EP3312307A1 - Method for efficient black chromium plating i - Google Patents

Abstract

The invention relates to an improved black chrome plating, for example, of auto parts.

Description

The present invention relates to a process for black chrome plating of surfaces, in particular for automotive interior surfaces.

Black chromium plating is a well-established and long-known method of surface coating in which, in addition to metallic chromium, more or fewer polychromates are deposited, depending on the morphology and the state of germination. It creates dark and especially black surfaces of decorative function. Black chromed surfaces generally suffer from limited abrasion resistance and also contain i. d. R. hexavalent chromium ions. Abrasion resistance varies depending on the application, but can be a significant disadvantage. The presence of hexavalent chromium is fundamentally disadvantageous because of the health hazard it poses and the environmental regulations prevailing, especially in the automotive industry (eg RoHS compliance).

With the in the European patent EP 1 876 268 B In the present invention, the described problem of the problem with hexavalent chromium ions has been solved or considerably reduced, the disclosure which follows in principle also referring to black chromium plating methods deviating from this patent.

In principle, black chromium plating next to a good thermal efficiency in terms of efficiency of the process and blackening is desired. On this basis, the present invention is the technical problem of providing an improved method for black chrome plating.

This problem is solved by a method for black chrome plating of surfaces in a bath with chromic acid, characterized by a fluoride content in the bath of between 0.03 g / l and 0.2 g / l.

The black chrome plating is generally carried out in chromic acid baths with other additives, such as potassium nitrate, potassium hexafluorosilicate and barium carbonate. The additives are individually different and fulfill purposes such. B. an increase in the current efficiency or z. B. elimination of sulfate impurities such as chromic acid (by precipitation with barium ions). These details are not of primary concern here. Rather, it is essential that fluoride ions are also present in a typical galvanic black plating bath, for example at equilibrium with silica (compare hexafluorosilicate as mentioned above). According to the invention, this fluoride content is increased or adjusted to a quantitative range between 0.03 g / l and 0.2 g / l (based on the fluoride mass and the volume of the electroplating bath), the lower limit of this interval being increasingly preferred in the following order: 0.035; 0.04; 0.045; 0.05; 0.055 and finally 0.06 g / l, and as upper limit also in the following order: 0.18; 0.16; 0.14; 0.12; 0.1; 0.095; 0.09; 0.085 and finally 0.08 g / l.

For example. The fluoride content can be increased by adding potassium fluoride or hydrofluoric acid.

The inventor has found that the current efficiency and rate of deposition are significantly improved, especially in an initial phase. It may happen that the layers according to the invention are deposited in the long term in a very similar or identical manner as in a comparable process without increased fluoride content. However, deposition takes place much more quickly in a phase which is decisive for practical cases, for example in some cases twice as fast.

Furthermore, it has been found that the increased fluoride content also significantly reduces the energy yield and thus, with otherwise comparable process control, the energy required for the electrical supply itself and for the frequently required cooling of the bath (because of the Joule heat loss), for example in individual cases Order of 20% less power and accordingly also lower cooling capacity. Taking into account the aforementioned faster process control, this results in substantial energy savings potential, a greater yield in the process in terms of time efficiency and lower requirements for the electrical supply and the cooling system.

A further embodiment of the black chrome plating process according to the invention relates to an increased value of trivalent chromium ions (chromium III) relative to the chromium ions having an oxidation number 6 (chromium VI), namely between 2 and 6%. Preferred lower limits of this interval are 2.2; 2.4; 2.8 and finally 3%, preferred upper limits at 5.5; 5.0; 4.6; 4.4; 4.2 and finally 4%, each in that order.

Formulated in absolute terms (and also meant independently of the above relative values), a preferred chromium III content is between 6 g / l and 50 g / l, with preferred lower limits being 8; 10; 11 and 12 g / l and preferred upper limits at 45; 40; 35; 30; 25 and finally 20 g / l, each in that order. The inventor has found that this can improve the degree of blackening and decrease the tendency of the layers to a slight browning. A brownish tint may be bothersome for aesthetic reasons, but is also associated with some increased effort independently, if other (especially decorative) surfaces must be matched appropriately, eg. B. black painted parts.

