METHOD FOR CONTROLLING THE CHROMIUM FEED IN AN ELECTROLYSIS PROCESS FOR PRODUCING CHROMIUM LAYERS, AND AN ELECTROLYSIS CELL FOR THIS PURPOSE

Description

TECHNICAL FIELD

The invention relates to a method for controlling the chromium feed in an electrolysis process for producing chromium layers, and an electrolysis cell for this purpose.

BACKGROUND OF THE INVENTION

Galvanic processes for coating the surfaces of objects have long been known in the art. The coated objects thus obtained have particularly advantageous surface properties, such as greater hardness, improved corrosion resistance, metallic appearance, gloss and the like. Using a galvanic bath that contains the metal to be deposited as a salt in solution, the metal is deposited onto the object connected as a cathode by means of direct current. The object to be coated is therefore usually a metallic material or is subjected to additional metallization of the surface in order to become electrically conductive.

One metal to be used for this purpose is chromium. The application of chromium layers using galvanic baths can be used for decorative purposes to form bright and highly reflective chromium layers. It is also possible to use chromium plating of objects for technical applications, for example, to increase wear resistance, improve abrasion resistance, and increase heat and corrosion resistance. This, for example, is used for chromium-plated pistons, cylinders, cylinder liners, axle bearings or the like.

It is well known that chromium(VI) salts, such as CrO₃, and sulphuric acid are used in galvanic baths. However, this has a number of disadvantages: For example, the generation of gas, particularly hydrogen, and to a lesser extent oxygen, leads to the formation of acidic, corrosive and partly toxic chromic acid mists. This requires intensive extraction from the surface of the galvanic bath and the use of surface-active substances or wetting agents to contain the chromic acid mist that occurs. In addition, the chromium(VI) electrolyte is highly toxic and carcinogenic. It would therefore be better to use non-toxic galvanic baths containing chromium(III) salt.

In case of galvanic chromium plating using chromium(III) salts, it must be taken into account that chromium layers can be deposited in a suitable, not too thin, layer thickness and that the system design is not too complex so that industrial use remains possible.

The deposition of chromium from the chromium(III) salt present in the electrolyte causes a decrease in the concentration of chromium(III) ions in the electrolyte. However, Cr(III) can only be added in the form of chromium(III) salts, which results in an undesirable gradual accumulation of the anion present in the salt in the electrolyte. This requires regular dilution with subsequent re-dosing of the other components and therefore constant control and monitoring of the system.

In addition, it is not possible to apply an anode voltage or an anode potential to a chromium metal anode, as chromium, as a very active metal, immediately forms an oxide layer on its surface, which passivates the chromium. When an anode voltage is applied to this passive chromium, only a small amount of chromium is dissolved. If a high anode voltage is applied, however, then the chromium dissolves as chromium(VI) ions, which are not only carcinogenic, but also represent an undesirable interference for chromium(III) electrolytes and impair the function of the electrolyte. It is therefore very difficult to make chromium continuously available to the electrolyte electrolytically as trivalent chromium.

The following proposed solutions have already become known from the prior art:

According to EP 2 640 873 A1 (WO 2012/067725 A1), a method for supplementing or increasing the chromium content of an electrolyte with trivalent chromium is described, the method comprising the steps of:

• • a) immersing an electrode containing chromium and a second electrode in an electrolyte that contains trivalent chromium ions; and
  • b) applying a pulsed alternating current to the chromium electrode and the second electrode; whereas chromium is electrolytically dissolved from the chromium electrode in the form of trivalent chromium ions, and the chromium(III) content of the electrolyte in which the chromium electrode is immersed is replenished again or accumulated. The duration of each forward pulse and each reverse pulse is typically between about 0.1 and about 2 seconds. For example, a square waveform can be used as the waveform, with the duration of the alternating current pulse being an about 400 ms cathodic forward pulse and 400 ms anodic reverse pulse. The disadvantage of this proposal is the complex technology, where permanent polarity reversal requires the use of an expensive pulse rectifier.

Furthermore, GB 414 939 discloses a method for electroplating chromium, wherein a direct current is conducted from a chromium anode to a cathode to be plated, and an alternating current is superimposed on the plating current in order to activate and dissolve the chromium of the anode. For example, FIG. 2 shows a circuit diagram of an arrangement that can be used when the alternating current is only superimposed on the anode. H is an auxiliary electrode, where the alternating current generator WG is connected to the anode A and the auxiliary electrode H via a transformer T.

The present invention is based on the object of avoiding the disadvantages of the prior art and providing a method or an electrolysis cell which enables a controlled supply of chromium metal during electrolysis without causing an accumulation of undesirable anions and without the formation of chromium(VI) ions. The chromium layers provided should also meet the requirements placed on chromium coatings, particularly on gravure cylinders.

BRIEF DESCRIPTION OF THE INVENTION

The object described is achieved in accordance with the invention by the teachings of the independent claims. The teachings of the sub-claims represent advantageous embodiments.

