EP2635724B1 - Process for electroplating hard chromium from a cr(vi) free electrolyte - Google Patents

Description

The invention relates to a process for the electrodeposition of hard chromium on a substrate from an electrolyte solution containing Cr (II) or mixtures of Cr (II) and Cr (III) and halides.

Hard chrome is produced as a wear and corrosion protection according to the current state of the art in high layer thicknesses almost exclusively of chromic acid electrolyte. Depending on the deposition conditions, hardnesses of between 800 and 1150 HV _{0.1 are} achieved. A further increase in hardness can be achieved with conventional hard chrome only by post-treatment. Because of the carcinogenic potential of Cr (VI) compounds and the resulting limitations, such as Regulation (EC) No 1907/2006 (REACH Regulation), increased efforts are underway not only for products but also for manufacturing processes Cr (VI ) -free. Particularly affected are the aerospace and automotive industries and their suppliers.

Despite considerable research efforts, the deposition of hard chromium from Cr (III) electrolytes in layer thicknesses> 50 μm has so far not been satisfactorily solved. Among others, because of the desired avoidance of unwanted anode reactions for the use of dimensionally stable (gas evolving) electrodes, depositions predominantly of sulfate electrolytes have been proposed in the literature, often using additives.

The GB 1 488 381 and the GB 1 144 913 see the use of dimethylformamide (DMF), the US 4,107,004 the use of hypophosphite before. The deposition of thin Cr layers for decorative purposes is often made of formate oxalate or maleate-based Cr (III) electrolytes with glycine as an additive.

In the US 4,804,446 For example, an electrolyte based on CrCl ₃ with additions of boric acid (as buffer), KBr (as conducting salt) and various organic acids (formic acid, glycolic acid and citric acid) or their Na or K salts is disclosed.

The WO 2007/115030 A1 describes the chromium deposition from trivalent chromium sulfate / chloride electrolyte. The published US patent application. 2008/0169199 A1 describes the use of a trivalent chromium electrolyte based on sulfate and / or chloride with various bromides as additives.

The main disadvantages of the methods described so far in comparison to the deposition of hard chromium from conventional hexavalent electrolytes are usually due to a successful maximum layer thickness d (usually d <15 .mu.m) as well as inadequate layer quality or adhesion, etc. Sulfate-based electrolytes frequently have the disadvantage of formation of Cr (VI) as a side reaction at the anode, which is undesirable for Cr (III) electrolytes. The disadvantage of halide-based electrolytes is the anodic reaction of the electrolyte under halogen evolution when using dimensionally stable electrodes.

Object of the present invention is therefore to provide a method of the type mentioned, in which these disadvantages are avoided. In particular, a method is to be provided in which a chromium layer is deposited on a substrate, which has good adhesion and very high hardness and no upper limit for the achievable layer thicknesses. At the same time the chrome layer should have high gloss. Most importantly, the disadvantages of the prior art regarding the toxicity and carcinogenicity of Cr (VI) compounds should be avoided.

This object is achieved by a method for electrodepositing hard chrome on a substrate from an electrolyte solution containing Cr (II) or mixtures of Cr (II) and Cr (III) and halides by preventing the formation of elemental halogen by using a solution anode or by complexing the elemental halogen formed during the deposition with a complexing agent which is a quaternary ammonium compound.

The preferred halides are iodides and bromides, although preference is given to the bromides or mixed solutions of iodide / bromide.

In one embodiment, it can be provided that the preparation of the electrolyte solutions by dissolution of Cr metal takes place in acidic halide solutions.

Alternatively, it can be provided that the production of the electrolyte solutions by mixing a solution of Cr (III) halide and a Cr (II) -containing solution. In this embodiment, on the one hand, it may be provided that the preparation of the Cr (II) -containing solution is carried out discontinuously by dissolving Cr metal and admixing it with the electrolyte solution. On the other hand, however, it can also be provided that the preparation of the Cr (II) -containing solution by dissolving Cr metal and the admixing with the electrolyte solution is carried out continuously by means of a bypass.

In one embodiment, it can be provided that the preparation of the electrolyte solutions by dissolution of Cr (III) halides and the in situ - reduction of Cr (III) to Cr (II) takes place directly on the substrate or by means of at least one auxiliary electrode.

Alternatively, however, it may also be provided that the preparation of the electrolyte solutions by dissolution of Cr (III) halides and the ex situ - reduction of Cr (III) to Cr (II) by means of at least one auxiliary electrode.

