oxidizable by an inserted electrolyte solution, preferably a polyhydroxy compound, is applied to the body before the surface coating is applied by electroplating, with a viscosity of at least 1000 mPas at 25 ° C is applied. A method according to claim 1, characterized in that the polyhydroxy compound is selected from the group consisting of glycerol, carbohydrates and polyethylene glycol. A method according to claim 1 or 2, characterized in that the body (4) is cleaned before applying the layer of a polyhydroxy compound with an alcohol, preferably ethanol. Process for the galvanic application of a surface coating, in particular a chromium coating, to a body (4), for example a machine component, wherein the surface coating is carried out in a preferably closed reactor (2) in an at least two-stage, preferably three-stage process, characterized in that an electrolyte solution containing a temperature T1 for carrying out a subsequent process stage in the reactor (2) is replaced by an electrolyte solution having a temperature T2 ≠ T1. A method according to claim 4, characterized in that the replacement of the electrolyte solution at a temperature T1 by an electrolyte solution having a temperature T2 ≠ T1 by introducing the electrolyte solution having a temperature T2 ≠ T1 in the reactor (2) and thereby forced out the electrolyte solution with a Temperature T1 occurs. A method according to claim 4 or 5, characterized in that during a process stage in the reactor (2) contained electrolyte solution is continuously circulated by herausbefördern from the reactor (2) and replacing with the same electrolyte solution. Method according to one of claims 4 to 6, characterized in that the surface coating is carried out in a three-stage process in the reactor (2), wherein the first process stage is carried out with an electrolyte solution having a temperature T1, then the second process stage with an electrolyte solution with a Temperature T2 ≠ T1 is performed, and the third process stage is performed with an electrolyte solution having a temperature T3 ≠ T2. A method according to claim 7, characterized in that the temperature T3 is equal to the temperature T1. Method according to one of claims 4 to 8, characterized in that on the body (4) prior to the electroplating of the surface coating, a layer of an oxidizable by an inserted electrolyte solution compound, preferably a polyhydroxy compound having a viscosity of at least 1000 mPas at 25 ° C is applied. Method according to one of claims 4 to 9, characterized in that the polyhydroxy compound is selected from the group consisting of glycerol, carbohydrates, preferably glucose, and polyethylene glycol. Method according to one of claims 4 to 10, characterized in that the body (4) before the application of the layer of a polyhdryoxy compound with an alcohol, preferably ethanol, is purified. Method according to one of claims 4 to 11, characterized in that the body (4) is rotationally symmetrical. A method according to claim 12, characterized in that the body (4) rotates during the surface coating. Method according to one of claims 4 to 13, characterized in that during the surface coating of the reactor (2) is operated by means of a ventilation system for the removal of emerging gases. Apparatus for electroplating a surface coating, in particular a chromium coating, in particular for carrying out a method according to one of claims 4 to 14, comprising a reactor (2) for receiving a body (4) to be coated, for example a machine component, an anode (5) and at least two, preferably two, electrolyte containers (7, 8), characterized in that the electrolyte containers (7, 8) are connected via connecting lines (7a, 7c, 8a, 8c) to the interior of the reactor (2) through separate inlets and outlets.

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