In particular, a method for controlling the chromium feed in an electrolysis process for producing a chromium layer is provided, wherein the chromium layer is produced by an electrolytic deposition of chromium from an electrolyte by means of direct current and use of an anode and a cathode, comprising the steps of:

• • (A) providing a first auxiliary electrode that includes chromium metal or consists of it;
  • (B) providing a second auxiliary electrode in the form of an inert electrode,
  • (C) immersing both auxiliary electrodes in the electrolyte that contains at least one chromium(III) salt;
  • (D) interconnecting the first auxiliary electrode and the second auxiliary electrode in an electric circuit separate from the cathode and anode; and
  • b) during the electrolytic deposition of chromium with formation of a chromium layer:
  • (E) applying a cathode voltage to the first auxiliary electrode, as a result of which the passivation layer of the chromium metal dissolves and chromium metal in the form of chromium(III) ions begins to go into solution in the electrolyte;
  • (F) following dissolution of the passivation layer, terminating the current supply to or switching off the voltage on the first auxiliary electrode; and
  • (G) solubilizing the chromium metal from the first auxiliary electrode in the form of chromium(III) ions without current by the action of the electrolyte.

The process can also be terminated again in a targeted manner, for example, when the object to be coated has been coated completely. According to one embodiment of the invention, the process can be terminated during step (G) by:

• • applying an anode voltage to the first auxiliary electrode, or
  • pulling the first auxiliary electrode out of the electrolyte or pumping the electrolyte out of the electrolysis cell so that the first auxiliary electrode comes into contact with ambient air, or
  • no replenishing of the chromium metal of the first auxiliary electrode, so that solubilization of the chromium metal from the first auxiliary electrode in the form of chromium(III) ions without current ends by the action of the electrolyte as soon as the chromium has completely gone into solution.

The subject matter of the invention is also an electrolysis cell for controlling the chromium feed to an electrolyte, comprising:

• • an anode;
  • a cathode;
  • an electrolyte that contains at least one chromium(III) salt;
  • the anode and cathode are immersed in the electrolyte;
  • a first circuit which connects the anode and cathode and causes the deposition of a chromium layer by an electrolytic deposition of chromium from an electrolyte by means of direct current on the cathode;
  • a first auxiliary electrode that includes chromium metal or consists of it;
  • a second auxiliary electrode in the form of an inert electrode;
  • both auxiliary electrodes are immersed in the electrolyte;
  • a second circuit which connects the first auxiliary electrode and the second auxiliary electrode in an electric circuit separate from the cathode and anode; wherein either
  • a cathode voltage is applied to the first auxiliary electrode so that the passivation layer of the chromium metal on the first auxiliary electrode dissolves; or
  • no voltage is applied to the first auxiliary electrode, so that, following dissolution of the passivation layer, the chromium metal from the first auxiliary electrode in the form of chromium(III) ions goes into solution into the electrolyte without current; or
  • an anode voltage is applied to the first auxiliary electrode so that the passivation layer forms again on the first auxiliary electrode.

The method and electrolysis cell according to the invention are therefore based on the chemical dissolution of chromium in the form of chromium(III) ions in the electrolyte, which is activated by a current flow, continues without a current flow and, if desired, can be terminated again by a current flow or in another way.

BRIEF DESCRIPTION OF THE DRAWINGS

The described and other aspects, advantages and features according to the present invention will also be described in more detail in the following paragraphs by means of the accompanying drawings, in which:

FIG. 1 shows a flow chart for illustrating an embodiment of the method according to the invention;

FIG. 2a shows a schematic representation of an embodiment of the electrolysis cell according to the invention during a galvanization phase, with a cathode voltage being applied to the first auxiliary electrode;

FIG. 2b shows a schematic representation of the same embodiment as in FIG. 2a, with no voltage being applied to the first auxiliary electrode;

FIG. 2c shows a schematic representation of the same embodiment as in FIG. 2a, with an anode voltage being applied to the first auxiliary electrode;

FIG. 3a shows a schematic representation of an embodiment of a circuit according to the invention, comprising a first, second and third auxiliary electrode, with a cathode voltage being applied to the first auxiliary electrode;

FIG. 3b shows a schematic representation of the same embodiment as in FIG. 3a, with no voltage being applied to the auxiliary electrodes;

FIG. 3c shows a schematic representation of the same embodiment as in FIG. 3a, with an anode voltage being applied to the first auxiliary electrode;

FIG. 4a shows a schematic representation of an embodiment of a circuit according to the invention, comprising interconnected units of auxiliary electrodes in order to illustrate a series connection, with a cathode voltage being applied to the first auxiliary electrodes;

FIG. 4b shows a schematic representation of the same embodiment as in FIG. 4a, with no voltage being applied to the auxiliary electrodes;

FIG. 4c shows a schematic representation of the same embodiment as in FIG. 4a, with an anode voltage being applied to the first auxiliary electrodes;

FIG. 5a shows a schematic representation of another embodiment of the electrolysis cell according to the invention during a galvanization phase, with a cathode voltage being applied to the first auxiliary electrode;

FIG. 5b shows a schematic representation of the same embodiment as in FIG. 5a, with no voltage being applied to the auxiliary electrodes;

FIG. 5c shows a schematic representation of the same embodiment as in FIG. 5a, with an anode voltage being applied to the first auxiliary electrode;

FIG. 6a shows a schematic representation of another embodiment of the electrolysis cell according to the invention during a galvanization phase, with a cathode voltage being applied to the first auxiliary electrode;

FIG. 6b shows a schematic representation of the same embodiment as in FIG. 6a, with no voltage being applied to the auxiliary electrodes; and

FIG. 6c shows a schematic representation of the same embodiment as in FIG. 6a, with an anode voltage being applied to the first auxiliary electrode.

