EP1097261B1 - Galvanic bath, method for producing structured hard chromium layers and use thereof - Google Patents

Abstract

The present invention relates to a galvanic bath, to a process for the precipitation of chromium onto objects, and to the use of the process for the generation of textured hard-chrome layers on machine components. The galvanic bath contains in aqueous solution at least one compound delivering chromium(VI)-ions, and it comprises a) chromium(VI)-ions in an amount that corresponds to 100 to 600 g/ltr of chromic acid anhydride, b) sulfate ions in the form of sulfuric acid and/or of a soluble salt thereof in a molar concentration ratio of chromium(VI)-ions to sulfate ions (SO4-2) ranging from 90:1 to 120:1, and c) 2-hydroxyethane sulfonate ions in an amount that corresponds to 0.01 to 3.0 g/ltr of the sodium salt.

Description

The present invention relates to a galvanic bath and a method for producing structured hard chrome layers and its use for producing structured hard chrome layers on components. It has long been common state of the art to provide objects of technology or general use with surface coatings by means of galvanic processes. This is necessary in order to give the objects special functional and / or decorative surface properties, such as hardness, corrosion resistance, metallic appearance, gloss, etc. In the case of galvanic surface coating, a bath that contains at least the metal to be deposited as a salt is converted into metal deposited by means of direct current on the object connected as cathode. The object to be coated usually consists of a metallic material. If the base material is not electrically conductive per se instead, the surface can be made conductive by a thin metallization. Galvanic baths containing nickel or chrome are mostly used in technical applications to produce particularly hard, mechanically resistant layers.

In certain cases, it is necessary or desirable that objects which are provided with an electroplated hard chrome layer have a rough surface structure. With decorative tops, for example, a matt appearance or a pleasant, not smooth "handle" should be created. In the technical field, rough hard chrome layers or structured chrome layers fulfill certain functional properties. For machine components that are in sliding contact with each other, such as pistons, cylinders, liners, axle bearings, etc., rough hard chrome layers are advantageous because the structure forms lubricant deposits so that dry running is prevented. In the graphics industry, for example, printing drums, ink rollers and, in particular, dampening cylinders with a special, rough surface are required for printing machines. Chromium-plated tools can be used in forming technology to give the workpiece to be machined a structured surface.

According to conventional technology, objects with a hard chrome coating and a rough surface structure are obtained by mechanical processing, such as grinding, sandblasting, spark erosion, etc., or by chemical etching processes before, between or after the chrome plating. However, due to the large number of different working techniques required, corresponding methods are complex, complicated and cost-intensive.

DE 42 11 881 discloses a galvanic method for applying surface coatings to machine components, in which, for example, chromium is deposited in a structured form. In this case, at least one initial pulse and at least one subsequent pulse of voltage or current and certain guidance of the voltage or current function initially cause nucleation on the surface of the machine component and subsequent growth of the germs of the separating material.
The chromium is deposited in the form of statistically evenly distributed, dendritic or approximately hemispherical (dome-shaped) elevations.

EP 0 722 515 contains a further development of the method according to DE 42 11 881, the increase in electrical voltage or current density taking place in stages.

Known, conventional galvanic baths are used in these processes.

DE 34 02 554 C2 describes a saturated aliphatic sulfonic acid with a maximum of two carbon atoms and a maximum of six sulfonic acid groups or salts or halogen acid derivatives thereof for increasing the current efficiency in the electrodeposition of hard chromium on a workpiece made of steel or aluminum alloy from an aqueous, chromic acid and Use non-caustic electrolytes containing sulfuric acid.

US-A-5 176 813 discloses a process for the electrodeposition of chromium from a galvanic bath with a lead-containing anode in the absence of monosulfonic acid, the galvanic bath containing chromic acid, Contains sulfate ions and at least one optionally halogenated alkylpolysulfonic acid or its salt with 1 to 3 carbon atoms.

However, the known methods in which structured chromium layers are generated galvanically have disadvantages. They require a complicated multi-layer structure, in which a nickel strike layer is applied before the actual structural chrome layer, then a thicker sulfamate nickel layer, followed by a low-crack chrome layer on the base material of the component, and finally the structural chrome layer has to be coated with a low-crack hard chrome layer. These different layers require specific, differently composed galvanic baths and different deposition conditions matched to them. The process control is thus complex, complicated and very expensive due to the necessary work steps. Furthermore, only layers with roughness values Rz up to about 10 µm are apparently available with this method. In addition, the uniformity of the distribution and the formation of dome-shaped surveys is still in need of improvement.

The object of the present invention is therefore to simplify the production of structured hard chrome layers considerably and, in particular, to enable structure layers with a more uniform surface topography and significantly higher roughness values.

