JP2012506496A - Method for galvanic electrodeposition of hard chrome layers - Google Patents

Abstract

The present invention relates to a method for depositing a hard chromium layer on a substrate surface at high speed.
The method according to the invention is such that the substrate surface to be coated is contacted with a chromium-containing electrolyte suitable for galvanic electrodeposition under reduced pressure relative to ambient pressure, and during deposition of the chromium layer on the substrate surface, A relative movement of the substrate surface and the electrolyte takes place.
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Description

  The present invention relates to a method for depositing a hard chromium layer on a substrate surface. In particular, the present invention relates to a method for depositing a hard chromium layer at a high deposition rate.

  Hard chrome layers are widely used as coatings for technical parts. Thus, for example, it is known how to apply a hard chrome layer to a valve body, bushing, brake piston, or accelerator hub. The deposited chrome layer is used on the one hand for preventing corrosion of the underlying substrate surface and on the other hand because the deposited hard chrome layer has a high hardness, and is also used as a protective layer against friction and wear.

For galvanic deposition of the chrome layer, the coated substrate surface after appropriate pretreatment to condition the surface is contacted with an electrolyte having at least a deposited metal (chromium), during which the deposition voltage is reduced. Applied between the cathode (cathode) contact substrate surface and the anode (anode). As a result, chromium dissolved in the electrolyte is deposited as a layer on the substrate surface.
Such deposited layers produce internal tensile or compressive stresses. The compressive internal stress creates microcracks in the deposited layer, i.e. the coating layer is not continuously closed, but rather forms a network of microcracks.

Tensile internal stress, on the other hand, causes deep cracks in the deposited layer, and moisture and corrosive substances penetrate into it, inducing a corrosive effect on the substrate surface under the chromium layer, ultimately breaking the chromium layer. As a result, peeling occurs.
In addition, the tensile internal stresses that occur in such layers are disadvantageous for many applications, such as chrome plating on accelerator hubs, which has a negative effect on fatigue strength under the opposite bending stresses of structural components and substrates. It is. Further, gaseous hydrogen is inevitably generated during the deposition of the chromium layer, hydrogen is brought into the layer and the substrate, and instead a crack is generated in the layer, damaging the substrate.

  In order to reduce the tensile internal stress generated in the deposited chromium layer, the coated substrate surface is subsequently processed, for example by cutting or honing, to remove the internal tensile stress generated in the layer according to the prior art. Applied. In addition to the secondary processing costs that this requires, the processing damages the deposited chrome layer and thus ultimately significantly reduces their properties as corrosion protection. Chromium itself is relatively base metal from a chemical point of view, but thanks to the formation of a thin oxide layer on the surface and the enormous advantageous potential associated therewith, the chromium layer is related to their corrosion and haze protection. Compared to precious metals such as gold, silver, and platinum, it protects against corrosion and acts to exhibit corrosion protection.

  To guarantee an economically reasonable production process in industrial secondary machining of galvanically coated mass-produced products such as valves, shock absorbers, accelerator hubs, or similar mechanical parts of a four-cylinder internal combustion engine In addition, it is necessary to deposit a chromium layer having a sufficiently high deposition rate on the substrate surface. Fast deposition rates are generally achieved by setting a high current density in the galvanic coating method. However, hydrogen production at the cathode occurs as a reaction on one side in the galvanic deposition of the chromium layer. Since the coated substrate surface acts as a cathode in the galvanic electrocoating process, the resulting hydrogen induces bubble formation on the substrate surface, which strongly affects the product of galvanic electrochromic deposition. In this way, pores and cracks are formed due to hydrogen bubbles, which adversely affects the corrosion resistance of the deposited chromium layer.

  Increasing the current density to achieve a sufficiently fast deposition rate results in greatly promoting hydrogen formation on the substrate surface.

  However, the network of cracks generated in the galvanic electrodeposition layer due to internal compressive stress not only adversely affects the anticorrosion properties of the deposit layer, but instead improves the properties of the working parts so coated. Guide as much as possible. This is because the lubricant that reduces the friction between the working parts is embedded in the microcracks and thus has a replenishment effect for the lubricant. This ability of the layer is known as oil carrying capacity and is absolutely desirable for such machine parts. For example, this is important to maintain fire stability in the case of piston rings.

GB 1 551 340 A discloses that a hard chromium layer is deposited on a substrate surface by passing the substrate through a low pressure chamber containing a chromium deposition electrolyte at 60 ° C. and a current density setting of 80 A / dm ² .