TERMINOLOGY AND DEFINITIONS

An “electrolysis process” or “electrolysis” is understood to mean the deposition of metal, in this case chromium, from a solution that contains the corresponding metal ions, using electric current. This serves to produce metal layers, in this case chromium plating.

The terms “cathode voltage” or “cathode potential” are used synonymously and interchangeably in the present invention and are to be understood to mean that a voltage is applied to an electrode or auxiliary electrode so that the electrode or auxiliary electrode serves as a cathode.

The terms “anode voltage” or “anode potential” are also used synonymously and interchangeably in the present invention and are to be understood to mean that a voltage is applied to an electrode or auxiliary electrode so that the electrode or auxiliary electrode serves as an anode.

DETAILED DESCRIPTION OF THE INVENTION

The method according to the invention for controlling the chromium feed takes place during an electrolysis process for producing a chromium layer. The electrolysis process for producing a chromium layer is therefore described first, since the method according to the invention is closely related to this:

The electrolytic deposition of chromium layers is usually carried out in an electrolysis cell filled with the electrolyte. The container for the electrolysis cell can be any vessel that is suitable for a person skilled in the art, such as those used in galvanic technology, in particular.

The cathode usually serves as the object to be coated onto which the chromium layer is to be deposited, for example, a gravure cylinder.

Anodes known to a person skilled in the art can be used as the anode. In particular, an inert electrode is used as the anode, which is composed of one or more electrically conductive materials that are insoluble in the electrolyte. Materials used for an insoluble anode or inert electrode include, for example:

• • platinized titanium,
  • an expanded metal made of titanium, coated with a mixed oxide or coated with graphite, where required,
  • carbon materials such as graphite,
  • titanium coated with indium and/or tantalum,
  • mixed metal oxides: in particular, iridium-ruthenium mixed oxide, iridium-ruthenium-titanium mixed oxide, or iridium-tantalum mixed oxide;
  • mixed metal oxides, with titanium serving as the anode base material that is coated with platinum, iridium, tantalum and/or palladium oxide;
  • titanium, niobium or tantalum sheet coated with mixed metal oxides;
  • titanium, tantalum or niobium coated with an iridium transition metal mixed oxide;

and combinations of these materials.

The shape of the anode can be adjusted according to the respective purpose by a person skilled in the art. For example, the anode can be a flat material, plate material, sintered material or an expanded material.

The anode and cathode are immersed in an electrolyte. Any electrolyte known to a person skilled in the art of galvanic technology can be used as an electrolyte. When a DC voltage is applied to the two electrodes—anode and cathode—, chromium(III) ions are deposited from the electrolyte onto the object, i.e., the cathode. If the object is not metallically conductive, it can be made electrically conductive by pre-treatment.

As disclosed in WO 2008/014987 A2, the set-up described above can also be varied to the extent that the electrolyte in the electrolysis cell is separated by a semi-permeable membrane into a catholyte (electrolyte in the cathode compartment) and an anolyte (electrolyte in the anode compartment). The cathode, as the object to be coated, is immersed in the catholyte that contains the chromium ions to be deposited. When a voltage is applied, a current passes via the anolyte through the membrane into the catholyte.

The anode system, for example, can also be one in which the anode is in direct contact with a membrane, i.e., the anode is coated with a membrane. This is a so-called direct contact membrane anode, as is known from DE 10 2010 055 143 A1.

The chromium layer can be produced at a temperature of 20° C. to 60° C., whereas the temperature of the electrolyte is set by appropriate heating and cooling devices. The chromium layer can be produced at a current density of, for example, 5 to 60 A/dm².

The electrolyte can be stirred or mixed while the electrodes are immersed in it. In particular, circulation can also occur. Preferably, five bath volumes, i.e., volumes of the electrolyte, are circulated per hour.

The object to be coated can also be moved. If a gravure cylinder is to be coated, it can be moved at a rotational speed of 0.5 to 1.5 m/min, for example.

In the method according to the invention, it is now possible to control the chromium feed in an electrolysis process for producing a chromium layer. For this purpose, a first auxiliary electrode (step (A)) and a second auxiliary electrode (step (B)) are provided, both of which are immersed in an electrolyte (step (C)).

