EP2975162A1 - Method for providing a workpiece with a protective coating - Google Patents

Abstract

The present invention relates to a method of protective coating a workpiece with the following steps in the order of their mention: Depositing a first nickel layer by electrodeposition in a first nickel sulphate-based bath which is free from sulphate-containing additives apart from the sulphate; - applying a second nickel layer by electrodeposition in a second bath based on nickel sulphate, which apart from the sulphate has a sulfur-containing additive; - Applying a cover layer.

Description

The invention relates to the protective coating of a workpiece with nickel.

The workpiece may, for example, be a metallic workpiece which is to be protected against corrosion by the protective coating.

The present invention has for its object to provide an advantageous method for protective coating of a workpiece.

• Aufbringen einer ersten Nickel-Schicht durch galvanische Abscheidung in einem ersten Bad auf Nickelsulfat-Basis, das von dem Sulfat abgesehen frei von schwefelhaltigen Zusätzen ist;
• Aufbringen einer zweiten Nickel-Schicht durch galvanische Abscheidung in einem zweiten Bad auf Nickelsulfat-Basis, das von dem Sulfat abgesehen einen schwefelhaltigen Zusatz aufweist;
• Aufbringen einer Deckschicht.

According to the invention, this object solves a method with the following steps, which are carried out in the order of their naming:

• Depositing a first nickel layer by electrodeposition in a first nickel sulfate-based bath which is free of sulfur-containing additives apart from the sulfate;
• Depositing a second nickel layer by electrodeposition in a second nickel sulphate-based bath containing, in addition to the sulphate, a sulphurous additive;
• Applying a cover layer.

Preferred embodiments can be found in the dependent claims and the present description, wherein in the presentation is not always distinguished in detail between process or use aspects and device features.

The decisive factor is therefore that the first bath is free of sulfur-containing additives, resulting in a sulfur-free first nickel layer. In contrast, the then deposited on this second nickel layer on sulfur; the inventor has observed a potential difference of more than 160 mV between the first and the second nickel layer.

This protective coating, which is then still provided with the cover layer, preferably a black chromium layer, exhibits a surprisingly good corrosion resistance, for example in salt spray tests. Correspondingly protection-coated workpieces can even reach failure-free up to 1000 hours in a salt spray test (for comparison: stress level 3 corresponds to 96 hours salt spray test, stress level 4 to 144 hours). As already mentioned, the workpiece is preferably a metallic workpiece, for example of a light metal or a light metal alloy, for example based on aluminum, for example aluminum, or of an iron or steel material, for example of non-stainless steel , In general, the workpiece per se, for example, be provided from a non-conductive plastic material, which is provided with a metal coating, which is then coated according to the invention.

Both the first and second nickel layers are deposited in a nickel sulfate-based bath, so at least some of the nickel ions in the bath of nickel sulfate (NiSO ₄ ) are provided. The first bath is free of sulphurous additives, with no consideration of sulphate. Thus, the first bath has sulphate and nickel ions, but apart from the sulphate no further sulphurous constituent (so it is free).

In contrast, the second bath to a sulfur-containing additive, wherein the sulfate is again disregarded. In other words, therefore, the second bath has nickel and sulfate ions and, in addition, a sulfur-containing additive which is different from the sulfate.

The inventor has found that the corresponding (sulfur-free) deposited first nickel layer grows rather irregular, for example, with a columnar structure. It may be, for example, that due to a irregular growth of the first nickel layer results in a protective coating, which then has a non-planar surface as a whole. In particular, the first nickel layer may be non-planarizing in nature. Although this may be disadvantageous, for example, in decorative terms, but a corresponding protective coating system just shows a very good corrosion resistance.

The first and the second bath are both provided on a water basis, so it is in each case an aqueous solution of the ions mentioned in more detail below. So far as this is the case of the fact that a bath consists exclusively of certain ingredients, this refers to these ingredients in aqueous solution and the water is not always listed as a component again.

