EP2180088B2 - Method for electroplating hard chrome layers - Google Patents

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 hard chromium layers at high deposition rates.

Hard chrome layers are widely used as coatings of engineering components. For example, it is known to provide valve bodies, liners, brake pistons or axle hubs with hard chrome layers. Here, the deposited chromium layer serves on the one hand as a corrosion protection layer for the underlying substrate surface, on the other hand, as a tribological wear protection layer, since the deposited hard chrome layers have a high hardness.

For the electrodeposition of chromium layers, the substrate surfaces to be coated are brought into contact with an electrolyte having at least the metal (chromium) to be deposited after a suitable pretreatment for the surface, wherein a deposition voltage is applied between the cathodically contacted substrate surface and an anode. As a result, the chromium dissolved in the electrolyte is deposited as a layer on the substrate surface.

The layers thus deposited may have tensile or compressive residual stresses. Compressive stresses can cause the deposited layers to be microcracked, which means that the layers are not continuous, but have a network of microcracks.

Inherent tensile stresses, on the other hand, can lead to deep cracks in the deposited layers into which moisture or aggressive substances migrate and thus can lead to corrosion phenomena of the substrate surface located beneath the chromium layer, as a result of which damage to the chromium layer up to its flaking off can occur.

Moreover, the inherent tensile stress exhibited by such layers is detrimental to many applications, such as the chrome-plating of axle hubs, as this has a negative effect on the bending fatigue strength of the substrate or component. In addition, it is believed that the advent of gaseous H ₂ unavoidable in the deposition of Ab chromium layers leads to the incorporation of hydrogen into the layer and the substrate, which in turn can lead to the formation of cracks in the layer and damage to the substrate.

In order to relieve the deposited chromium layers with respect to their inherent tensile stresses, the coated substrate surfaces in the prior art are mechanically reworked, for example by grinding or honing, in order to break down the inherent tensile stresses occurring in the layers. In addition to the associated production costs, the processing can also lead to a violation of the deposited chromium layers, which ultimately drastically reduces their property as a corrosion protection layer.

Although chromium is itself a chemically relatively non-noble metal, chromium layers have a corrosion-protective effect due to the formation of a thin oxide layer on the surface and the associated very positive potential. Corrosion and tarnish protection with precious metals such as gold, silver or platinum shows comparable corrosion protection properties.

In the industrial production of electroplated mass-produced articles, such as valves for four-stroke internal combustion engines, shock absorbers, axle hubs or similar mechanical components, it is necessary to deposit chromium layers on substrate surfaces with a sufficiently high deposition rate in order to be able to ensure economically viable production. Higher deposition rates are usually achieved by setting higher current densities in the electrodeposition process. However, as a side reaction in the electrodeposition of chromium layers, the formation of hydrogen at the cathode occurs. Since the substrate surfaces to be coated serve as the cathode in the galvanic coating processes, the resulting hydrogen can cause blistering on the substrate surfaces, as a result of which the deposition result the galvanic chromium deposition is strongly influenced. Thus, due to the resulting hydrogen bubbles pores or defects may form, which significantly adversely affect the anti-corrosion properties of the deposited chromium layers.

Increasing the current density in order to achieve sufficiently high deposition rates also leads to significantly increased hydrogen formation at the substrate surfaces.

However, the crack network occurring in the electrodeposited chromium layers due to residual compressive stresses not only has a negative influence on the anticorrosive property of the deposited layer, but also leads to improved mechanical properties of the so coated running parts, since any lubricants to reduce the tribological resistance between moving components in the Microcracks can store the so have a depot effect for the lubricant. This ability of the layers is termed oil carrying capacity and is consistently desired for corresponding mechanical components. This is important, for example, in the case of piston rings to maintain the fire stability.

GB 1 551 340 A discloses the deposition of a hard chromium layer on a substrate surface at a temperature of 60 ° C and a set current density of 80 A / dm ² in a vacuum chamber through which a chromium deposition electrolyte flows.

US 2,706,175 A discloses a device for internal coating of hollow cylinders, wherein a chromium layer is deposited under negative pressure.

EP 1 191 129 A discloses a method for depositing a hard chromium layer under negative pressure, wherein the electrolyte and substrate are moved relative to each other at a relative speed of 0.4 m / s.

US 2001/054557 A1 discloses a method for the electrodeposition of hard chromium layers, in which the chromium layer is also under reduced pressure at a Current density of 30 to 40 A / dm ² and a pulse frequency of 5 to 700 Hz is deposited.