The first auxiliary electrode includes or consists of chromium metal and is therefore also referred to herein as a ‘chromium electrode’. The first auxiliary electrode or chromium electrode, for example, is made of chromium molds that can be held in a framework or holder. The molds can have a regular or irregular shape and can be smooth or porous. These, for example, are nuggets, chunks, lumps, platelets, bars, wires and/or grids; a powder is not to be included. The framework or holder is a material resistant to the acidic electrolyte and may or may not conduct current. Such a conductive material can be a metal, such as titanium. Such a non-conductive material is a plastic, for example, polypropylene or polyvinyl chloride. If the framework or holder does not conduct current, a conductive plate is additionally attached, for example, in order to be able to energize the chromium molds.

According to one embodiment, chromium metal pieces, which are also referred to herein as ‘chromium nuggets’, are accommodated in a plastic framework as chromium molds.

The shape of the first auxiliary electrode can be selected accordingly by a person skilled in the art. As regards the shape, in particular, the molds made of chromium metal, it is important that a larger surface area of the chromium metal causes a higher dissolution rate in the electrolyte. A person skilled in the art can therefore select a suitable shape.

In one embodiment, the surface area of the metallic chromium can be 1% to 50% or 1% to 100% of the surface area of the first auxiliary electrode. In this way, a particularly good dissolution of the chromium and a subsequent supply of chromium(III) can be achieved.

The second auxiliary electrode is an inert electrode, which is composed of one or more electrically conductive materials that are insoluble in the electrolyte. The material for the inert electrode is not further restricted, provided it has the properties described. For example, the same materials as for the anode in the electrolysis process described above can be used for chromium coating.

The shape of the second auxiliary electrode can be selected by a person skilled in the art in accordance with the structural conditions. The second auxiliary electrode, for example, can be a flat material, plate material, sintered material or an expanded material.

According to a preferred embodiment, the surface area of the first auxiliary electrode is selected to be the same size as the surface area of the second auxiliary electrode. In this case, it is expedient if the surface area of the metallic chromium is 100% of the surface area of the first auxiliary electrode.

There are two separate electric circuits in the electrolysis cell: In a first electric circuit, the anode and cathode are interconnected in order to apply the chromium(III) ions dissolved in the electrolyte in the form of a chromium layer to an object connected as a cathode. Direct current is used and there is no polarity reversal between the anode and the cathode. The anode always remains the anode, and the cathode always remains the cathode.

In a second electric circuit, a first auxiliary electrode and a second auxiliary electrode are, in one embodiment, interconnected in an electric circuit separate from the cathode and anode (step (D)). The first electric circuit of the anode and cathode and the second electric circuit of the first and second auxiliary electrodes are not connected to each other, but are connected completely separately from each other. The second electric circuit is therefore controlled independently of the first electric circuit. Direct current is used in the second electric circuit, but the polarity is reversed after specific time intervals, which, however, are significantly longer than with pulsed alternating current (for example, with a duration of 0.1 to 2 s). This means that the first auxiliary electrode initially functions as a cathode and the second auxiliary electrode as an anode, and, at a later point in time, the first auxiliary electrode functions as an anode and the second auxiliary electrode as a cathode. This type of polarity reversal is known from the art, albeit in a different context and for other purposes, so that the technical implementation is readily possible for a person skilled in the art. For example, the polarity reversal can be achieved by a rectifier with a pole inverter.

The second electric circuit can be expediently activated during the electrolytic deposition of chromium with formation of a chromium layer, as it works independently of the first electric circuit. For this purpose, a cathode voltage is initially applied to the first auxiliary electrode. The first auxiliary electrode is therefore the cathode, and the second auxiliary electrode is the anode. The cathode voltage is also referred to herein as the cathode potential and has a reducing effect on the chromium metal. As a result, the passivation layer that has formed on the surface of the chromium metal of the first auxiliary electrode begins to degrade.

Although chromium is actually chemically less noble than iron, it behaves almost like a precious metal when corroded by air and water. This is due to a very thin, practically invisible chromium oxide layer, a few nanometers thick (about 50 atomic layers thick for chromium-nickel steel, about 5 atomic layers thick for pure chromium), which protects the metal from the atmosphere and oxidation. The passivating layer also hinders diffusion to the metal, so that further corrosion of the metal is prevented. The passivation layer is therefore dissolved by applying a cathode and thus a voltage that has a reducing effect.

A cathode (i.e., reducing) voltage is therefore initially applied to the first auxiliary electrode (step (E)). Accordingly, the counter electrode, the second auxiliary electrode, becomes the anode. The cathode voltage reduces the passivation layer in the form of the present chromium oxide layer that has formed on the surface of the chromium metal. In this process, chromium oxide is reduced to metallic chromium (Cr₂O₃→Cr_metalic). This cathode voltage or cathode potential is selected so high that degradation of the chromium oxide layer is achieved. The reducing direct current therefore dissolves the passivation layer, and chromium(III) ions begin to go into solution.