The chromium-III ions can be added as such or can also be produced by reducing the chromium VI, which is already present in large quantities because of the chromic acid. Preferably, a known per se reducing agent is used. A particularly inexpensive and simple solution is the addition of sugar, which is thereby oxidized to carbon dioxide.

As already explained, is in connection with this invention, as already in the cited EP 1 876 268 B described ultrasonic cleaning in alkaline solution, ie from pH 8, preferably. Depending on the field of application and legal In general, the resulting reduction in chromium VI stress makes black chromium plating significantly more attractive or even usable, with details referring to the cited patent.

This cleaning step after black chromium plating is preferably carried out in an alkaline solution in a pH range between 9 and 13, with pH values below 12 or below 11 being preferred, for example at pH 10. The solution may optionally also contain surfactants. These improve the cleaning properties, but do not make the ultrasonic treatment unnecessary. Preferably, the ultrasonic treatment is carried out in a temperature range between 50 and 60 ° C. The ultrasound treatment itself preferably lasts at least 30 s. The ultrasound power can range between 0.5 and 2 W / l.

This procedure according to the invention is suitable for a wide variety of surfaces to be coated, with preferred embodiments of the method for plastic surfaces on the one hand and metal surfaces, on the other hand, being discussed in more detail below. The black chrome plating requires a metallic surface, which in individual cases, for example, in metallic copper workpieces or stainless steel workpieces, even in an uncoated workpiece surface itself can exist. However, an additional metallization, preferably a galvanic metallization, is preferred. Here are particularly nickel metallizations into consideration.

Furthermore, a cathodic activation of the metallized surface is preferably additionally provided before the black chrome plating. A cathodic activation significantly improves the quality of the black chrome plating, above all it leads to a lower crack formation and thus supports the cleaning options by the ultrasonic cleaning step according to the invention. In principle, the cathodic activation of the metallized surface before the black chromium plating also ensures a particularly efficient cleaning of the surface, also with regard to organic impurities remaining from previous metallization steps. This also offers a special cathodic activation Fine grained germination during the following black chrome plating. This may be related to the reduction of particles on the surface by the cathodic circuit. In any case, the layers according to the invention show a good and even improved abrasion resistance on the substrate compared to conventional layers and at the same time a significantly reduced cracking of the black chromium plating layer.

The initial metallization prior to cathodic activation is preferably a galvanic metallization, even in the case of materials other than nickel, such as in the case of copper. Particular preference is given to a galvanic high-gloss nickel layer, that is to say a nickel layer deposited from a galvanic bath with brightener additives.

The procedure according to the invention for black chrome plating is also particularly suitable for plastic surfaces. Here, metal is preferably first germinated, preferably with palladium. Germination may be preceded by a chemical pretreatment step, such as sulfonation or pickling in chromic acid solution.

The germination is followed by a chemical nickel coating, ie a nickel layer deposited without external current. This nickel layer can then be galvanically reinforced, in particular with nickel or copper. Preference is given to pure galvanic nickel layers, ie without brightener additives in the galvanic bath, or copper layers of acidic electroplating solution, ie. H. based on sulfuric acid and not on cyanide.

Furthermore, the procedure according to the invention is also suitable for metal surfaces. Particularly suitable metals are: non-ferrous metals, zinc die casting, light metals and light metal alloys, iron and steel materials.

On metal surfaces, an initially conventional layer structure is predetermined, such as with a galvanic metallization, in particular copper, and a then the following galvanic high-gloss metallization, in particular copper coating of acidic solution or nickel coating. The high-gloss layers have the function of leveling the surface. The following procedure then continues as already explained in connection with the plastic surfaces; so it follows the cathodic activation.

Another aspect of the invention relates to possibilities of adjusting the gloss or mattness of the black-chromed surface. Here are to be created by fotomatten to high-gloss layers of play. To set certain degrees of matting here is provided to produce the initial metallization of the surface by applying a matte nickel layer on a smooth surface of the workpiece by electrodeposition without organic matting additives and further applying a Sulfamatnickelschicht.

The basic idea of this aspect of the invention is to apply a matt nickel layer on a smooth workpiece surface and adjust the mattness over the thickness of the nickel layers. This is aimed at galvanic nickel layers, where no organic matting additives are used. Rather, in a preferred embodiment, it may be a known Wattsche nickel layer, which is technically simple and easy to control.

