EP1876268A1 - Black chromium process free of Cr-VI - Google Patents

Abstract

Preparation of black chrome-plating of a surface comprises black chrome-plating the metal surface and cleaning the black chrome-plated surface using an alkaline solvent by ultrasound treatment.

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).

The present invention is based on the technical problem of providing an improved process for black chromium plating with reduced Cr-VI loading of the resulting surface and good and possibly even improved abrasion resistance.

This problem is solved by a process for black chromium plating of surfaces with the steps: black plating of a metallic surface, cleaning of the black chromed surface in alkaline solution with ultrasonic treatment.

Preferred embodiments are specified in the dependent claims and are also explained in more detail below.

The black chrome plating of metallic surfaces is conventional in itself. For this purpose, the invention proposes an ultrasonic cleaning step after black chromium plating in an alkaline solution, ie from pH 8 onwards.

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.

The cleaning step of the invention following the black chromium plating becomes efficient only through the combination of ultrasonication with the alkaline pH. It has surprisingly been found that both chemical cleaning and heat, such as cook-out, could not effectively reduce the Cr-VI loading of the black plating layer alone, but the use of ultrasound in alkaline setting provides quite significant advances. In this case, with the ultrasonic cleaning step, the Cr-VI load can be reduced to practically insignificant values, as can be verified in subsequent standard tests, such as by boiling with Diphenylcarbazidtest.

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. Also, the cathodic activation provides a particularly fine-grained germination in the course of 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 simultaneously a significantly reduced cracking of the black 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.

An initially conventional layer structure is predefined on metal surfaces, for example with a galvanic metallization, in particular copper, and then a subsequent 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.

The cathodic activation, both on top as plastic coated original plastic surfaces so also metal surfaces, preferably takes place from an acidic solution, preferably at a pH between 1 and 2. Preferred are aqueous solutions based on sodium sulfate and sulfuric acid or on the Base of sodium hydrogen sulfate. The solution may optionally also contain surfactants and / or fluorides. Preferably, amounts of about 30-100 g / l, more preferably 50-75 g / l sodium hydrogen sulfate. In practice, a correspondingly converted amount of sodium sulfate can be dissolved and adjusted with sulfuric acid, the pH.

Preferred electrical parameters are 1 - 5 A / dm ² at treatment times in the order of 10 - 300 s. The voltage is adjusted to give the desired current density.

The cleaning step according to the invention 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.

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 still by a more or less pronounced residual gloss emerging smooth surface on the workpiece with the imparted by the matte nickel layer dullness and finally the explained rounding by the Sulfamatnickelschicht a visually attractive and especially good reproducible silk shine. 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 layer 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. Commercial solutions are available, which may include, for example, nickel sulfate, organic brighteners, and so-called levelers. Suitable current densities in this range are 1 to 3 A / dm ² . It may also be advantageous under the bright nickel layer already a shiny metal layer, such as a copper layer to provide.

The matte galvanic nickel layer is preferably applied as a known and technically well-controlled Wattsche nickel layer, ie as a 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. Current strengths in the range of 0.1 to 2 A / dm ² , better 0.1 to 1 A / dm ² , are preferred here. The layer thickness of the matt layer should be comparatively low and may be between 0.05 and 5 .mu.m, with upper limits of 4 .mu.m, 3 .mu.m, 2 .mu.m and more preferably 1 .mu.m and lower limits of 0.075 .mu.m and more preferably 0.1 .mu.m even cheaper. The layer thickness is ultimately determined by optical / aesthetic considerations.

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, but 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 favorable thickness for the sulfamate nickel layer is in the range of 5 and 20 microns, with a lower limit of 10 and an upper limit of 15 microns are more preferred.

A particular advantage of this embodiment is that it can be set by galvanic parameters in a very simple manner, the degree of gloss or matte degree and even after the black chrome plating in the desired manner effect. 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.

A particularly favorable application of the invention in coatings of parts whose surfaces occur in the interior of automobiles and other vehicles. Here, because of the contact with human skin, the Cr-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.

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 can be coated according to the invention by initially treating in the case of ABS and ABS PC a dressing with chromic acid or sulfonation in the case of polyamides.

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 aluminum alloy automotive headrest support strut, first becomes electrolytically thick of 3 - 10 microns coppered. This is followed by an acidic high-gloss copper layer of thickness 20-25 μm and optionally another high-gloss nickel layer.

Both examples are then 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 time of 30 s. The solution contains relatively small amounts of surfactants and fluorides to support the cleaning and activation function.

Thereafter, a black chromium plating layer known per se may be electrolytically deposited from a bath of about 450 g / l chromic acid, about 7.5 g / l chromium-III sulphate, of the order of 2-8 g / l sodium or Potassium carbonate or nitrate and about 1 g / l hexafluorosilicate. This black plating layer has a thickness of about 0.3-2 μm.