In a preferred embodiment, the first bath, in order to obtain a correspondingly high-purity first nickel layer, during the electrodeposition (during the application of the first nickel layer) is filtered, preferably by means of an activated carbon filter. For example, organic contaminants can be removed. The first bath is ideally pumped continuously through the filter, preferably through a filter with activated charcoal (plates).

The flow through the filter is preferably chosen such that the first bath is recirculated at least 3 times in one hour, more preferably at least 5 times, 7 times and 9 times in this order. Possible upper limits may be, for example, at a maximum of 30, 25, 20, 15, 13 or 11 revolutions per hour. The "circulating" means that the x-fold volume (corresponding to the x-times circulation) of the first bath is pumped through the filter in one hour, so just a corresponding flow is set.

Preferably, a plurality of batches are successively coated in the first bath, and the first bath is continuously filtered during that time, preferably also between batches. Each batch can then of course Also, several workpieces may be provided, so several workpieces can be coated in parallel.

Also with regard to a high-purity first nickel layer, a cleaning of the first bath before the electrodeposition is preferred, wherein a metal part (as a cleaning part) is introduced into the first bath and energized. Thus, for example, metallic impurities can be removed from the first bath, which may have been introduced, for example, during the coating of the preceding batch. The current density on the cleaning metal part is preferably at least 0.05 A / dm ² , more preferably at least 0.08 A / dm ² , and possible upper limits are for example at most 0.2 A / dm ² , preferably at most 0.15 A. / dm ² , more preferably at most 0.12 A / dm ² (an upper limit may also be independent of a lower limit of interest, and vice versa).

This cleaning with the cleaning metal part does not have to be done before each batch, but preferably a plurality of batches may be successively coated without cleaning therebetween, and a cleaning may be performed before these plurality of batches and another then afterwards before a further plurality of batches.

Before being introduced into the first bath, the workpiece is chemically pre-nickel-plated in a preferred embodiment, particularly preferably a workpiece based on aluminum.

The first nickel layer preferably has a thickness of at least 5 μm, in this order increasingly preferably at least 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm or 13 μm; independent, preferred upper limits are for example in this order increasingly preferably at most 20 microns, 19 microns, 18 microns and 17 microns. The provision of a lower limit can generally also be disclosed independently of the provision of an upper limit, and vice versa.

The second nickel layer preferably has a thickness of at least 5 μm, in this order increasingly preferably at least 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm or 13 μm; preferred upper limits are for example in this order increasingly preferably at most 20 microns, 19 microns, 18 microns and 17 microns. Also in this case, the provision of an upper limit may generally be preferred independently of the provision of a lower limit, and vice versa.

In a preferred embodiment, in the case of the first and / or the second nickel layer, the current density in the electrodeposition on the workpiece is at least 0.1 A / dm ² , preferably at least 0.3 A / dm ² , especially at least 0.4 A / dm ² . Advantageous upper limits, for example, in this order increasingly preferably at most 2 A / dm ² , 1.8 A / dm ² , 1.6 A / dm ² , 1.4 A / dm ² , 1.2 A / dm ² , 1 A / dm ² , 0.8 A / dm ² and 0.6 A / dm ² are. The provision of an upper limit should again generally be disclosed independently of the provision of a lower limit, and vice versa.

In a preferred embodiment, the sulfur-containing additive in the second bath is a brightener additive, that is to say a so-called brightener, with saccharin being particularly preferred as brightener additive. On the other hand, the first bath is correspondingly free of saccharin or sulfur-containing brightener additives or preferably of brightener additives in general.

In the second bath is the brightener, preferably the saccharin, in a preferred embodiment with a concentration of at least 2.5 g / l, preferably at least 3.5 g / l, more preferably at least 4 g / l before. Advantageous upper limits may be, for example, at most 15 g / l, in this order increasingly preferably at most 13 g / l, 11 g / l, 9 g / l, 7 g / l and 6 g / l, respectively (the provision of an upper limit again generally be disclosed independently of the provision of a lower limit, and vice versa).