For the degradation of the passivation layer, the cathode voltage, for example, can be set in a range from 1.0 to 10.0 volts, preferably 2.0 to 9.0 volts, more preferably 2.5 to 8.5 volts, even more preferably 2.5 to 8.0 volts, most preferably 3.0 to 7.0 volts. The current density, for example, lies in the range from 2.5 to 4 A/dm², preferably 3.4 A/dm². It has proven to be preferable if the current density is not set higher than 4 A/dm², as this prevents the formation of undesirable chromium(VI) in any case. Typically, the passivation layer can then be degraded within about 5 to 60 seconds, preferably about 5 to 45 seconds, more preferably about 5 to 30 seconds. However, this can also be shorter or longer in individual cases and depends on numerous parameters of the process, such as the set pH value, the current density, the thickness of the chromium metal layer to be deposited, the temperature, the selected voltage and the type of auxiliary electrodes used. The specified range only serves as a guide for a person skilled in the art, who can determine the optimum level of voltage and duration for the cathodic current flow on the first auxiliary electrode for the respective application by means of a few experiments.

In the method according to the invention, it is now possible to degrade the passivation layer on the first auxiliary electrode, which has formed at or on the chromium metal as a secondary reaction and has a passivating effect, and thus brings chromium(III) ions into solution without the formation of chromium(VI) ions. It has been found that the degradation of the passivation layer is a prerequisite for avoiding the formation of chromium(VI) ions during the electrolytic dissolution of chromium.

A purely chemical dissolution of chromium by acids would only be possible at a very low pH-value of <0.5, but this would be disadvantageous for the process conditions in galvanic baths. In accordance with the invention, a pH value in the range from 2.0 to 3.5, in particular 2.1 to 3.4, preferably 2.2 to 3.3, more preferably 2.3 to 3.2, even more preferably 2.4 to 3.1, most preferably 2.5 to 3.0, is usually given in the electrolyte. The preferably used pH value of the electrolyte alone is therefore not sufficient to remove the passivation layer present on the chromium and to start the chemical dissolution. For this reason, the dissolution of the chromium metal as chromium(III) ions is therefore virtually triggered and started by applying a cathode voltage.

Dissolution of the passivation layer on the first auxiliary electrode can be easily observed by the formation of bubbles occurring on the surface of the auxiliary electrode. As soon as chromium(III) ions go into solution in the electrolyte, hydrogen gas is formed, which becomes visible in the form of bubbles. In simplified terms, it can therefore be assumed that the passivation layer has dissolved when bubbles form over the entire surface of the chromium metal on the first auxiliary electrode. This occurs, for example, after about 5 to 30 seconds, as already explained.

Surprisingly, experiments have now shown that after applying the cathode voltage to the first auxiliary electrode as a cathode and dissolving the passivation layer of the chromium metal, the reaction of dissolution that can be recognized by the formation of hydrogen (bubbles), does not stop when the voltage is switched off, but continues without current. Dissolution of the metallic chromium can therefore be observed. Without being tied to this, it is assumed that if chromium(III) is present, chromium(II) is formed cathodically during the reduction. This should favor the dissolution of the metallic chromium. At the same time, it prevents the formation of chromium(VI) during oxidation, i.e., no chromium(VI) is formed when metallic chromium is dissolved during electrolysis.

In the method according to the invention, the current supply to the first auxiliary electrode is therefore terminated after dissolution of the passivation layer (step (F)); the first and second auxiliary electrodes are then no longer live. If the cathode voltage were maintained after dissolution of the passivation layer, chromium(VI) ions would form, which should be avoided for the reasons already mentioned. The formation is prevented by switching off the voltage.

Solubilization of the chromium metal from the first auxiliary electrode in the form of chromium(III) ions then occurs without current by the action of the electrolyte (step (G)). As soon as the chromium oxide layer has degraded, the chromium is therefore attacked by the acidic electrolyte and is dissolved chemically. For this purpose, it is advantageous for the electrolyte to have an acidic pH value in the range from 2.0 to 3.5, for example. In this pH range, the electrolyte is acidic enough to bring the metal into solution without current after the passivation layer has dissolved. By applying a relatively short cathodic current, the passivation layer is therefore degraded to such an extent that the acidic electrolyte can attack the elemental chromium of the first auxiliary electrode. In case of an electrolyte in the specified pH range, the passivation of the chromium metal does not build up again on its own after the passivation layer has degraded, but dissolution of the chromium continues until the dissolution process is discontinued or the chromium metal in the first auxiliary electrode is depleted and no longer replenished.

In one embodiment, the chromium metal in the first auxiliary electrode is replenished during step (G), i.e., during solubilization of the chromium(III) ions into the electrolyte without current, and, subsequently, steps (E), (F) and (G), as disclosed, are performed successively in that order, preferably without intermediate steps. This embodiment therefore comprises:

• • replenishing the chromium metal in the first auxiliary electrode during step (G);
  • (E) applying a cathode voltage to the first auxiliary electrode, as a result of which the passivation layer of the chromium metal dissolves and chromium metal in the form of chromium(III) ions begins to go into solution in the electrolyte;
  • (F) following dissolution of the passivation layer, terminating the current supply to or switching off the voltage on the first auxiliary electrode; and
  • (G) solubilizing the chromium metal from the first auxiliary electrode in the form of chromium(III) ions without current by the action of the electrolyte.