  US 2,706,175 A discloses an apparatus for coating the inside of a hollow tube, whereby the chromium layer is deposited at low pressure.

  EP 1 191 129 A discloses a method for the deposition of a hard chromium layer under low pressure, the electrolyte and the substrate moving at a rate of 0.4 m / sec relative to each other.

  US 2001/054557 A1 discloses a method of galvanic electrodeposition of a hard chromium layer, which is likewise deposited under low pressure at a pulse frequency of 30-40 A / dm 2 and 5-700 Hz.

EP 0 024 946 A discloses a method of depositing a hard chromium layer with the addition of relative motion of the electrolyte and the substrate to be coated at a current density of 200 A / dm ² under low pressure.

  US 5,277,785 discloses a method and apparatus for depositing a hard chromium layer by brush deposition.

  In view of the above description, the subject of the present invention therefore relates to a method of depositing a hard chromium layer, which deposits a hard chromium layer having high anticorrosion properties and excellent mechanical properties at a high deposition rate.

  Thus, in summary, the present invention has, in part, a step of contacting between a substrate surface to be coated and a chromium-containing electrolyte suitable for galvanic electrodeposition, for galvanic electrodeposition of a hard chromium layer on the substrate surface. Applying a voltage between the surface of the substrate to be coated and the counter electrode, wherein the deposition takes place in a container that is essentially gas-impermeable to the surroundings and is at least essential to the surroundings during voltage application A low pressure is created in a gas-impermeable container, and the substrate surface and the chromium-containing electrolyte are moved relative to each other at a speed of 0.1-5 m / sec, preferably greater than 1-5 m / sec. In particular, it relates to a method of galvanic electrodeposition of a hard chromium (ie chromium based) layer on a substrate surface.

  Other objects and features will be clarified and pointed out in part below.

This application claims the priority of European patent application 080184462.5 filed on October 22, 2008, the entire disclosure of which is incorporated by reference.
The problem of the present application is solved by a method comprising the following steps for galvanic electrodeposition of a hard chromium layer on a substrate surface.
Making contact between the substrate surface to be coated and a chromium-containing electrolyte suitable for galvanic electro-deposition;
Applying a voltage between the counter electrode for galvanic electrodeposition of the substrate surface to be coated and a hard chromium layer on the substrate surface;
Here, the deposition takes place in an essentially gas-impermeable container with respect to the surroundings, at least during the application of a voltage, a low pressure is created in the surroundings, in particular in a gas-impermeable container, and the substrate surface and chromium-containing The electrolyte is moved relative to each other at a speed of 0.1 to 5 m / sec, preferably 1 to 5 m / sec, a second hard chromium layer is deposited on the first hard chromium layer, and the first hard chromium A pulse current is applied between the substrate surface and the counter electrode for the deposition of the layer, and a direct current is applied to the first hard chromium layer for the galvanic electrodeposition of the second hard chromium layer.

  The decrease in pressure relative to ambient pressure during galvanic electrodeposition induces improved separation of hydrogen bubbles on the substrate surface that are formed during galvanic electrodeposition. This separation is aided by the relative movement of the substrate surface and the electrolyte, and at the same time induces the deposition of a hard chromium layer that is essentially free of pores and cracks even at high deposition current densities.

  A moderate low pressure is created by suitable means such as a pump. Advantageously, the pressure difference achieved is between 10 mbar and 800 mbar, preferably between 20 and 200 mbar.

  In the method of the present invention, the second hard chromium layer is deposited on the first hard chromium layer, and a pulse current is passed between the substrate surface and the counter electrode for the deposition of the first hard chromium layer. For galvanic deposition of the second hard chrome layer, a direct current is passed through the first hard chrome layer.

  In one embodiment of the method according to the invention, a first hard chromium layer free of internal stress and microcracks is deposited thanks to the applied pulsed current. Subsequently, a direct current is passed between the surface of the substrate to be coated and the counter electrode, and a second hard chrome layer is deposited on the already deposited first hard chrome layer free of cracks and internal stress, The hard chromium layer has internal tensile stress and mechanically desirable microcracks.

The resulting compound layer structure exhibits excellent anticorrosion properties and excellent mechanical properties as a running or sliding surface thanks to microcracks generated in the upper chromium layer.
For the deposition of the first chromium layer, the pulse current can be adjusted with a pulse frequency of 5 to 5000 Hz, preferably 50 to 1000 Hz. On the other hand, the current density is adjusted to 25 to 1000 A / dm ² , preferably 50 to 500 A / dm ² .