In the second bath, the nickel concentration is preferably at least 40 g / l, more preferably at least 50 g / l or 60 g / l, particularly preferably at least 65 g / l. Advantageous upper limits may be, for example, in this order increasingly preferably at most 150 g / l, 130 g / l, 110 g / l, 90 g / l, 80 g / l and 75 g / l (the provision of an upper limit in general again be disclosed independently of the provision of a lower limit, and vice versa). These figures relate to the nickel ions in the bath, which are thus at least partially provided by the nickel sulfate available.

Preferably, the nickel ions in the second bath are provided not only by the nickel sulfate but also by nickel chloride (NiCl ₂ ), wherein the chloride concentration (the chloride ion) is preferably at least 10 g / L, more preferably at least 15 g / l, more preferably at least 20 g / l should be. Advantageous upper limits may be, for example, at most 50 g / l, preferably at most 40 g / l, more preferably at most 30 g / l (the provision of an upper limit should in turn also be disclosed independently of the provision of a lower limit, and vice versa). Preferably, the nickel in the second bath is provided solely by nickel sulfate and nickel chloride.

In a preferred embodiment, boric acid is also provided in the second bath, preferably at a concentration of at least 40 g / l, more preferably at least 50 g / l, particularly preferably at least 55 g / l. Advantageous upper limits may be, for example, in this order increasingly preferably at most 100 g / l, 90 g / l, 80 g / l, 70 g / l and 65 g / l (the provision of an upper limit may in turn also be independent of the provision a lower limit, and vice versa).

The second bath therefore preferably has nickel, chloride and sulfate ions, boric acid and a brightener, preferably saccharin. Preferably, the second bath consists exclusively of these components, optionally one apart from a better wetting of the surface to be coated wetting agents (and the water).

In the first bath, the nickel concentration is preferably at least 40 g / l, more preferably at least 45 g / l, particularly preferably at least 50 g / l. Advantageous upper limits may be, for example, in this order increasingly preferably at most 80 g / l, 70 g / l and 60 g / l (the provision of an upper limit may generally be preferred regardless of providing a lower limit, and vice versa).

The chloride concentration in the first bath may be, for example, at least 10 g / l, preferably at least 20 g / l, more preferably at least 25 g / l, with advantageous upper limits (in the order of their naming increasingly preferred) at about 50 g / l , 40 g / l and 30 g / l respectively (and in general the provision of an upper limit may also be of interest irrespective of the provision of a lower limit, and vice versa).

In the first bath, the concentration of boric acid is preferably at least 20 g / l, more preferably at least 30 g / l, more preferably at least 35 g / l, with favorable upper limits (in the order of their nomination increasingly preferred) about at most 70 g / l, 60 g / l, 50 g / l and 45 g / l respectively (and the provision of an upper limit may again be of interest irrespective of the provision of a lower limit, and vice versa). Apart from the water of the aqueous solution and optionally a (sulfur-free) wetting agent, the second bath is preferably composed exclusively of nickel, sulfate and chloride ions and boric acid.

The black chromium layer provided in a preferred embodiment as a cover layer preferably has a thickness of at least 0.5 μm and of at most 1.5 μm (the upper limit may in turn be independent of the lower limit of interest, and vice versa). Insofar as layer thicknesses are generally referred to in the present disclosure, these relate in each case to the finished layer, ie the layer thickness at the end / after the deposition (ie not at an intermediate thickness during the deposition). The thickness is taken perpendicular to the surface of the workpiece, and in the case of a layer having varying thickness over the surface of the workpiece, an average value is considered.

Although a further layer could generally also be applied to the cover layer, this is preferably the final layer, ie the layer which then forms the outer surface of the protective-coated workpiece.

The invention also relates to a correspondingly protective-coated workpiece, that is to say a workpiece having a first nickel layer, a second nickel layer on the first nickel layer (which adjoins the first nickel layer) and a cover layer on the second nickel layer (US Pat. which adjoins the second nickel layer). The first nickel layer is free of sulfur, whereas the second nickel layer has a mass fraction of at least 0.05 wt .-%, preferably at least 0.07 wt .-%, particularly preferably at least 0.08 wt .-%, Has. Possible upper limits are for example at most 0.15 wt .-%, preferably at most 0.12 wt .-%, particularly preferably at most 0.1 wt .-%.