Chromium VI oxides and polychromates are initially present on this 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.

The success of the invention can be controlled by a standard test by boiling with water and a diphenylcarbazide test of the cooking water. It can be z. For example, the method validated by Zentralverband Oberflächentechnik ZVO (ZVO-0101-UV-05) can be used. There, a test part is boiled under specified conditions and the Cr-VI content in the extraction solution according to DIN 38405 part 24 determined. Cr-VI oxidizes 1,5-diphenylcarbazide to 1,5-diphenylcarbazone, which forms a violet-colored complex with the resulting Cr-III. The absorbance of the dye at 540 nm is linearly related to the Cr-VI concentration, which can be evaluated by the calibration function or a comparative solution. With a standard test plate of 5 cm by 5 cm, it is allowed to boil out under the standard conditions for 10 minutes in 130 ml of water become. According to the invention, values below 300 μg / l Cr-VI can be achieved here.

If an adjustable dullness is desired in the manner already explained, follows the already mentioned galvanic high-gloss nickel layer of 15 to 25 microns, a matte Wattsche nickel layer. This is carried out at a current density of about 0.5 A / dm ² from an aqueous solution containing 210 g / l nickel sulfate, 35 g / l nickel chloride and 40 g / l boric acid without further additives. The preferred layer thickness range is between about 0.1 and 2 .mu.m, the mattness of the layer ultimately resulting being adjusted via the layer thickness. In this embodiment, 0.2 microns are deposited.

This layer thickness is meaningful only in terms of averaging. In fact, the growth is very grainy or "bulbous", the individual grains are larger with increasing averaged layer thickness and have decreasing average distances. At significantly greater layer thicknesses, the grains are finally close, resulting in a matte layer that does not let the gloss of the underlying bright nickel layer through.

In the next step, a sulfamate nickel layer is deposited on the Watt's nickel layer. The grains are thereby reinforced, rounded the corners a bit and in particular the niches and angles lying on the edge of the grains filled.

Here, a 12 μm thick layer is deposited at a galvanic current density of 1 A / dm ² . The aqueous solution contains 36% by volume of 60% by weight nickel sulphamate solution. The galvanic solution contains 5 g / l nickel chloride and 35 g / l boric acid. Considered, for example, the bathroom Schlötter MS.

This can then be followed by cathodic activation.

Claims (16)

Method of black chromium plating of surfaces with the steps: - black chrome plating of a metallic surface, - Clean the black-chromed surface in alkaline solution with ultrasonic treatment. The method of claim 1, wherein the surface is metallized prior to the black chrome plating, in particular by a nickel metallization. The method of claim 2, wherein the metallization of the surface prior to the black chrome plating is a galvanic metallization, in particular a galvanic high gloss nickel plating. The method of claim 2 or 3, wherein the initial surface is a plastic surface and the metallization of the plastic surface precede the steps of: - Metallbege germination, in particular Palladiumbekeimung, - chemical nickel metallization of the germinated surface, - Galvanic metal reinforcement of the nickel layer, in particular with a matte nickel metallization or a copper metallization of acidic solution. A method according to claim 4, wherein the metal germination is preceded by a chemical pretreatment, in particular a sulphonation or pickling with chromic acid. The method of claim 1, 2 or 3, wherein the initial surface is a metal surface and the metallization of the metal surface is preceded by the steps of: - galvanic metallization, in particular copper metallization, - Galvanic high-gloss metallization of the metallized surface, in particular high-gloss nickel metallization or high-gloss copper metallization of acidic solution. The method of claim 6, wherein the initial metal surface is selected from the group consisting of non-ferrous metals, die-cast zinc, light metals and alloys, iron and steel materials. Method according to one of the preceding claims with the additional step: - cathodic activation of the metallized surface before black chrome plating. The method of claim 8, wherein the cathodic activation is carried out in an acidic solution, in particular at a pH between 1 and 2. The method of claim 9, wherein the acidic solution contains substantially sodium sulfate and sulfuric acid in aqueous solution. The method of claim 10, wherein the acidic solution contains 30-100 g / l of sodium bisulfate and optionally surfactants and optionally fluorides. Method according to one of claims 8 - 11, wherein the cathodic activation takes place at a current density of 1-5 A / dm ² for 10 - 60 s. A method according to any one of the preceding claims wherein the final purification step in the alkaline solution is at a pH of between 9 and 13. A process according to any one of the preceding claims, wherein the final purification step in the alkaline solution is at 50-60 ° C. Method according to one of the preceding claims, wherein the final cleaning step in the aqueous solution at an ultrasonic power of 0.5 to 2 W / I takes place. A method according to any one of the preceding claims, wherein an automotive interior surface is black chrome plated.