For the deposition of the second chromium layer, the direct current is adjusted with a current density in the range of 25 to 1000 A / dm ² , preferably 50 to 500 A / dm ² .
According to the present invention, the surface of the substrate to be coated is brought into contact with a chromium-containing electrolyte at a temperature of 30 ° C. to 85 ° C., and the electrolyte has a pH value in the range of less than pH 3, preferably less than pH 1.
Also according to the present invention, the chromium-containing electrolyte can have a dielectric constant K 2 of 200 mS / cm to 550 mS / cm at 20 ° C.
Advantageously, the method can be carried out with only one electrolyte in a single coated cell.

According to the present invention, the relative movement between the electrolyte and the coated substrate surface can be performed at least temporarily. According to the present invention, the relative movement is performed in the range of 0.1 m / second to 5.0 m / second.
To cause relative movement between the electrolyte and the coated substrate surface, the substrate surface is moved or the electrolyte is moderately supplied. A stirrer or pump is suitable for supplying the electrolyte.
The relative movement between the electrolyte thus created and the coated substrate surface facilitates the separation of the hydrogen bubbles that are generated, in addition to the low pressure applied.

  In a particularly advantageous embodiment of the method according to the invention, the surface of the substrate to be coated is brought into contact with the electrolyte in one cell, in which the chromium-containing electrolyte is poured from below and flows over the drain, A sufficient flow rate is adjusted to maintain the separation of the generated hydrogen bubbles.

  For carrying out the process according to the invention, a coating reactor is particularly suitable which has a cylindrical shape and is externally fitted with a cylindrical internal anode of platinum-coated metal such as platinum-coated titanium, niobium or tantalum. The top and bottom of the coating reactor can be equipped with cradles for chromed structural parts. This type of coating reactor is suitable for coating cylindrical parts. One of the at least two pedestals supplies current to the part to be coated, thus making electrical contact.

  By suitable pumping means, the electrolyte is drawn from the reservoir, through the reactor to the top of the reactor, and from there back to the reservoir. In the reservoir, the electrolyte is degassed with a suitable device. The gas mixture thus separated is removed to the outside through a drop separator. Alternatively, a separate degassing tank can be installed.

  An apparatus for temperature regulation of the electrolyte, such as a heating and / or cooling system, may be provided in the reservoir. The reservoir can be connected to another reservoir via a discharge pump, and as long as electrolyte discharge is required, the additional reservoir contains a composition that replenishes the electrolyte in the reservoir. In order to reduce the capacity, the electrolyte heated by the applied deposition voltage passes through the evaporator unit, where it removes water from the electrolyte simultaneously with cooling.

  Advantageously, such a reactor constructed in accordance with the invention is fitted with an end face (single port) from the outside, which enables at least one movable, covered part to be lifted and picked up from below. . In addition, conventional handling systems and seals may be provided for automation of this process.

In one embodiment of such a coating reactor, the parts to be coated in the reactor are cleaned with rinse water or steam, or at least pre-cleaned. For this reason, the supply of electrolyte to the reactor is interrupted and replaced by rinsing water or steam. In the case of a simple precleaning of the reactor coating parts, the final cleaning is performed in a second reactor, which is basically the same design as the first reactor, but with any anode ( Anode) and current supply are not required.
The method of the present invention is described below as a sample embodiment, but the concept of the present invention is not limited to that sample embodiment.

[Example 1]
A chrome-plated specimen (steel type CK45 piston rod) is contacted with an electrolyte comprising 370 g / l chromic acid and 5.3 g / l sulfuric acid for depositing a hard chromium layer in a reactor constructed according to the present invention; The electrolyte flows from the bottom of each reactor and is removed from the drain at the top of the reactor. The relative velocity between the coated substrate surface and the electrolyte of the test piece thus obtained was 4 m / s. The electrolyte was 70 ° C. With appropriate equipment, the reactor was brought to 50 mbar. The specimen was properly conditioned and activated using a suitable current ramp and then a hard chromium layer was deposited by adjusting at a current density of 235 A / dm ² at 300 second intervals. The substrate was then cleaned.
The obtained chromium layer had a layer thickness of 11 μm, had about 40 cracks per cm, and had a corrosion resistance of less than 100 hours in a neutral salt spray test.

[Example 2]
The chrome-plated specimen was contacted with the electrolyte in a reactor constructed according to the present invention as in Example 1. The electrolyte contained 370 g / l chromic acid, 5.3 g / l sulfuric acid and 6 g / l methanesulfonic acid. The deposition conditions were the same as in Example 1. A bright chromium layer with a layer thickness of 11 μm was obtained, which had cracks of about 250 / cm and a corrosion resistance of less than 100 hours in a neutral salt spray test.