On the matte nickel layer then another sulfamate nickel layer should be applied. This has the advantage and the function of slightly rounding off the more or less grain-like structure of the matt nickel layer and thus making it less rough and susceptible to soiling.

Overall, the result of the interaction of depending on the strength of the matte nickel layer even by a more or less pronounced residual gloss emerging smooth surface on the workpiece with the conferred by the matte nickel layer dullness and finally the explained Rounding by the Sulfamatnickelschicht an optically attractive and above all well reproducible silk gloss. This is in no way inferior in its optical quality to the known nickel layers with organic matting additives. In addition, in this method, the degree of gloss or degree of matting can be easily and easily controlled by various galvanic parameters, in particular by treatment time and / or current intensity. The need to filter the solutions for filtering organic matting additives is also eliminated. However, the use of conventional nickel layers with a degree of mattness which can be adjusted by means of organic matting additives is likewise possible within the scope of the present invention.

The smooth surface on the workpiece under the matt nickel layer may, for example, be a polished workpiece surface itself or else an applied metal layer. If a preferred bright nickel coating is used here, this has the particular advantage of very well leveling any surface defects and defects. Thus, it can improve the quality of the final gloss content of the finished metal surface according to the invention. Galvanic processes for bright nickel coatings are well known and need not be detailed here.

The matte galvanic nickel layer is preferably applied as a known and technically well-controlled Wattsche nickel layer, d. H. as galvanic nickel layer without organic matting additives. This results in a microscopically bulbous layered structure in which the tuber sizes and tuber intervals can be set via the current intensity and / or coating time, which ultimately determine the dullness. A maximum dullness arises when the tubers are practically close to each other.

The electrodeposition of a sulfamate nickel layer is also conventional and known. The corresponding solutions contain nickel sulfamate, that is, the salt of amidosulfuric acid. The sulfamate nickel coating rounds and reinforces the aforementioned nodular or otherwise matt nickel layer something, however, does not really level it. In particular, the sulfamate nickel layer also enhances the grain size without changing fundamentals at the granularity referred to above as "nodular". It thus receives the matte character, possibly only slightly increases the gloss, but above all provides increased material strength for reasons of stability and resilience and for better wiping sensitivity or better dirt-repellent properties. The roughness reduced by the rounding provides less grip to soiling.

A particular advantage of this embodiment is that the degree of gloss or degree of matting can be adjusted by galvanic parameters in a very simple manner and also has the desired effect after black chromium plating. Different optical properties can be generated with one and the same basic process, that is, the same solution compositions, identical baths, etc. In particular, can be adjusted from batch to batch simply on the current, or even cheaper over the treatment time, the dullness. The thicker the matt nickel layer is, the higher the degree of matting results. This also applies after the application of the following black chrome layer.

An economically quite interesting alternative is to resort to the production of already chrome parts, which then under the chrome layer z. B. are constructed according to the above description. Such parts are often available at very reasonable cost. There, the chrome layer z. B. are removed galvanically and then the black chrome plating invention are applied, preferably after cathodic activation. This cathodic activation can be carried out (also in the above process) in alkaline or acidic solution, with a pH between 11 and 13 or 1 and 2. Preferably, a combination, because the alkaline cathodic activation gives good results, a subsequent acidic cathodic activation but helps to avoid occasional difficulties with the carryover of residues of the alkaline solution into the acidic black plating bath.

A particularly favorable application of the invention in coatings of parts whose surfaces occur indoors, but also outdoors, of automobiles and other vehicles. Here, because of the contact with human skin, chromium-VI freedom is particularly important. Furthermore, in this area increased demands are placed on the abrasion resistance of surface layers. But also applications, for example in medical technology come into consideration.