EP 0 024 946 A discloses a process for the deposition of hard chromium layers in vacuum at a current density in the range of 200 A / dm ² and the generation of a relative movement between the electrolyte and the substrate to be coated.

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

In view of the above, it is therefore an object of the present invention to provide a method for the deposition of hard chrome layers, with which can be deposited at high deposition rate hard chrome layers with high corrosion resistance and good mechanical properties.

• Kontaktieren der zu beschichtenden Substratoberfläche mit einem zur galvanischen Abscheidung geeigneten chromhaltigen Elektrolyten;
• Anlegen einer Spannung zwischen der zu beschichtenden Substratoberfläche und einer Gegenelektrode zur galvanischen Abscheidung einer Hartchromschicht auf der Substratoberfläche,
  wobei die Abscheidung in einem gegenüber der Umgebung im Wesentlichen gasdichten Behälter erfolgt, wobei zumindest während des Anlegens der Spannung in dem im Wesentlichen gegenüber der Umgebung gasdichten Behälter ein Unterdruck eingestellt wird und wobei Substratoberfläche und chromhaltiger Elektrolyt mit einer Relativgeschwindigkeit von > 1 m/s bis 5 m/s zueinander bewegt werden, dadurch gekennzeichnet, dass auf eine erste abgeschiedene Hartchromschicht eine zweite Hartchromschicht abgeschieden wird, wobei zur Abscheidung der ersten Hartchromschicht ein Pulsstrom zwischen Substratoberfläche und Gegenelektrode angelegt wird und zur Abscheidung der zweiten Hartchromschicht auf der ersten Hartchromschicht ein Gleichstrom angelegt wird,
  wobei eine Druckdifferenz zum Umgebungsdruck von zwischen 20 mbar und 200 mbar eingestellt wird und wobei zur Abscheidung der ersten Hartchromschicht eine Pulsspannung mit einer Frequenz von 5 Hz bis 5000 Hz, bevorzugt zwischen 50 Hz und 1000 Hz angelegt wird und wobei zur Abscheidung der Hartchromschicht eine Stromdichte zwischen 25 A/dm² und 1000 A/dm², bevorzugt zwischen 50 A/dm² und 500 A/dm² eingestellt wird und wobei der chromhaltige Elektrolyt eine Leitfähigkeit K von 200 mS/cm bis 550 mS/cm bei 20°C aufweist.

This object is achieved by a method for the galvanic deposition of a hard chrome layer on a substrate surface, comprising the method steps:

• Contacting the substrate surface to be coated with a chromium-containing electrolyte suitable for electrodeposition;
• Applying a voltage between the substrate surface to be coated and a counter electrode for electrodepositing a hard chrome layer on the substrate surface,
  wherein the deposition takes place in a relative to the environment substantially gas-tight container, wherein at least during the application of the voltage in the substantially gas-tight environment a negative pressure is set and wherein the substrate surface and chromium-containing electrolyte with a relative speed of> 1 m / s to 5 m / s are moved to each other, characterized in that a second hard chromium layer is deposited on a first hard chromium layer deposited, wherein for the deposition of the first hard chrome layer, a pulse current between Substrate is applied and applied to deposit the second hard chromium layer on the first hard chrome layer, a direct current,
  wherein a pressure difference to the ambient pressure of between 20 mbar and 200 mbar is set and wherein for depositing the first hard chrome layer, a pulse voltage having a frequency of 5 Hz to 5000 Hz, preferably between 50 Hz and 1000 Hz is applied and wherein for depositing the hard chrome layer, a current density between 25 A / dm ² and 1000 A / dm ² , preferably between 50 A / dm ² and 500 A / dm ² , and wherein the chromium-containing electrolyte has a conductivity K of from 200 mS / cm to 550 mS / cm at 20 ° C having.

The reduction of the pressure compared to the ambient pressure during the electrodeposition leads to an improved detachment of the hydrogen bubbles on the substrate surface, which are formed during the electrodeposition process. This detachment is supported by the relative movement of substrate surface and electrolyte to each other. Together, this leads to the deposition of a hard chromium layer, which is essentially free of pores or defects even at high separation effluent densities.

By appropriate measures such as pumps, a corresponding negative pressure is generated. Advantageously, the pressure difference to be set is in a range from 20 mbar to 200 mbar.

