EP3564411A1 - Method for black chromium plating - Google Patents

Abstract

The invention relates to a process for black chrome plating surfaces in a bath with chromic acid, a water-soluble silver compound being added to the bath with the chromic acid.

Description

The present invention relates to a method for black chrome plating of surfaces.

Black chrome plating is a process known per se, black chrome baths are commercially available. A key component is chromic acid, which is added in a fresh mixture in water and dissolved with stirring. This can be about 1 kg of chromic acid, or a little less, come to 1 liter of water. In order to optimize or adjust the bath, further additives are usually added, such as, for example, potassium hexafluorosilicate and barium carbonate. In detail, the additives fulfill different functions, they can serve, for example, an increase in the current efficiency or elimination of sulfate impurities of chromic acid (by precipitation with barium ions).

The present invention is based on the technical problem of specifying a particularly advantageous method for black chrome plating of surfaces.

This is inventively achieved by a method according to claim 1, the bathroom with the chromic acid is thus added a water-soluble silver compound. The inventor has found that chloride ions, which are usually present from the bath batch, but can also be introduced by impurities, with regard to the throwing power in the deposition and also the color can be disadvantageous. The former concerns the coating of surfaces of more complex shape, such as undercuts. With regard to the color of the black chrome layer, the chloride ions can ultimately give less black, rather grayish, surfaces.

By adding the water-soluble silver compound according to the invention, the chloride ions are bound in the form of silver chloride (AgCl). This is poorly soluble (solubility product ~ 1.7 × 10 ^-10 ), it settles in the bath or is filtered out.

In experiments with baths prepared in accordance with the addition of the water-soluble silver compound, the inventor on the one hand has observed a significantly improved scattering. This can be investigated with a so-called Hull cell, ie a calibrated angular cell. Simply put, the black chromium can be deposited better into "corners" by precipitating the chloride ions. In addition, the resulting black chrome layers are jet black, so they show no unwanted gray cast.

Preferred embodiments can be found in the dependent claims and the entire disclosure, wherein in the representation of the features is not always distinguished in detail between the working and a manufacturing process; In any case, implicitly, the disclosure must be read with regard to all categories of claims.

In a preferred embodiment, silver nitrate is added to the bath as a water-soluble silver compound. The nitrate is not further detrimental to the bath in terms of the resulting black chrome layer (scattering and deep black color).

In a preferred embodiment, the bath after adding the water-soluble silver compound is free of chloride ions, so there are no longer chloride ions in it. In general, a particular challenge in the development of the present subject matter was that chloride ions can only be detected with limited accuracy (~100 mg / l). The inventor has aligned his researches that in a series of experiments, based on the calculated amount of chloride ions present in the bath (which results from the new formulation), silver nitrate was added to the "saturation". The best results were observed when all chloride ions were precipitated, so the bath was free.

In a preferred embodiment, the water-soluble silver compound is added to the bath at a concentration of at least 5 mg / l, 10 mg / l and 15 mg / l, respectively. The stated composition relates to the concentrated water-soluble silver compound, based on 1 l of the bath with the chromic acid. Advantageous upper limits may, for example, be at most 100 mg / l, 80 mg / l, 60 mg / l, 40 mg / l and 20 mg / l, respectively. In general, upper and lower limits should also be disclosed independently of each other.

In a preferred embodiment, the water-soluble silver compound is added to the bath following a fresh approach. For such a new approach of the bath appropriate formulations and chemicals are commercially available, for example. From the company Schlötter. In principle, chromic acid is added to water, resulting in the chloride ion addressed by the present invention. The amount of chromic acid in the new batch can, for example, be at least 350 g / l or 400 g / l, with possible (independent) upper limits at z. B. at most 550 g / l or 500 g / l.

Furthermore, additives can be used, for example to increase the current efficiency or eliminate sulphate impurities, for example, potassium hexafluorosilicate and barium carbonate are added. Initially, a chromium III content of about 16 g / l is advantageous, which can be achieved by appropriate addition of sugar. Ideally, the initial proportion of fluoride is increased, for example. To about 0.06 g / l fluoride. The current density during the deposition may, for example, be at least 5 A / dm ² , preferably at least 15 A / dm ² , with upper limits at, for example, at most 40 A / dm ² or 25 A / dm ² .

In a preferred embodiment, the water-soluble silver compound is added to the bath at intervals during operation. The latter means that some surfaces have already been black-chromed before being added to the bath, the water-soluble silver compound need not necessarily be added during the actual deposition itself (this is generally possible, but not preferred). Preferably, the surfaces become in several batches sequentially chromed black and the water-soluble silver compound is added to the bath between the black chrome plating of two batches (each batch of components are introduced into the bath, black chrome and then removed from the bath).

In general, the adding during operation may be advantageous insofar as chloride ions can be introduced into the bath by impurities or dirt effects. One possibility of the entry may, for example, result from flushing errors before chrome plating. Preferably, a combination of re-addition addition and on-the-fly addition may be used to account for both chromic acid and impurities.