The deposition of metallic chromium apparently proceeds via Cr (II) ions as an intermediate. When using conventional Cr (VI) or commercial Cr (III) electrolytes, these are formed "in situ", ie in the reaction vessel and, moreover, directly on the cathode surface, ie on the workpiece as an intermediate.

Experience has shown that although a certain minimum concentration of these (very volatile) ions is indispensable for the deposition, in this situation it causes an increasing disturbance of the deposition (due to so-called oleation of the Cr (III) hydrates) which is the probable cause of the limited layer thickness of commercial Represent Cr (III) processes. This oleate ion, which prevents further chromium deposition, is favored by the simultaneous concurrence of (1.) high Cr (II) concentrations and (2.) high pH values, a consequence of the likewise immanent evolution of hydrogen.

1. 1. direkte Auflösung von Chrommetall zu Cr(II), welches teilweise unmittelbar durch die Säure oder Sauerstoff zu Cr(III) weiterreagiert. Das Ergebnis ist eine (bezüglich Oleation) ausreichend saure Cr(II)/Cr(III)Lösung mit hohem Cr(II)-Gehalt.
2. 2. die elektrochemische Erzeugung von Cr(II) an Hilfselektroden innerhalb (in situ) oder außerhalb (ex situ) des Reaktionsgefäßes bei relativ positiven Potentialen (-450 mV). Die (lokale) Alkalisierung ist an solchen Hilfelektroden geringer als am Werkstück bei der Cr-Metallabscheidung (bei ca. -1100 mV) und überdies nicht relevant, da sie in weiterer Entfernung von der Hilfselektrode durch die Einwirkung der überschüssigen Säure kompensiert wird.
3. 3. In konventionellen Cr(VI)-Elektrolyten wird eine zu hohe lokale Konzentration von Cr(II)-Ionen an der Kathode durch die hohe Oxidationskraft der im hohen Überschuss vorliegenden Cr(VI)- Ionen weitgehend unterbunden.

In the process according to the invention, a high Cr (II) content is produced at the same time as the pH is low, which apparently prevents this oleate ion. This is guaranteed either by:

1. 1. direct dissolution of chromium metal to Cr (II), which partly further reacted directly by the acid or oxygen to Cr (III). The result is a sufficiently acidic Cr (II) / Cr (III) solution (in terms of oleate ion) with a high Cr (II) content.
2. 2. the electrochemical generation of Cr (II) at auxiliary electrodes within (in situ) or outside (ex situ) of the reaction vessel at relatively positive potentials (-450 mV). The (local) alkalization is lower at such auxiliary electrodes than at the workpiece in the Cr metal deposition (at about -1100 mV) and, moreover, not relevant, since it is compensated for further away from the auxiliary electrode by the action of the excess acid.
3. 3. In conventional Cr (VI) electrolytes, a too high local concentration of Cr (II) ions at the cathode is largely prevented by the high oxidation power of the high excess Cr (VI) ions.

If an auxiliary electrode is used, this preferably has a (negative) hydrogen overvoltage of at least 450 mV in the range of technical current densities. For this purpose, a sufficiently high hydrogen overvoltage may be provided when using an auxiliary electrode that the auxiliary electrode has a surface of Pb, Hg, amalgam or preferably of conductive (e.g., boron doped) diamond.

Particularly in the embodiments in which no solution anodes are provided, it is provided that the electrolyte solutions have complexing agents for elemental halogen in order to prevent the release of toxic halogens.

The complexing agent is a quaternary ammonium compound or a mixture of such compounds. Examples of suitable ammonium compounds are N-methyl-ethyl-pyrolidinium bromide and N-methyl-ethyl-morpholinium-bromide.

Furthermore, it is preferably provided that the complex which is formed from a reaction of the complexing agent with halogen, preferably bromine or iodine during the metal deposition, by reduction reaction to the complexing agent and halide, preferably bromide or iodide, regenerated and can be recycled in this way , This regeneration can be done for example by recombination with cathodically formed hydrogen or by dissolution of chromium metal. The aforementioned examples are suitable complexing agents.

In variants with solution anode, it is preferably provided that the solution anode consists of chromium metal or comprises chromium alloys.

As the embodiments will also show in detail, it has surprisingly been found to be favorable when the galvansiche deposition takes place at temperatures below 40 ° C, preferably between 20 ° C and 37 ° C. With these process conditions, it was possible to achieve qualitatively very high-quality layers.

As individual embodiments also show, by this method, in particular when using complexing agents, hardnesses up to 1650 HV were achieved, which thus significantly exceed the hardness range of chromium from conventional Cr (VI) baths (up to about 1150 HV).