In the method according to the invention, a second hard chromium layer is deposited on a first deposited hard chrome layer, a pulse current being applied between the substrate surface and counterelectrode for depositing the first hard chrome layer and a direct current being applied to deposit the second hard chrome layer on the first hard chrome layer.

In one embodiment of the method according to the invention, a first hard chrome layer is deposited, which has no residual stresses due to the applied pulse current and is free of microcracks. By the subsequent application of a direct current between the substrate surface to be coated and the counter electrode is deposited on the already deposited intrinsic and crack-free first hard chrome layer, a second hard chrome layer, which has tensile residual stress and the mechanically desired microcracking.

The layer composite obtained in this way exhibits excellent corrosion resistance and moreover has excellent mechanical properties as running or sliding surfaces due to the microcracks occurring in the upper chromium layer.

For depositing the first chromium layer, the pulse current is applied at a pulse frequency of 5 Hz to 5000 Hz, preferably 50 Hz to 1000 Hz. Here, a current density between 25 A / dm ² and 1000 A / dm ² , preferably 50 A / dm ² to 500 A / dm ^{2 is} set.

For the deposition of the second chromium layer, a direct current with a current density in the range between 25 A / dm ² and 1000 A / dm ² , also with a preferred range between 50 A / dm ² and 500 A / dm ^{2 is} set.

The substrate surface to be coated can with the chromium-containing electrolyte. According to the invention be contacted at a temperature between 30 ° C and 85 ° C, wherein the electrolyte may have a pH in the range ≤ pH 3, preferably ≤ pH 1.

According to the invention, the chromium-containing electrolyte has a conductivity K of from 200 mS / cm to 550 mS / cm (at 20 ° C.).

Advantageously, the process can be carried out with only one electrolyte in a single coating cell.

In this case, it is provided according to the invention to generate a relative movement at least temporarily between the electrolyte and the substrate surface to be coated. According to the invention, the relative speed here is in a range between 0.1 m / s and 5.0 m / s.

To generate the relative movement between the electrolyte and the substrate surface, the substrate surfaces can be moved or the electrolyte can be conveyed accordingly. Among others, agitators or pumps are suitable for conveying the electrolyte.

Due to the relative movement between substrate surface and electrolyte thus generated, a detachment of the resulting hydrogen bubbles is promoted in addition to the applied negative pressure.

In a particularly advantageous embodiment of the method according to the invention it is provided that the substrate surface to be coated is contacted with the electrolyte in a cell in which the chromium-containing electrolyte flows from below and can flow off via an overflow, with a sufficient flow rate is adjusted to the peeling to support the resulting hydrogen bubbles.

To carry out the process according to the invention, a coating reactor is particularly suitable, which is cylindrical and is equipped with a cylindrical inner anode made of platinized metal such as platinum-plated titanium, niobium or tantalum. At the top and bottom of the coating reactor can be recordings for the component to be chromed. A coating reactor designed in this way is particularly suitable for coating cylindrical components. At least one of the two receptacles serves to supply power to the component to be coated and is accordingly designed as an electrical contact.

From the bottom, an electrolyte is sucked from a reservoir through the reactor to the upper part of the reactor by means of a suitable pump and conveyed by this back into the reservoir. In the reservoir, the electrolyte can be degassed by means of suitable facilities. The gas mixture to be separated off is discharged to the outside via a mist eliminator. Alternatively, a separate degassing container may be provided.

In the reservoir, means for controlling the temperature of the electrolyte, ie heaters and / or cooling can be provided. The reservoir can be connected via metering with other reservoirs, which receive compositions for supplementing the electrolyte contained in the reservoir, if a re-dosing of the electrolyte is necessary. To reduce the volume of the heated by the applied deposition voltage electrolyte can be passed through an evaporator unit, wherein the electrolyte is deprived of water and this is cooled simultaneously.

Advantageously, such a reactor designed according to the invention is equipped with at least one movable end face, which facilitates the supply and removal of the component to be coated. In addition, conventional handling systems and seals may be provided to automate the process.

In one embodiment of such a coating reactor, the coated component can be rinsed in the reactor with rinsing water or steam or at least pre-rinsed. For this purpose, the electrolyte supply to the reactor can be interrupted and replaced by rinsing water or steam. In the case of a mere pre-rinsing of the coated component in the reactor, the final rinse can take place in a second reactor, which is essentially identical in construction to the first reactor, but has no anode and power supply.