This information regarding sulfur content should also be expressly disclosed with regard to the process category, that is, a first nickel layer which is free of sulfur and a second nickel layer is deposited, which has a corresponding minimum sulfur content (and preferably also a maximum content disclosed herein). On the other hand, the invention also expressly relates to a workpiece that has been protective coated in a method disclosed herein.

For a corresponding layer system, in which therefore the first nickel layer is actually free of sulfur (ie, not only has only a small amount of sulfur, but actually no sulfur), but the second nickel layer arranged thereon contains sulfur, the inventor has a comparatively observed large electrochemical potential difference between the two nickel layers. This is at least 160 mV, preferably at least 170 mV, with possible upper limits, for example, at most 260 mV, 240 mV, 220 mV, 200 mV or 190 mV can be. Preferably, the first nickel layer consists exclusively of nickel.

This large potential difference corresponds with the observed good corrosion resistance. The latter is not impaired by the black chromium layer, which is preferably provided as a cover layer, and may even be improved to some extent. A particular advantage of this protective coating (preferably with electrolytically deposited black chrome as the cover layer) is that it protects against corrosion on the one hand and is electrically conductive on the other hand.

Even with this background, the workpiece in a preferred embodiment is an electrical connection part that can be screwed and / or plugged in, that is, for example, a connector for a power and / or data line. The inventor has found that the protective coating is relatively ductile, which can help prevent cracking and spalling. According to current knowledge, this is at least attributable to the highly pure deposited first nickel layer. In a connector that is also bolted, the thread can represent the most mechanically stressed area. In a test on such a connector, the protective coating was also there after 500-times Schraubverbinden and loosening still intact, which is also due to the high ductility of the protective coating. Cracking / spalling would otherwise be weak spots that could lead to corrosion.

Also, due to the black or more generally dark surface of the preferred black chrome layer, preferred uses are in the military field. For example, the black chrome layer may contain unwanted reflections avoid helping. For example, "in the military field" means as part of a military vehicle or aircraft.

In the following, the invention will be explained in more detail with reference to an embodiment, wherein the individual features may be essential to the invention in another combination and should be disclosed in this form.

A connector made of aluminum is first chemically pre-nickel-plated before the protective coating according to the invention and then fed to the galvanic coating.

In this case, the pre-nickel-plated surface of the connector is provided with a first nickel layer in a first step of electrodeposition. The first bath used in this case is based on nickel sulphate and, apart from the aqueous solution, consists exclusively of nickel, sulphate and chloride ions and boric acid.

To ensure the purity, the first bath is continuously filtered, even during the electrodeposition. A circulation pump pumps the first bath through an activated carbon filter; the flow rate through the filter is adjusted so that the first bath circulates approximately 10 times per hour. Furthermore, the first bath is also freed of metallic impurities with a cleaning metal part before the electrodeposition (the first layer on the connector); For this purpose, the cleaning metal part is introduced into the first bath and energized for a few hours. The current density on the cleaning metal part is about 0.1 A / dm ² .

The proportion of nickel ions in the first bath is around 55 g / l, the proportion of chloride ions around 26 g / l and the boric acid content around 40 g / l. The galvanic deposition of the first nickel layer is then carried out at a current density of about 0.5 A / dm ² until the first nickel layer reaches a thickness of about 15 microns Has. The growth of the first nickel layer is pronounced columnar, also because the first bath has no sulfur-containing (gloss) additive.

After the application of the first nickel layer, the connector is removed from the first bath, then preferably rinsed with water and then fed to a second bath. Also in the second galvanic step, a nickel layer is applied, but the second bath is provided with a sulfur-containing additive, namely with saccharin. The second bath consists of an aqueous solution of nickel, sulfate and chloride ions with boric acid and saccharin. The proportion of nickel ions, which is adjusted as in the case of the first bath via the added amount of nickel sulfate and nickel chloride, is 70 g / l in the second bath. The proportion of chloride ions is 25 g / l, that of boric acid at 60 g / l, and the saccharin content is 4.5 g / l.