This procedure can be repeated as often as required, whereas the chromium metal is replenished time and again and brought into solution as chromium(III) ions for the chromium layer to be deposited.

There are several ways to terminate the process, i.e., to stop dissolution of the chromium(III) ions into the electrolyte:

According to one embodiment, an anode voltage is applied to the first auxiliary electrode, as a result of which the passivation layer of the chromium metal forms again and dissolution of the chromium(III) ions in the electrolyte is thereby terminated. Dissolution of the chromium in the electrolyte can therefore be stopped at any desired time by applying an anode voltage (oxidizing potential) to the first auxiliary electrode. Accordingly, the second auxiliary electrode, preferably in the form of an inert electrode as counter electrode, then becomes the cathode. This initially causes the metallic chromium to go into solution as chromium(III) for a very short time, but parallel to the dissolution of the Cr(III) ions, the previously described passivation layer of Cr₂O₃ forms again, as the chromium surface reacts with the oxygen contained in the water. In this process, the anode voltage is selected in such a way that the passivation layer builds up in the form of a chromium oxide layer. This can be observed in a simple way by the disappearing bubble formation on the surface of the auxiliary electrode. As soon as no more chromium(III) ions go into solution into the electrolyte, no more hydrogen gas is formed, which would be visible in the form of bubbles. In simplified terms, it can therefore be assumed that the passivation layer has formed again after the bubble formation has disappeared over the entire surface of the chromium metal on the first auxiliary electrode.

The anode voltage, for example, can be set in a range from 1.0 to 10.0 volts, preferably 2.0 to 9.0 volts, more preferably 2.5 to 8.5 volts, even more preferably 2.5 to 8.0 volts, most preferably 3.0 to 7.0 volts, to build up the passivation layer again. The current density, for example, lies in the range from 2.5 to 4 A/dm², preferably 3.4 A/dm². Typically, the passivation layer is then generated again within about 5 to 60 seconds, preferably about 5 to 45 seconds, more preferably about 5 to 30 seconds. However, this can also be shorter or longer in individual cases and only serves as a guide for a person skilled in the art, who can determine the level of the anode voltage and the duration for the anodic current flow on the first auxiliary electrode for the respective application by means of a few experiments.

To terminate the process, it is also possible to pump the electrolyte out of the cell. Another possibility is to pull the chromium electrode out of the electrolyte so that it is exposed to ambient air, causing the passivation layer to form again and ending the chemical reaction. Alternatively, the chromium metal in the existing first auxiliary electrode can no longer be replenished, so that the chromium electrode is allowed to run empty.

This means that, in this process, chromium can be brought into solution without current and started in any way at a defined point in time by means of direct current used, using 2 auxiliary electrodes, continued as often as desired by replenishing the chromium metal and terminated again at a defined point in time. The method according to the invention in accordance with this embodiment is therefore based on the chemical dissolution of chromium in the form of chromium(III) ions in the electrolyte, which can be activated by a defined current flow or a defined voltage, continued without current and terminated by any one of the described ways.

According to another embodiment, a third auxiliary electrode is provided in addition to the first and second auxiliary electrodes. The third auxiliary electrode is an inert electrode. According to one embodiment, the first auxiliary electrode in the form of a chromium electrode, the second auxiliary electrode and the third auxiliary electrode in the 2^nd electric circuit are then interconnected to form one or more units. A unit is then composed as follows:

an inert electrode - a chromium electrode - an inert electrode
(2nd auxiliary electrode) (1st auxiliary electrode) (3rd auxiliary electrode)

In other words, the first auxiliary electrode is surrounded by the second and third auxiliary electrodes.

Two or more units of auxiliary electrodes can also be interconnected, in which case the following sequence is given for two units:

an inert electrode—a chromium electrode-an inert electrode—a chromium electrode—an inert electrode.

In this process, the units are preferably connected in series, similarly to a car battery. The series connection has the advantage that it allows the units to be accommodated in a particularly space-saving manner. Moreover, it provides a higher dissolving capacity for the chromium(III) ions.

The process with three or more auxiliary electrodes is carried out analog to the process with first and second auxiliary electrodes already described in detail:

To dissolve the passivation layer on the chromium electrode(s), the surrounding inert electrodes serve as anodes and the intermediate chromium electrode(s) as cathode(s). Following dissolution of the passivation layer, the chromium(III) ions from the chromium electrode(s) dissolve in the electrolyte without current. To continue the process, the chromium metal in the chromium electrodes can be replenished again during the dissolution without current. The process can then be continued by degrading the passivation layer again and dissolving the chromium(III) ions into the electrolyte again. The chromium metal can in each case be replenished again shortly before it is used up. These process steps can be repeated as often as required.