• a) Chrom(VI)-ionen in einer Menge, die 100 bis 600 g/l Chromsäureanhydrid entspricht,
• b) Sulfationen in Form von Schwefelsäure und/oder eines löslichen Salzes davon in einem molaren Konzentrationsverhältnis von Chrom(VI)-ionen zu Sulfationen (SO₄ ^2-) von 90:1 bis 120:1, und
• c) 2-Hydroxyethansuifonationen in einer Menge, die 0,01 bis 3,0 g/l des Natriumsalzes entspricht, umfaßt

It has now been found that structured hard chrome layers corresponding to the requirements can be obtained from a galvanic bath which contains at least one compound which provides chromium (VI) ions in aqueous solution and

• a) chromium (VI) ions in an amount corresponding to 100 to 600 g / l chromic anhydride,
• b) sulfate ions in the form of sulfuric acid and / or a soluble salt thereof in a molar concentration ratio of chromium (VI) ions to sulfate ions (SO ₄ ^2- ) of 90: 1 to 120: 1, and
• c) 2-hydroxyethanesulfonate ions in an amount corresponding to 0.01 to 3.0 g / l of the sodium salt, comprises

It was surprisingly found that the combination of the components sulfate and 2-hydroxyethanesulfonate according to the invention results in particularly advantageous properties of the chromium bath.

The galvanic bath according to the invention preferably contains chromium (VI) ions in an amount which corresponds to 200 to 250 g / l chromic acid anhydride. The chromium (VI) ion-providing compound is preferably selected from chromic anhydride (CrO ₃ ) and / or alkali dichromates such as Na ₂ Cr ₂ O ₇ and K ₂ Cr ₂ O ₇ . Of the alkali dichromates, K ₂ Cr ₂ O _{7 is} preferred. In a particularly preferred embodiment, the chromium (VI) ion-providing compound is chromic anhydride. In a further embodiment, part of the chromium (VI) ion-providing compound is one or more alkali dichromate (s), preferably potassium dichromate. In this embodiment, preferably less than 30% and particularly preferably less than 15% of the chromium (VI) ions are supplied by alkali dichromate.

The molar concentration ratio of chromium (VI) ions to sulfate ions in the electroplating bath is preferably 100: 1 to 105: 1. The soluble salts of sulfuric acid which can be used are preferably selected from sodium sulfate, potassium sulfate, lithium sulfate, ammonium sulfate, magnesium sulfate, strontium sulfate, aluminum sulfate and potassium aluminum sulfate. Strontium sulfate is particularly preferred

In a preferred embodiment, the bath comprises 2-hydroxyethanesulfonate ions in an amount corresponding to 0.07 to 1.5 / l of the sodium salt. The 2-hydroxyethanesulfonate ions contained in the galvanic bath according to the invention can be provided by the 2-hydroxyethanesulfonic acid itself or a salt thereof, preferably the sodium salt.

The electroplating chrome bath according to the invention can be used in the electroplating systems usually used in this technology and with the usual working methods as well as for the usual coating purposes on the normally provided basic materials. Such base materials can be, for example, objects made of conductive materials such as metal, in particular steel, and metallized, non-conductive objects.

The galvanic bath according to the invention is expediently used at temperatures between 30 and 70 ° C.

If the galvanic deposition from such a bath is carried out at a temperature of 50 50 ° C., chrome layers with a largely uniform dome-shaped microstructure and roughness values Rz of up to approximately 40 μm can be produced. Such a deposition is preferably carried out in the temperature range 40 and 50 ° C., preferably between 42 and 48 ° C. and particularly preferably between 44 and 46 ° C.

If the galvanic deposition from such a bath is carried out at a temperature of ≤ 50 ° C, smooth chrome layers with little cracks can be produced. Such a deposition is preferably carried out in the temperature range between 51 and 61 ° C., preferably between 53 and 59 ° C. and particularly preferably between 55 and 57 ° C.

It is thus possible in this way without further ado to make a three-layer build-up on the substrate from one and the same chrome bath according to the invention only by varying the bath temperature during the electrodeposition, expediently as a first layer a low-crack, smooth base layer, subsequently a structured chrome layer and finally a low-crack smooth functional layer are provided. The chrome bath according to the invention can be used to deposit directly onto the base material, for example steel. Galvanic precoatings, especially with nickel, are not necessary.

In order to deposit a structured hard chrome layer on an object, the latter, when connected as a cathode, is introduced into the galvanic bath according to the invention. It is sufficient if the object is ground to size. Further surface processing and in particular electroplating are not necessary. For a particularly uniform coating, it is advantageous to continuously circulate the bath and / or to keep the object to be coated rotating in the bath.