In connection with the present invention, in particular also in the EP 1 876 268 B described ultrasonic cleaning to reduce the burden of the coated components with hexavalent chromium ions. Depending on the field of application and the legal framework conditions, the resulting reduction of chromium VI stress makes black chromium plating significantly more attractive or usable, with details relating 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 the most varied surfaces to be coated, with preferred embodiments of the method for plastic surfaces on the one hand and metal surfaces on the other will be discussed in more detail. 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. 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 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. The germination can be a chemical Pre-treatment step are preceded, such as a sulfonation or a 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 invention also relates to a method for producing a black-chrome-plated component that is black-chrome-plated in a method disclosed here. The component is preferably an automobile part. This may, for example, be provided for the interior, it may, for. B. act a fan blade, a switch or a panel or cover. In general, the automotive part is preferably a plastic part.

In a preferred embodiment, the automobile part is provided as part of the vehicle body, so as an automobile outer part. Particularly preferably, it is an apron in the bumper area. In this can, for example, the air intake be integrated into the engine compartment. In general, the application of the method described herein in the automotive environment can be of particular advantage insofar as the resulting black chrome layers are also particularly easy to polish, namely, the inventor has observed a particularly uniform removal. This is advantageous, for example, with regard to the surfaces which are comparatively large in the automotive environment (irregularities are more pronounced than in the case of a small surface, eg of a grid).

In the following the invention will be explained in more detail with reference to an embodiment, wherein the individual features in the context of the main claim in other combinations may be essential to the invention in other combinations and continue to distinguish not in detail between the different claim categories.

The component to be coated is preferably an apron for mounting in the bumper region of a motor vehicle, also referred to as a "bumper". Typically, such a skirt is made of a plastic material. The surface to be coated is pretreated before the black chrome plating, for example by pickling in chromic acid. Subsequently, z. B. germinated with palladium and then chemically pre-nickeled. The chemical nickel layer (layer thickness typically less than 1 μm) is then galvanically reinforced, for example with a galvanic nickel layer having a thickness in the range of a few 10 μm.

However, plastic components are not mandatory, the method of the invention can of course also be applied to metal parts. An example from the automotive sector are head restraint struts, but also grids (grille) can be coated, which can also be used outside the automotive sector. Examples are covers for electronic devices called, for example. Grille for jukeboxes, black chrome screened for microwave ovens are possible.

In their further treatment, the nickel-plated plastic component and the metal parts do not differ. Furthermore, the embodiment with "component" refers to both variants. The components are cathodically activated before black chromium plating (in aqueous solution of 60 g / l sodium hydrogen sulfate at pH ~1.8 and 3 A / dm ² for 30 s). To support the cleaning and activation function, the solution may contain surfactants and fluorides.

Then it is black chromed in a bathroom. For its new approach corresponding formulations and chemicals are commercially available, for example. From the company Schlötter. As already mentioned, chromic acid is added to water (~450 g / l chromic acid), and further, for example, potassium hexafluorosilicate and barium carbonate are added. Initially, a chromium III content of about 16 g / l is advantageous, which can be achieved by appropriate addition of sugar. Ideally, the initial proportion of fluoride is increased, for example. To about 0.06 g / l fluoride.

Furthermore, a water-soluble silver compound is added to the bath, namely silver nitrate, and based on the bath to about 20 mg / l. The silver forms with the present of the chromic acid in the bath chloride ions silver chloride, which can be deposited or filtered out. This will set the bath so that it will no longer contain chloride ions. With regard to the advantages, reference is expressly made to the introduction to the description, it is possible to improve the throwing power and obtain a layer with a deep black color, in addition, the resulting surfaces are well polishable.

The current density during the deposition may, for example, be at least 5 A / dm ² , preferably at least 15 A / dm ² , with upper limits at, for example, at most 40 A / dm ² or 25 A / dm ² . After the black chrome plating is preferably cleaned in an alkaline solution, cf. the description introduction in detail.

Claims (12)

Method for black chrome plating surfaces in a bath of chromic acid,
characterized in that a water-soluble silver compound is added to the bath with the chromic acid. The method of claim 1, wherein the water-soluble silver compound added to the bath is silver nitrate. The method of claim 1 or 2, wherein the bath is free of chloride ions after adding the water-soluble silver compound. A method according to any one of the preceding claims, wherein the water-soluble silver compound is added relative to the bath at a concentration of at least 5 mg / L. A method according to any preceding claim, wherein the water-soluble silver compound is added to the bath at a concentration of at most 100 mg / L. A method according to any one of the preceding claims, wherein the water-soluble silver compound is added to the bath with the chromic acid after a re-batch of the bath. A method according to any one of claims 1 to 5, wherein the water-soluble silver compound is added to the bath at intervals during operation. The method of claim 7, wherein the surfaces are black-chromed sequentially in multiple batches, the water-soluble Silver compound is added to the bath between the black chrome plating of two batches. 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, in particular at a Ultrasonic power from 0.5 W / l to 2 W / l. A method of producing a component having a black-chrome-plated surface, wherein the component is black-chromed in a method according to one of the preceding claims. The method of claim 10, wherein the component is an automotive part. The method of claim 11, wherein the automobile part is provided as part of a vehicle body, preferably as a skirt.