The deposition of the second nickel layer is again carried out at a current density of 0.5 A / dm ² , up to a layer thickness of about 15 microns.

Subsequently, the connector is removed from the second bath and preferably rinsed again with water. In a last step, a black chromium layer with a thickness of about 1 μm is then applied as a cover layer. For this purpose, the connector is first cathodically activated in an aqueous solution of 60 g / l sodium hydrogen sulfate at a pH of about 1.8, namely at 3 A / dm ² for a period of 30 s. The solution contains relatively small amounts of surfactants and fluorides to support the cleaning and activation function.

Thereafter, a black chromium layer known per se can be electrodeposited, namely from a bath of about 450 g / l chromic acid, about 7.5 g / l chromium sulfate, of the order of 2-8 g / l sodium or potassium carbonate or nitrate and about 1 g / l hexafluorosilicate. This black chrome layer has a thickness of about 1 micron.

The first nickel layer is sulfur-free (sulfur content of 0 wt .-%), and the sulfur content in the second nickel layer is about 0.09 wt .-%. Potential measurements on the correspondingly applied protective coating show an electrochemical potential difference between the first and the second nickel layer of around 180 mV. In a salt spray test, appropriately coated connectors show good corrosion resistance over a period of at least 1000 hours.

In addition to the electrical conductivity of the protective coating distinguishes its black surface. This helps avoid light reflections, which is why the corresponding protective-coated connector can be used particularly advantageously in a military vehicle or aircraft. The protective coating is relatively ductile, which helps prevent cracking or chipping.

Claims (15)

A method of protective coating a workpiece with the following steps in the order of their designation: Depositing a first nickel layer by electrodeposition in a first nickel sulphate-based bath which is free from sulphate-containing additives apart from the sulphate; - applying a second nickel layer by electrodeposition in a second bath based on nickel sulphate, which apart from the sulphate has a sulfur-containing additive; - Applying a cover layer. The method of claim 1, wherein the first bath is filtered during the electrodeposition, preferably by means of an activated carbon filter. The method of claim 1 or 2, wherein the first bath is cleaned prior to the electrodeposition by a metal part is introduced and energized. A method according to any one of the preceding claims, wherein the cover layer is a black chrome layer. A method according to any one of the preceding claims, wherein at least one of the first nickel layer and the second nickel layer is at least 5 μm and at most 20 μm thick. A method according to any one of the preceding claims, wherein the current density on the workpiece in the electrodeposition of at least one of the first nickel layer and the second nickel layer is at least 0.1 A / dm ² and at most 2 A / dm ² . A method according to any one of the preceding claims, wherein the sulfur-containing additive in the second bath is a brightener additive. The method of claim 7, wherein the brightener is saccharin. A method according to claim 7 or 8, wherein the brightener is provided in the second bath at a concentration of at least 2.5 g / l and at most 15 g / l. A method according to any one of the preceding claims, wherein the second bath comprises nickel at a concentration of at least 40 g / l and at most 150 g / l. A method according to any one of the preceding claims, wherein the second bath comprises chloride at a concentration of at least 10 g / L and at most 50 g / L. A method according to any one of the preceding claims, wherein the second bath comprises boric acid at a concentration of at least 40 g / l and at most 100 g / l. Workpiece which is protective coated with a first nickel layer of galvanic nickel which is free of sulfur; - On a second nickel layer of galvanic nickel, which has sulfur at a mass fraction of at least 0.05 wt .-%; - on top of it
in which workpiece the electrochemical potential difference between the first nickel layer and the second nickel layer is at least 160 mV, preferably at least 170 mV. A workpiece according to claim 13, which is an electrical connection part which is at least one of screwable and pluggable. Use of a workpiece according to claim 13 or 14 in the military field.