The process according to the invention can be carried out in the following way:

In a first step, a base layer in the form of a smooth, low-crack chrome layer at a temperature in the range from 50 to 70 ° C, preferably from 51 to 61 ° C, particularly preferably from 53 to 59 ° C and most preferably from 55 to 57 ° C deposited. The current density can be up to 50 A / dm ² . With a deposition time TP of 10 to 15 minutes, a base layer thickness of 6-9 µm can be achieved. A waiting time TW is expediently inserted before the start of the deposition while the object takes on the temperature of the bath. This time can be 1 to 10 minutes depending on the size of the object and the temperature difference. It is advantageous to insert an activation step before the deposition, in which the object is poled positively. The current density can be up to 30 A / dm ² . 1 to 2 minutes is sufficient as the duration TP. The base layer obtained generally has a microhardness of 800 to 950 HV 0.1.

In the second step, the actual deposition of the structured chrome layer takes place from the same bath. For this, the bath temperature is to be set to 30 to 50 ° C., preferably to 40 to 50 ° C., particularly preferably to 42 to 48 ° C. and most preferably to 44 to 46 ° C. In this step, too, it is expedient to insert a waiting time TW and an activation step with the parameters already mentioned before the start of the deposition. The deposition is expediently carried out at a current density of 75 to 90 A / dm ² . With a deposition time TP of 10 to 30 minutes, a thickness of the structural layer of 14 to 40 µm can be achieved. The structure layer obtained normally has a microhardness of 850 to 900 HV 0.1. The structure layer has a roughness Rz of up to about 40 μm.

In the third step, the structural chrome layer is coated with a thin, smooth hard chrome layer, the functional layer, again from the same bath. For this purpose, the bath is brought to a temperature in the range from 50 to 70 ° C., preferably from 51 to 61 ° C., particularly preferably from 53 to 59 ° C. and most preferably from 55 to 57 ° C. and then at a current density up to 50 A / dm ² deposited. With a deposition time TP of 5 to 15 minutes, a layer thickness of the functional layer of 3 to 9 µm can be achieved. The functional layer normally has a microhardness of 1,000 to 1050 HV 0.1. The final thin hard chrome layer practically does not change the roughness of the structural layer. In this step, too, it is advisable to use the At the beginning of the separation, insert a waiting time TW and an activation step with the parameters already mentioned.

In all separation steps, it is also advantageous to provide a ramp time TR before the respective separation times, in which the current density is regulated to the corresponding value. The ramp time TR can be 1 to 5 minutes each.

In contrast to methods according to the prior art, the method is distinguished by a particularly simple current density control. In order to produce thin, uniform, well-structured hard chrome structure coatings, it is simply sufficient to linearly increase the current density to the respective setpoint or the respective descent in the respective steps. As a result, otherwise required, technically complex and thus expensive current and voltage regulating units and their complex programming are not required. However, it can also be favorable and advantageous in individual cases to regulate the current density in steps to the maximum value or down again.

With this procedure, a structured hard chrome layer is obtained on the surface of the object, which is characterized by a particularly dense and uniform distribution of very well-formed dome-shaped elevations. A layer with a peak number of 75 to 100 / cm can be obtained. Depending on the selection of the deposition conditions, especially in the step of the structural coating, roughness values Rz up to 40 µm can be achieved.

The method according to the invention can be used to generate a chrome layer on components, in particular machine components. In a preferred embodiment, the method is used to produce a structured hard chrome layer on machine components in sliding contact with one another, in particular pistons, cylinders, bushings and axle bearings, on rollers, drums or cylinders in the graphics industry, in particular ink rollers and dampening cylinders, and on tools ,

In an exemplary embodiment of the invention, 100 l of bath contain 20.450 kg of chromic anhydride, 2.500 kg of potassium dichromate, 0.550 kg Strontium sulfate and 3.5 g of 2-hydroxyethanesulfonic acid sodium salt. This results in the concentration values in the bath of 222 g / l chromic anhydride, 2.2 g / l free sultate and 0.035 g / l 2-hydroxyethanesulfonic acid sodium salt. The following process parameters are selected for structural chrome plating using the example of a rolling cylinder made of the basic material steel St 52 Deposition of the base layer (bath temperature 55 to 57 ° C): TW 5.0 min Activation (30 A / dm ² ) TR 1.0 min TP 0.5 min Separation (50 A / dm ² ) TR 2.0 min TP 10.0 min Deposition of the structural layer (bath temperature 44 to 46 ° C): TW 0.5 min Activation (30 A / dm ² ) TR 1.0 min TP 0.5 min Separation (75 A / dm ² ) TR 3.0 min TP 10.0 min TW 3.0 min Separation (80 A / dm ² ) TR 1.0 min TP 10.0 min TW 30 min Separation (90 A / dm ² ) TR 1.0 min TP 10.0 min Deposition of functional layer (bath temperature 55 to 57 ° C) TW 3.0 min Activation (30 A / dm ² ) TR 1.0 min TP 0.5 min Separation (50 A / dm ² ) TR 2.0 min TP 10.0 min

The structure chrome layer obtained has a roughness Rz of 35 to 40 μm and a peak number of 75-100 / cm with an extremely uniform distribution of very well-formed dome-shaped elevations.