1. 1. Verfahren zur Schwarzverchromung von Oberflächen in einem Bad mit Chromsäure, gekennzeichnet durch einen Fluorid-Anteil in dem Bad zwischen 0,03 g/l und 0,2 g/l.
2. 2. Verfahren nach Aspekt 1 mit einem Anteil von Chrom III, der relativ zu dem Anteil von Chrom VI in dem Bad zwischen 2 % und 6 % beträgt.
3. 3. Verfahren nach Aspekt 1 oder 2 mit einem Anteil von Chrom III, der zwischen 6 g/l und 50 g/l beträgt.
4. 4. Verfahren nach Aspekt 2 oder 3, bei dem der Chrom-III-Anteil erzeugt wird durch Reduzieren von Chrom-VI-Ionen, insbesondere durch Zugabe eines Reduktionsmittels.
5. 5. Verfahren nach Aspekt 4, bei dem das Reduktionsmittel Zucker ist.
6. 6. Verfahren nach einem der vorstehenden Aspekte, bei dem das Schwarzverchromen auf einer Nickelschicht erfolgt, insbesondere einer galvanischen Nickelschicht, insbesondere einer Hochglanz-Nickelschicht.
7. 7. Verfahren nach Aspekt 6, bei dem die Nickelschicht vor der Schwarzverchromung kathodisch aktiviert wird, und zwar in alkalischer Lösung, insbesondere bei einem pH-Wert zwischen 11 und 13.
8. 8. Verfahren nach Aspekt 6 oder Aspekt 7, bei dem die Nickelschicht vor der Schwarzverchromung kathodisch aktiviert wird, und zwar in saurer Lösung, insbesondere bei einem pH-Wert zwischen 1 und 2.
9. 9. Verfahren nach einem der vorstehenden Aspekte, bei dem nach dem Schwarzverchromen die schwarzverchromte Oberfläche in alkalischer Lösung mit Ultraschallbehandlung gereinigt wird, insbesondere bei einem pH-Wert zwischen 9 und 13, insbesondere bei einer Temperatur zwischen 50 °C und 60 °C und insbesondere bei einer Ultraschallleistung von 0,5 W/l bis 2 W/l.
10. 10. Verfahren nach einem der vorstehenden Aspekte, bei dem ein bereits verchromtes Teil entchromt wird, insbesondere galvanisch, und danach schwarzverchromt wird.
11. 11. Verfahren nach einem der vorstehenden Aspekte, bei dem eine Automobilinnenoberfläche schwarzverchromt wird.
12. 12. Verfahren nach einem der Aspekte 1 bis 10, bei dem eine Automobilaußenoberfläche schwarzverchromt wird.

The invention thus relates to the following aspects as a whole:

1. A method of black chrome plating surfaces in a bath of chromic acid characterized by a fluoride content in the bath of between 0.03 g / l and 0.2 g / l.
2. 2. Method according to aspect 1 with a proportion of chromium III which is between 2% and 6% relative to the proportion of chromium VI in the bath.
3. 3. The method according to aspect 1 or 2 with a proportion of chromium III, which is between 6 g / l and 50 g / l.
4. 4. A process according to aspect 2 or 3, wherein the chromium III portion is produced by reducing chromium VI ions, in particular by adding a reducing agent.
5. 5. The method of aspect 4, wherein the reducing agent is sugar.
6. 6. Method according to one of the preceding aspects, wherein the black chrome plating takes place on a nickel layer, in particular a galvanic nickel layer, in particular a high-gloss nickel layer.
7. 7. The method of aspect 6, wherein the nickel layer is cathodically activated prior to the black chrome plating, in alkaline solution, in particular at a pH between 11 and 13.
8. 8. A process according to aspect 6 or aspect 7, wherein the nickel layer is cathodically activated prior to black chromium plating, in acidic solution, in particular at a pH between 1 and 2.
9. 9. The method according to any one of the preceding aspects, wherein after black chrome plating the black-chromed surface is cleaned in alkaline solution with ultrasound treatment, in particular at a pH value between 9 and 13, in particular at a temperature between 50 ° C and 60 ° C and in particular at an ultrasonic power of 0.5 W / l to 2 W / l.
10. 10. The method according to any one of the preceding aspects, in which an already chrome-plated part is chromed, in particular galvanically, and then black-chromed.
11. 11. A method according to any preceding aspect, wherein an automotive interior surface is black chrome plated.
12. 12. The method of any one of aspects 1 to 10, wherein an automobile exterior surface is black chrome plated.

In the following, the invention will be explained in more detail with reference to a concrete exemplary embodiment, wherein the disclosed features can also be essential to the invention in other combinations.