Likewise, in contrast to commercial Cr (III) -based processes in which a maximum layer thickness of only 15 μm is often achieved, the process according to the invention has the advantage of virtually unlimited layer thicknesses, similar to the Cr (VI) -based processes.

Consequently, it can be provided according to the invention that the hardness of the deposited chromium is above 1150 HV and / or that the layer thickness of the deposited chromium is above 15 μm.

In the following, further details and advantages of the invention will be explained with reference to exemplary embodiments and more detailed descriptions.

Comparative Example:

A piece of copper plate with 1 cm ² free surface was ground (grade 600), degreased and as a substrate in a simple 2 l reaction vessel with a Pt-coated titanium expanded metal anode in 1 liter of a freshly prepared solution of 52 g (1 mol) chromium powder, dissolved in 2.6 mol HBr, at 47 ° C for 40 min. coated at a constant current density of 110 [A.dm ^-2 ]. It was a uniform, shiny layer with a thickness of 8 microns obtained, the hardness was 950 HV (Vickers hardness).

Lowering the bath temperature to 37 ° C under otherwise identical conditions (110 [A.dm ^-2 ], same solution and sample preparation, 40 min deposition time) resulted in a layer thickness of 25 μm and a hardness of 1200 HV

Example 1:

Steel sheet with 1 cm ² free surface was after mechanical and chemical pretreatment in an arrangement analogous to Example 1 under inert gas with an earlier (2 weeks before) prepared solution of 52g (1 mol) chromium powder, dissolved in 2.6 mol HBr and an addition of 0.2 mol N-methyl-ethyl-pyrolidinium bromide and N-methyl-ethyl-morpholinium bromide at 57 [A.dm ^-2 ] constant current density for 120 minutes at 35 ° C coated. Instead of the development of Br ₂ , the formation of a heavy, semi-liquid organic complex phase was observed at the anode, which was precipitated as a soil body. The bromine odor of the purge gas removed was perceptible, but compared to the experiment without complexing agent significantly reduced.

The chromium layer obtained had a thickness of 60 .mu.m with very good layer quality and a hardness of 1400 HV.

Example 2:

A solution of 1 mol freshly precipitated chromium (III) hydroxide dissolved in 2.6 mol HBr with an addition of 0.2 mol N-methyl-ethyl-pyrolidiniumbromid and N-methyl-ethyl-morpholinium bromide was used for adjustment of sufficient Cr (II) content is pre-electrolyzed using a 10 cm ² lead lead cathode for 1 h at a current density of 1 [A.dm ^-2 ].

Subsequently, a piece of copper sheet with 1 cm ² free surface after mechanical and chemical pretreatment in an arrangement analogous to Example 2 under protective gas in this solution at 19 ° C and 30 [A.dm ^-2 ] constant current density for 120 minutes as a substrate galvanically coated. The evolution of free bromine was prevented as in Example 2 by formation of a bromine complex phase both during pre-electrolysis and during deposition. A matte layer with a layer thickness of 9 μm and a hardness of 1630 HV was obtained

In the process according to the invention, the formation of free halogen is avoided either by complex formation with elimination of organic, non-water-soluble halogen complexes of higher density, or by the possible use of solution anodes in the halide solutions.

The invention thus relates to a process for the galvanic production of chromium layers from Cr (VI) -free electrolytes based on acid halide solutions, in particular iodide and bromide.

1. 1. Die Herstellung einer Elektrolytlösung (Chromlösung), umfassend ein Gemisch von 2- und 3- wertigen Chromionen mit einem hohen gehalt an Cr(II), erfolgt durch:
   1. a. Mischen von 2-und 3- wertigen Chromlösungen oder -salzen,
   2. b. direkte Auflösung von Chrommetall
   3. c. eine Kombination von Auflösung dreiwertiger Chromsalze und Auflösung von Chrommetall oder
   4. d. Elektrochemische Reduktion von Cr(III) -Lösungen in situ oder mittels Hilfselektroden.
2. 2. Die Vermeidung der unerwünschten Bildung von Cr(VI) wird erzielt durch:
   1. a. Anodische Oxidation von Bromid oder Iodid oder
   2. b. Verwendung von Lösungsanoden (Metalloxidation).
3. 3. Die Vermeidung von freiem Brom oder Iod erfolgt durch Komplexbildung mittels Additiven, beispielsweise von quarternären Ammoniumverbindungen.