To build up the passivation layer on the chromium electrode(s) again, the inert electrodes are then connected as the cathode(s) and the chromium electrode(s) is(are) the anode(s). Alternatively, the chromium electrode(s) is(are) pulled out of the electrolyte or the electrolyte is removed during step (G); in both cases, a passivation layer builds up again on the chromium electrode(s) through contact with ambient air. It is also possible to no longer replenish the chromium metal in the chromium electrode(s) so that there is no more supply of chromium metal.

FIG. 1 illustrates an embodiment of the method according to the invention by means of a flow chart:

During the electrolytic deposition of chromium at the cathode, the passivation layer on the first auxiliary electrode or chromium electrode is removed in step (E). Following dissolution of the passivation layer, the current supply to the first auxiliary electrode is terminated or the voltage is switched off (step (F)) and chromium(III) ions from the first auxiliary electrode go into solution into the electrolyte without current (step (G)). Thereafter, the process can either be continued or terminated. This is depicted in FIG. 1 in the diamond which reads “Terminating the process?”. If the process is not to be terminated (junction “No” in FIG. 1), the one or more chromium electrodes can be replenished with chromium metal—if this is necessary—and process steps (E) to (G) can then be performed again. This can be repeated as often as required.

If the process is to be terminated (junction “Yes” in FIG. 1), then the passivation layer can be built up again by applying an anode voltage to the one or more chromium electrodes. Alternatively, the chromium metal in the first auxiliary electrode(s) can no longer be replenished, so that the chromium electrode is allowed to run empty. Another alternative for terminating the process is to pump the electrolyte out of the cell or to pull the chromium electrode(s) out of the electrolyte so that the chromium electrode(s) is(are) exposed to air and the passivation layer thereby forms again and the chemical reaction ends.

The electrolyte for the method according to the invention is not particularly restricted, provided that it is suitable for electrolysis. Any electrolyte known to a person skilled in the art can be used. The electrolyte contains water as a solvent. Preferably, the electrolyte has a pH value in the range from 2.0 to 3.5. In another embodiment, the electrolyte can also have a pH value in the range from 2.1 to 3.4, preferably 2.2 to 3.3, more preferably 2.3 to 3.2, even more preferably 2.4 to 3.1, most preferably 2.5 to 3.0.

According to one embodiment, the electrolyte comprises:

• • (a) one or more chromium(III) salts,
  • (b) a compound of the formula (I)

wherein R is selected from NH₂, OH or SO₃H, and/or their salts, in particular salts with monovalent cations, such as Na⁺ and/or K⁺, or bivalent cations, wherein n represents an integer from 1 to 3,

(c) formic acid and/or its salts, in particular salts with monovalent cations, such as Na⁺ and/or K⁺, or bivalent cations, and

(d) optionally, one or more additives.

Component (a) of the electrolyte according to this embodiment is one or more chromium(III) salts. In the present invention, the term “chromium(III) salt” is understood to mean any chromium(III) salt with which chromium can be deposited as a metal layer onto objects. The chromium(III) salt is selected from an inorganic or organic chromium(III) salt or mixtures thereof. The inorganic chromium(III) salt, for example, is selected from the group consisting of, but not limited to, potassium chromium alum, ammonium chromium alum, chromium sulfate, chromium hydroxy)sulfate (alkaline chromium sulfate), chromium sulfoacetate, chromium nitrate, chromium sulfamate (amidosulfonate), chromium chloride, chromium bromide, chromium iodide, chromium phosphate, chromium pyrophosphate (diphosphate), chromium phosphonate, and mixtures of two or more thereof. The organic chromium(III) salt, for example, is selected from the group consisting of, but not limited to, chromeitrate, chromium formate, chromium sulfoacetate, chromium oxalate, chromium methane sulfonate, chromium dimethanesulfonate and mixtures of two or more of these. Inorganic and organic chromium(III) salts can also be used in a mixture.

It is expedient, for example, if the amount of the chromium(III) salt is selected in the range from 0.25 mol/L to 2.0 mol/L relating to the electrolyte. This range has proven to be particularly advantageous for the production of chromium layers on metallic objects by an electrolytic deposition.

Component (b) of the electrolyte according to this embodiment is the compound of formula (I) and/or its salt. Preferably, the compound of formula (I) is selected from glycine, glycolic acid, sulfoacetic acid, sodium sulfoacetate, potassium sulfoacetate or a mixture of at least two of these compounds.

The amount of the compound of formula (I) in the electrolyte is preferably 0.5 mol/L to 1.5 mol/L relating to the electrolyte. This serves to set the pH value of the electrolyte.

Formic acid is present in the electrolyte as an additional component (c) according to the present embodiment, which is used to remove the oxygen released from the chromium(III) salt through chemical reaction into CO₂ and H₂O. The amount of formic acid in the electrolyte is advantageously 1.0 mol/L to 3.0 mol/L relating to the electrolyte before the deposition of chromium. This range has proven to be particularly useful for setting the pH value of the electrolyte.

Instead of or in addition to formic acid, its salts can also be used. Alkali and/or alkaline earth formates, in particular sodium formate, are mentioned by way of example.