The inventive method is shown below in the context of embodiments, wherein the inventive idea can not be limited to the embodiments.

embodiments Comparative Example 1

A work piece to be chromium plated (CK 45 steel piston rods) was contacted in a reactor constructed in accordance with the invention with an electrolyte for depositing a hard chromium layer comprising 370 g / l chromic acid and 5.3 g / l sulfuric acid, the electrolyte from below in the corresponding reactor flowed in and was discharged via an upper course at the top of the reactor. The relative velocity set here between the substrate surface of the workpiece to be coated and the electrolyte was 4 m / s. The electrolyte had a temperature of 70 ° C. By means of suitable devices, a pressure of 50 mbar was set within the reactor. After appropriate conditioning and activation of the workpiece by applying a suitable current ramp, a hard chrome layer was subsequently deposited by setting a current density of 235 A / dm ² within 300 seconds. Subsequently, the substrate was rinsed.

The chromium layer obtained had a layer thickness of 11 microns, showed about 40 cracks / cm and had a corrosion resistance in the neutral salt spray test of less than 100 h.

Comparative Example 2:

A workpiece to be chrome plated was contacted with an electrolyte as in Example 1 in a reactor constructed according to the invention, which had 370 g / l chromic acid, 5.3 g / l sulfuric acid and 6 g / l methanesulfonic acid. The deposition conditions corresponded to Example 1. A shiny chromium layer was obtained with a layer thickness of 11 microns, which showed about 250 cracks / cm and a corrosion resistance in the neutral salt spray test less than 100 h.

Example 1:

A workpiece to be chromium plated was contacted with the electrolyte according to Example 2 under the conditions mentioned in Example 2, wherein a pulse current with a current density during the pulse of 235 A / dm ² , a frequency of 1000 Hz and a duty cycle of 50% for 400 seconds was created. A bright, crack-free chromium layer with a layer thickness of 11 μm was obtained which showed 0 cracks / cm and a corrosion resistance in the neutral salt spray test of greater than 500 h.

Example 2:

A work piece to be chrome plated was coated under the deposition conditions of Example 1 except that first a pulse current with a current density of 235 A / dm ^{2 was applied} during the pulse, a frequency of 1000 Hz and a duty cycle of 50% for 400 seconds and then in the same electrolyte under otherwise identical conditions, a direct current with a current density of 235 A / dm ^{2 was applied} for 100 seconds.

The obtained shiny chromium layer showed a layer thickness of 17 μm and had about 25 cracks / cm, the layer having a corrosion resistance in the neutral salt spray test of greater than 500 h.

Claims (4)

1. A method for the galvanic deposition of a hard chromium layer on a substrate surface incorporating the following process stages:

   - bringing the substrate surface being coated into contact with a chromium-bearing electrolyte suitable for galvanic deposition;

   - applying a voltage between the substrate surface being coated and a counter-electrode for the galvanic deposition of a hard chromium layer on the substrate surface,

   wherein the deposition takes place in an essentially gas-tight vessel relative to the environment, wherein a negative pressure is set at least while the voltage is being applied in the vessel that is essentially gastight relative to the environment and wherein the substrate surface and chromium-bearing electrolyte are moved towards each other at a relative speed of > 1 m/s to 5 m/s, characterised in that a second hard chromium layer is deposited on a first deposited hard chromium layer, wherein a pulsed current is applied between the substrate surface and counter electrode to deposit the first hard chromium layer and a direct current is applied to deposit the second hard chromium layer on the first hard chromium layer, wherein a pressure differential between 20 mbar and 200 mbar is set relative to the ambient pressure, and wherein a pulsed voltage with a frequency of 5 Hz to 5000 Hz, preferably between 50 Hz and 1000 Hz, is applied for the deposition of the first hard chromium layer, and wherein a current density of between 25 A/dm² and 1000 A/dm², preferably between 50 A/dm² and 500 A/dm², is set for the deposition of the hard chromium layer, and wherein the chromium-bearing electrolyte comprises a conductivity K from 200 mS/cm to 550 mS/cm at 20 °C.
2. The method according to claim 1, wherein the substrate surface being coated is brought into contact with the chromium-bearing electrolyte at a temperature of between 30 °C and 85 °C.
3. The method according to one of the preceding claims, wherein a pH value in the electrolyte is set in the range < pH 3, preferably < pH 1.
4. The method according to one of the preceding claims, wherein the substrate surface being coated is brought into contact with the electrolyte in a cell in which the chromium-bearing electrolyte flows in from beneath and flows out through an overflow.