Method and device for the galvanic application of a surface coating

Abstract

A method for galvanic application of a surface coating, in particular a chromium coating, to a body, for example a machine component. Before the galvanic application of the surface coating, a layer of a compound that can be oxidized by an electrolyte solution that is used, preferably a polyhydroxy compound with a viscosity of at least 1000 mPas at 25° C., is applied to the body. A method for galvanic application of a surface coating, in particular a chromium coating, to a body, for example a machine component, wherein the surface coating is carried out in a closed reactor in an at least two-stage, preferably three-stage process, is also disclosed. An electrolyte solution contained in the reactor at a temperature T1 for carrying out a subsequent process stage is substituted by an electrolyte solution at a temperature T2≠T1. A device for carrying out this method is also disclosed.

Claims

1. A process for electrochemical application of a surface coating to a body, said surface coating being a chromium coating, where the electrochemical application of the surface coating is carried out in a reactor in at least a two-stage process comprising the steps of: providing a chromic acid containing solution having a temperature T1 as a first electrolyte solution, providing a chromic acid containing solution having a temperature T2≠T1 as a second electrolyte solution, electrochemically applying in a first process step a first portion of said surface coating being said chromium coating using said first electrolyte solution having the temperature T1, and thereafter electrochemically applying in a second process step a second portion of said surface coating being said chromium coating using said second electrolyte solution having the temperature T2≠T1, wherein said first electrolyte solution having the temperature T1 present in the reactor is replaced by the second electrolyte solution having the temperature T2≠T1 for carrying out said second process step, wherein said reactor is a closed reactor, and the replacement of the first electrolyte solution having the temperature T1 by the second electrolyte solution having the temperature T2≠T1 is carried out by introduction of the second electrolyte solution having the temperature T2≠T1 into the reactor through an inlet being arranged in a bottom plate of the reactor or in a lower third of the reactor, and resulting displacement of the first electrolyte solution having the temperature T1, due to introduction of the second electrolyte solution into the reactor and pushing the first electrolyte solution out of the reactor, through an outlet being arranged in an upper third of the reactor.

2. The process as claimed in claim 1, wherein said surface coating is carried out in a three-stage process.

3. The process as claimed in claim 2, wherein the first process step, of said three-stage process, is carried out using the first electrolyte solution having the temperature T1, the second process step is subsequently carried out using the second electrolyte solution having the temperature T2≠T1, and a third process step is carried out using an electrolyte solution having a temperature T3≠T2.

4. The process as claimed in claim 3, wherein the temperature T3 is equal to the temperature T1.

5. The process as claimed in claim 1, wherein during said electrochemical application of said surface coating, the electrolyte solution present in the reactor is continuously circulated by discharging the electrolyte solution from the reactor and replacing the electrolyte solution with the same electrolyte solution.

6. The process as claimed in claim 1, wherein a layer of a compound, which can be oxidized by the first or the second electrolyte solution and which has a viscosity of at least 1000 mPas at 25° C., is applied to the body before the electrochemical application of the surface coating.

7. The process as claimed in claim 6, wherein the compound to be applied to the body, before the electrochemical application of the surface coating, is a polyhydroxy compound.

8. The process as claimed in claim 7, wherein the polyhydroxy compound is selected from the group consisting of glycerol, carbohydrates and polyethylene glycol.

9. The process as claimed in claim 7, wherein the body is cleaned by an alcohol before said application of the layer of said polyhydroxy compound to said body.

10. The process as claimed in any of claim 1, wherein the body is rotationally symmetric.

11. The process as claimed in claim 10, wherein the body rotates during the electrochemical application of said surface coating.

12. The process as claimed in claim 1, wherein, during the surface coating, a ventilation system removes formed gases from the reactor.

Description

(1) The present invention will be illustrated with the aid of nonlimiting figures and examples.

A BRIEF DESCRIPTION OF THE DRAWINGS

(2) The FIGURE shows:

(3) FIG. 1 a schematic depiction of an apparatus according to the invention for carrying out the process of the invention

EXAMPLE 1

(4) FIG. 1 is a schematic depiction of an apparatus according to the invention for carrying out the process of the invention. The apparatus 1 comprises a reactor 2 for carrying out the electrochemical process. The reactor 2 is closed by a lid 3 which can be taken off.

(5) A body 4 to be coated, preferably a rotationally symmetric body, is introduced as cathode into the reactor 2. Furthermore, an anode 5 which preferably consists of platinated titanium is arranged in the reactor 2. The body 4 to be coated is connected via a rotatable rod 6 to the lid 3.