In addition to the use of halogens, preferably bromides or iodides, for the preparation of a trivalent chromium electrolyte, the following aspects are additional features of the invention:

1. 1. The preparation of an electrolyte solution (chromium solution) comprising a mixture of 2- and 3-valent chromium ions with a high content of Cr (II) is carried out by:
   1. a. Mixing 2- and 3-valent chromium solutions or salts,
   2. b. direct dissolution of chromium metal
   3. c. a combination of dissolution of trivalent chromium salts and dissolution of chromium metal or
   4. d. Electrochemical reduction of Cr (III) solutions in situ or by means of auxiliary electrodes.
2. 2. Avoidance of undesired formation of Cr (VI) is achieved by:
   1. a. Anodic oxidation of bromide or iodide or
   2. b. Use of dissolution anodes (metal oxidation).
3. 3. The avoidance of free bromine or iodine takes place by complex formation by means of additives, for example quaternary ammonium compounds.

Solution anodes are - in contrast to the inventive method - not usable in conventional acidic sulfate electrolytes because of the passivation of metallic chromium.

The deposition of chromium from Cr (III) iodide solutions was described in two patents from the 1930's ( DE 575450, 1933 . DE 579065, 1933 ) and could be reproduced in own experiments. Films with good appearance but moderate hardness were obtained on Cu substrates. A particular disadvantage of this method is the formation of free halogen at the anode.

The present invention is based, for example, on tests for the deposition of Cr from CrBr ₃ electrolytes, as well as from electrolytes based on Cr (II) / Cr (III) mixtures. On Cu and steel substrates, these gave very good results in terms of layer quality and high layer thicknesses> 150μm.

Even in comparison to conventional hard chrome, excellent hardness (up to approx. 1600 HV) is achieved (without heat treatment). Moreover, the halide-based Cr (III) electrolytes used have a remarkable diffusibility compared to known electrolytes, which facilitates the deposition of even layers on components of unfavorable geometry.

The solutions used preferably consist predominantly of halides, preferably bromides or iodides.

The complexing agents, for example, quaternary ammonium compounds, can effect the attachment of free halide to form separable organic bromine complexes.

Claims (13)

1. A method for the galvanic deposition of hard chromium on a substrate from an electrolyte solution containing Cr(II) or mixtures of Cr(II) and Cr(III), as well as halides, characterized in that the formation of elemental halogen is prevented by using a soluble anode, or that elemental halogen forming during the deposition is complexed by a complexing agent which is a quaternary ammonium compound.
2. A method according to claim 1, characterized in that the halides are iodides, preferably bromides.
3. A method according to claim 1 or claim 2, characterized in that the preparation of the electrolyte solutions is accomplished by dissolving Cr metal in acidic halide solutions.
4. A method according to any of claims 1 to 3, characterized in that the preparation of the Cr(II)-containing solution is accomplished by dissolving Cr metal and the admixture to the electrolyte solution occurs discontinuously.
5. A method according to any of claims 1 to 3, characterized in that the preparation of the Cr(II)-containing solution is accomplished by dissolving Cr metal and the admixture to the electrolyte solution occurs continuously by means of a bypass.
6. A method according to claim 1 or claim 2, characterized in that the preparation of the electrolyte solutions is accomplished by dissolving Cr(III) halides and the in situ reduction of Cr(III) to Cr(II) occurs directly on the substrate or by means of at least one auxiliary electrode.
7. A method according to claim 1 or claim 2, characterized in that the preparation of the electrolyte solutions is accomplished by dissolving Cr(III) halides and the ex situ reduction of Cr(III) to Cr(II) occurs by means of at least one auxiliary electrode.
8. A method according to claim 6 or claim 7, characterized in that, at technical current densities, the auxiliary electrode has, with regard to its value, a hydrogen overvoltage of > 450mV.
9. A method according to any of claims 6 to 8, characterized in that the auxiliary electrode has a surface of Pb, Hg, amalgam or, preferably, of conductive diamonds.
10. A method according to any of claims 1 to 9, characterized in that the complexing agent can be regenerated from the halogen complex formed, preferably from complexed bromine, by a reduction reaction.
11. A method according to claim 10, characterized in that the regeneration occurs through recombination with cathodically formed hydrogen or by dissolving chromium metal.
12. A method according to any of claims 1 to 11, characterized in that the soluble anode consists of chromium metal or comprises chromium alloys.
13. A method according to any of claims 1 to 12, characterized in that the galvanic deposition occurs at temperatures of below 40°C, preferably of between 20°C and 37°C.