[Example 3]
The chrome-plated test piece was brought into contact with the same electrolyte as in Example 2 under the same conditions as in Example 2, and the pulse current with a current density of 235 A / dm ^{2 during} the pulse was 1000 Hz frequency and 50% on-time. For 400 seconds.
A bright chromium layer with a layer thickness of 11 μm was obtained, which had a crack number of 0 / cm and a corrosion resistance of more than 500 hours in a neutral salt spray test.

[Example 4]
The chrome-plated specimens were run under the same deposition conditions as in Example 3 and initially a pulse current with a current density during the pulse of 235 A / dm ² was supplied for 400 seconds at a frequency of 1000 Hz and an on-time of 50%. Thereafter, direct current was supplied for 100 seconds at a current density of 235 A / dm 2 in the same electrolyte, and the other conditions were the same.
A bright chromium layer with a layer thickness of 17 μm was obtained, which had cracks of approximately 25 / cm and had a corrosion resistance of more than 500 hours in a neutral salt spray test.

When introducing an element in the present invention or a preferred embodiment thereof, the indefinite article and definite article are intended to mean one or more elements. “Comprising”, “including”, “having” are inclusive, meaning that there are additional components / elements in addition to the listed components / elements.
In view of the above, several objects of the invention have been achieved and other advantageous results have been obtained.
Since various changes may be made in the above compositions and processes without departing from the scope of the present invention, all matters included in the above description are interpreted in an illustrative manner and not in a limiting sense. .

Claims (9)

A method for galvanic electrodeposition of a hard chromium layer on a substrate surface comprising the following steps:
Contacting the substrate surface to be coated and a chromium-containing electrolyte suitable for galvanic electro-deposition;
Applying a voltage between the counter electrode for galvanic electrodeposition of the substrate surface to be coated and a hard chromium layer on the substrate surface;
Here, the deposition occurs in a container that is essentially gas impermeable to the surroundings, and at least during the application of voltage, a low pressure is created in the container that is essentially gas impermeable to the surroundings, and further the substrate surface And the chromium-containing electrolyte are moved relative to each other at a speed of 0.1 to 5 m / sec, preferably 1 to 5 m / sec.   A second hard chrome layer is deposited on the first hard chrome layer, and for the deposition of the first hard chrome layer, a pulse current is supplied between the substrate surface and the counter electrode, and the second hard chrome layer. The method according to claim 1, wherein a direct current is supplied to the first hard chromium layer for the galvanic electrodeposition.   2. A method according to claim 1, wherein a pressure difference of 10 mbar to 800 mbar, preferably 20 to 200 mbar, is provided for the ambient pressure.   4. The method according to claim 2, wherein a pulse voltage is applied to the first hard chromium layer deposition at a frequency of 5 Hz to 5000 Hz, preferably at a frequency of 50 Hz to 1000 Hz. The method according to claim 1, wherein a current density of 25 A / dm ^{2 to} 1000 A / dm ² , preferably 50 A / dm ^{2 to} 500 A / dm ² is adjusted to deposit the hard chromium layer.   The method according to any one of claims 1 to 5, wherein the substrate surface to be coated is contacted with the chromium-containing electrolyte at a temperature of 30C to 85C.   The method according to claim 1, wherein the pH of the electrolyte is adjusted to 3 or less, preferably 1 or less.   The method according to any of claims 1 to 7, wherein the surface of the substrate to be coated is brought into contact with the electrolyte in one cell, and the chromium-containing electrolyte is flowed from below and flows out beyond the drain. A method for galvanic electrodeposition of a hard chromium layer on a substrate surface comprising the following steps:
Contacting the substrate surface to be coated and a chromium-containing electrolyte suitable for galvanic electro-deposition;
Applying a voltage between the counter electrode for galvanic electrodeposition of the substrate surface to be coated and a hard chromium layer on the substrate surface;
Here, deposition occurs in a container that is essentially gas impermeable to the surroundings, and at least during the application of voltage, a low pressure is created in the container that is essentially gas impermeable to the surroundings, and further the substrate surface and The chromium-containing electrolyte is moved relative to each other at a speed of 0.1 to 5 m / sec, preferably 1 to 5 m / sec, and a second hard chromium layer is deposited on the first hard chromium layer, A pulse current is supplied between the substrate surface and the counter electrode for the deposition of the hard chromium layer, and a direct current is supplied to the first hard chromium layer for the galvanic electrodeposition of the second hard chromium layer. .