For example, a plastic door handle for automobiles made of glass fiber or mineral fiber reinforced polyamides, ABS or ABS-PC according to the invention In the case of ABS and ABS-PC, a dressing with chromic acid or, in the case of polyamides, sulfonation first takes place.

Then it is germinated in a conventional manner with palladium and a likewise conventional chemical nickel layer of the thickness of 0.1 - 1 microns deposited.

This is followed by a reinforcement with a matt nickel plating of thickness 3 - 15 μm followed by a galvanic high-gloss nickel plating to level the thickness 15 - 25 μm.

In another example, a metal part, namely an automotive headrest support strut made of an aluminum alloy, is first coppered electrolytically with a thickness of 3 - 10 microns. This is followed by an acidic high-gloss copper layer of thickness 20-25 μm and optionally another high-gloss nickel layer.

Also, an already chrome-plated trim part of an instrument panel or in the front area under a bumper can be galvanized.

In all examples, then in an aqueous solution of 60 g / l of sodium bisulfate at a pH of about 1.8 is cathodically activated, 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.

Thereupon, a black-chromium-plating layer can be deposited electrolytically with baths which are generally known. For example. The company Schlötter provides appropriate formulations and the necessary chemicals. In addition to, for example, 450 g / l chromic acid, about 1 g / l potassium nitrate and (preferably to saturation) potassium hexafluorosilicate and barium carbonate can be added. The latter in an amount sufficient to precipitate the sulphate impurities present in the chromic acid (as sparingly soluble barium sulphate). So far that is Process however conventional. In addition, 0.2 g / l potassium fluoride (equivalent to a little 0.06 g / l fluoride) is then added, and sugar in an amount that produces about 16 g / l chromium III (which can be measured and gives the amount of sugar). As a result, the desired layers z. B. generated in 5 minutes instead of 10 minutes at 20% reduced current and correspondingly lower cooling demand, the layers show a very good and little tendency to browning blackening. Depending on the individual part can be used for z. B. 12 simultaneously treated automotive interior trim strips or the like instead of 1,800 A only 1,500 A are used or even only 1,200 A, the process time is still shortened.

Chromium VI oxides and polychromates are initially present on the black chromium plating layer. These impurities can be removed very successfully by a first simple water bath and then an ultrasound assisted cleaning in an alkaline solution at pH 10 and about 50-60 ° C for preferably at least 1-2 minutes. According to experience, treatments beyond 5 minutes do not bring any significant improvements. After the ultrasonic treatment in the alkaline solution again a water bath is run through.

Claims (12)

A method of black chrome plating surfaces in a bath of chromic acid, the bath having a fluoride content between 0.03 g / l and 0.2 g / l,
characterized in that an already chromed part is chromed and then black chromed with the bath. Process according to claim 1, having a proportion of chromium III which is between 2% and 6% relative to the proportion of chromium VI in the bath. A process according to claim 1 or 2, with a chromium III content of between 6 g / l and 50 g / l. The method of claim 2 or 3, wherein the chromium-III portion is generated by reducing chromium-VI ions, in particular by adding a reducing agent. The method of claim 4, wherein the reducing agent is sugar. Method according to one of the preceding claims, in which the black chrome plating takes place on a nickel layer, in particular a galvanic nickel layer, in particular a high-gloss nickel layer. The method of claim 6, wherein the nickel layer is cathodically activated prior to black chromium plating, in alkaline solution, especially at a pH between 11 and 13. A method according to claim 6 or claim 7, wherein the nickel layer is cathodically activated prior to black chromium plating, in acidic solution, in particular at a pH of between 1 and 2. Method according to one of the preceding claims, wherein after the black chrome plating the black-chromed surface is cleaned in alkaline solution with ultrasound treatment, in particular at a pH value between 9 and 13, in particular at a temperature between 50 ° C and 60 ° C and especially at one Ultrasonic power from 0.5 W / l to 2 W / l. Method according to one of the preceding claims, in which the already chromed part is electroplated by chromium plating. A method according to any one of the preceding claims, wherein an automotive interior surface is black chrome plated. A method according to any one of claims 1 to 10, wherein an automobile exterior surface is black chromed.