1 shows the SEM image of the surface of the roll cylinder, which is chromium-plated by way of example with the chrome bath according to the invention and according to the method according to the invention, at a magnification of 30: 1. The dense and even distribution of dome-shaped elevations can be clearly seen.

2 shows a section of this area at a magnification of 400: 1, in which the topography of the structure is clarified.

3 shows the SEM image of a cross section through the layer at an enlargement of 400: 1.

Claims (17)

1. Galvanic bath which contains in aqueous solution at least one compound providing chromium (VI) ions and comprises

   a) Chromium (VI) ions in an amount which corresponds to 100 to 600 g/l chromic acid anhydride

   b) Sulphate ions in the form of sulphuric acid and/or a soluble salt thereof in a molar concentration ratio of chromium (VI) ions to sulphate ions (SO₄ ^2-) of 90:1 to 120:1, and

   c) 2-hydroxyethane sulphonate ions in an amount which corresponds to 0.01 to 3.0 g/l of the sodium salt.
2. Galvanic bath according to claim 1 characterised in that the bath comprises chromium (VI) ions in an amount which corresponds to 200 to 250 g/l chromic acid anhydride.
3. Galvanic bath according to claim 1 or 2 characterised in that the compound providing chromium (VI) ions is selected from chromic acid anhydride and/or alkali dichromates.
4. Galvanic bath according to one of claims 1 to 3 characterised in that the molar concentration ratio of chromium (VI) ions to sulphate ions amounts to 100:1 to 105:1.
5. Galvanic bath according to one of claims 1 to 4 characterised in that it comprises sulphate ions in the form of sulphuric acid, sodium, potassium, lithium, ammonium, magnesium, strontium, aluminium and/or potassium aluminium sulphate.
6. Galvanic bath according to claim 5 characterised in that it comprises sulphate ions in the form of strontium sulphate.
7. Galvanic bath according to one of claims 1 to 6 characterised in that the bath contains 0.07 to 1.5 g/l 2-hydroxyethane sulphonate, calculated as sodium salt.
8. Process for chromium coating characterised in that chromium is precipitated from a galvanic bath according to one of claims 1 to 7 on to an article connected as cathode.
9. Process according to claim 8 comprising the following steps:

   a) Precipitation of a base chromium layer at a temperature of ≥50°C,

   b) precipitation of a structured chromium layer at a temperature of ≤50°C, and

   c) precipitation of a functional chromium layer at a temperature of ≥50°C.
10. Process according to claim 9 characterised in that independently of one another step a) is carried out at a temperature in the range between 51 and 61°C, step b) at a temperature in the range between 40 and 50°C, and step c) at a temperature in the range between 51 and 61°C.
11. Process according to claim 9 or 10 characterised in that independently of one another the deposition of the basic chromium layer in step a) is carried out at a current density of up to 50A/dm², the deposition of the structured chromium layer in step b) at a current density of 70 to 90A/dm², and the deposition of the functional chromium layer in step c) at a current density of up to 50A/dm².
12. Process according to one of claims 9 to 11 characterised in that the rise and/or fall of the current density in the steps a), b), and/or c) from the starting to the finishing value or vice versa is carried out linearly in each case.
13. Process according to one of claims 9 to 11 characterised in that the rise and/or fall of the current density in the steps a), b), and/or c) from the starting to the finishing value or vice versa is carried out stepwise in each case.
14. Process according to one of claims 9 to 13 characterised in that independently of one another before one or more of the steps a), b) and c) an activation is carried out at a current density of up to 30 A/dm² and positive polarisation of the article.
15. Use of the process according to one of claims 9 to 14 to generate a chromium layer on a component.
16. Use according to claim 15 characterised in that a structured hard chromium layer is generated on machine components in sliding contact with one another, particularly pistons, cylinders, running bearings and axle bearings on rollers in the graphic industry, particularly inking rollers and damping distributor cylinders, or on tools.
17. Use according to claim 15 characterised in that a structured hard chromium layer is generated on sheet guiding cylinders or drums in the graphic industry.