(6) Electrolyte solution from the electrolyte containers 7, 8 can be introduced via connecting conduits 7a, 8a into the reactor 2. In FIG. 1, only two containers 7, 8 with respective connecting conduits 7a, 8a are shown; however, additional containers and connecting conduits can also be provided if required. The connecting conduits 7a, 8a can be opened and closed by means of shut-off devices 7b, 8b, which are preferably valves, so that only one particular electrolyte goes, in a targeted manner, into the reactor 2.

(7) The connecting conduits 7a, 8a end in inlets which are arranged in the bottom plate of the reactor 2. Outlets via which electrolyte can flow out and flow back via connecting conduits 7c, 8c into the electrolyte containers 7, 8 are arranged in the upper third of the reactor 2. The connecting conduits 7c, 8c can be opened and closed by means of shut-off devices 7d, 8d, which are preferably valves, so that only one particular electrolyte goes, in a targeted manner, from the reactor 2 into the electrolyte container 7, 8 provided.

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

(9) A rectifier 9 operated using an alternating voltage supplies the cathode 4 and anode 5 with the direct current necessary for the process via electric conductors 9a, 9b.

(10) The apparatus 1 is controlled by means of an electronic process control unit (not shown).

(11) According to the invention, the rotationally symmetric 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 firstly cleaned using a cleaning cloth impregnated with ethanol. A film of polyethylene glycol 1500 (from Merck) is subsequently applied to the surface of the body 4 by means of a vibratory grinder.

(12) The body 4, for example a steel cylinder, which has been pretreated in this way is introduced into the reactor 2 and the reactor 2 is closed by means of the lid 3. A mixture of 250 g of CrO.sub.3 and 2.5 g of sulfuric acid in 1 l of water is then pumped as electrolyte from the container 7 into the reactor 2. The electrolyte is heated to 50° C. beforehand. The body 4 is rotated, electric current is applied and a first chromium layer is formed. During this first process step, the shut-off devices 7b and 7d are opened and the shut-off devices 8b, 8d are closed, and the electrolyte from the container 7 is circulated continuously.

(13) After the first process step is complete, the shut-off device 7b is closed and the shut-off device 8b is opened instead. The shut-off device 7d remains open, while the shut-off device 8d is closed. A mixture of 250 g of CrO.sub.3 and 2.5 g of sulfuric acid in 1 l of water is then pumped as electrolyte from the container 8 into the reactor 2. The electrolyte is heated to 37° C. beforehand. The electrolyte from the container 8 displaces the hotter electrolyte originating from the container 7 back into the container 7 via the conduit 7c. As soon as the electrolyte from the container 7 has been completely displaced from the reactor 2, the shut-off device 7d is closed and the shut-off device 8d is opened. The electrolyte from the container 8 is now present in the reactor 2. The body 4 is rotated, electric current is applied and a second chromium layer (structured layer) is formed. During this second process step, the shut-off devices 8b and 8d are opened, and the electrolyte from the container 8 is recirculated continuously.

(14) After the second process step is complete, the shut-off device 8b is closed and the shut-off device 7b is opened instead. The shut-off device 8d remains open, while the shut-off device 7d is closed. A mixture of 250 g of CrO.sub.3 and 2.5 g of sulfuric acid in 1 l of water is then pumped as electrolyte from the container 7 into the reactor 2. The electrolyte is heated to 50° C. beforehand. The electrolyte from the container 7 displaces the hotter electrolyte originating from the container 8 back into the container 8 via the conduit 8c. As soon as the electrolyte from the container 8 has been completely displaced from the reactor 2, the shut-off device 8d is closed and the shut-off device 7d is opened. The electrolyte from the container 7 is then present in the reactor 2. The body 4 is rotated, electric current is applied, and a third chromium layer (covering layer) is formed. During this third process step, the shut-off devices 7b and 7d are opened, and the electrolyte from the container 7 is circulated continuously.

(15) During all process steps, the gas atmosphere in the reactor 2 can be drawn off by means of a pump (not shown) in order to prevent formation of a hydrogen/oxygen gas mixture.

(16) After the third process step is complete, the shut-off device 7b is closed, while the shut-off device 7d remains open. The entire electrolyte is removed from the reactor 2. The coated body 4 is cleaned using water or an aqueous solution which is introduced from a conduit (not shown) into the reactor 2. The cleaning water is subsequently discharged from the reactor 2 and purified. The reactor 2 is then opened and the coated body 4 is taken out.