One or more additives are optionally used as a component (d) of the electrolyte. Any compound or mixture of compounds that can give the galvanic bath advantageous properties can be used as an additive. These compounds are known to a skilled person from the art.

For example, the additives are selected from complexing agents, alkali or alkaline earth salts, wetting agents, catalysts or mixtures thereof.

The complexing agents are preferably compounds with short-chain alkyl chains (e.g., 1-5 C atoms) which have 1 or 2 carboxyl groups or their derivatives, or 1 or 2 thio and/or sulfone groups. The following compound is used by way of example:

• • wherein
  • R₁ represents a C_1-5 alkyl radical, in particular CH₃CH₂;
  • X represents one or more metal cations to balance the negative charge, such as Na⁺, K⁺; wherein
  • n is an integer from 1 to 5, in particular 3.

The compound N,N-Dimethyl-dithiocarbamyl propyl sulfonic acid sodium salt (DPS) is particularly preferred as a complexing agent. The use of DPS can be advantageous since particularly good chromium layers can be obtained.

Wetting agents cause the reduction of the surface tension so that H₂ bubbles formed can detach from the cathode. This prevents the formation of pores in the chromium layer, and more uniform chromium layers can be obtained. Preferred wetting agents, for example, are polyfluorinated mono and/or di-alkyl phosphates and PEG (polyethylene glycol) derivatives of salts, or esters of phosphoric acid, in particular PEGylated phosphates.

To increase the conductivity, sulfates or acetosulfates, such as alkali or alkaline earth salts, in particular sodium sulfate, sodium sulfoacetate, potassium sulfate or magnesium sulfate, can be used.

The amount of sulfate or acetosulfate can be 5 mM to 30 mM, for example, 10 mM to 20 mM.

The amount of additive(s) present in the electrolyte can be 0.01 g/L to 2.0 g/L relating to the electrolyte. Using PEG 6000 as a wetting agent, for example, results in a substance concentration of 0.001 mMol/L to 0.3 mMol/L.

As already explained, a pH value in the range from 2.0 to 3.5 is preferably set in the electrolyte. The pH value, for example, can be set by means of the compound of formula (I), formic acid and/or its salts.

The electrolyte is essentially free of chromium(VI) ions, i.e., only unavoidable impurities of chromium(VI) ions are present in the electrolyte composition. In the method according to the invention, the content of chromium(VI) ions is below the detection limit.

According to one embodiment, it can be preferable if the electrolyte does not contain a nitrogen-containing compound. In this case, the chromium layer formed also contains no nitrogen-containing compound, which results in a coating with particularly advantageous properties.

The subject matter of the invention is also an electrolysis cell for controlling the chromium feed to/in an electrolyte, comprising:

• • an anode;
  • a cathode;
  • an electrolyte that contains at least one chromium(III) salt;
  • the anode and cathode are immersed in the electrolyte;
  • a first circuit which connects the anode and cathode and causes the deposition of a chromium layer by an electrolytic deposition of chromium from an electrolyte by means of direct current on the cathode;
  • a first auxiliary electrode that includes chromium metal or consists of it;
  • a second auxiliary electrode in the form of an inert electrode;
  • both auxiliary electrodes are immersed in the electrolyte;
  • a second circuit which connects the first auxiliary electrode and the second auxiliary electrode in an electric circuit separate from the cathode and anode.

The electrolysis cell assumes one of the following 3 states in each case:

• • a cathode voltage is applied to the first auxiliary electrode so that the passivation layer of the chromium metal on the first auxiliary electrode dissolves; or
  • no voltage is applied to the first auxiliary electrode, so that, following dissolution of the passivation layer, the chromium metal from the first auxiliary electrode in the form of chromium(III) ions goes into solution into the electrolyte without current; or
  • an anode voltage is applied to the first auxiliary electrode so that the passivation layer forms again on the first auxiliary electrode.

The container that can be used as an electrolysis cell may be any container, vessel or tank that is suitable for a person skilled in the art, in particular those commonly used in galvanic technology.

The above statements regarding the method for controlling the chromium feed in an electrolysis process for producing a chromium layer apply equally with regard to the electrolysis cell and are therefore not repeated.

The electrolysis cell must not only be in one piece, it can also be in two pieces: In one embodiment, the chromium plating bath can be located in a first cell in which a direct current passes between an anode and a cathode immersed in an electrolyte. In a second cell, which is connectable to the first cell, a chromium electrode and an inert electrode, for example, both of which are immersed in electrolyte, are interconnected. Alternatively, one or more units, each composed of an inert electrode—a chromium electrode—an inert electrode, can be interconnected in the second cell. The chromium plating of an object takes place in the first cell. In the second cell, the electrolyte is accumulated with chromium(III) ions up to a desired concentration and can then be returned to the chromium plating bath again. Other configurations are also possible.

The process described or the electrolysis cell are used, in particular, to replenish the Cr³⁺ used up in an electrolyte. Currentless dissolution of the chromium, for example, occurs over a period of time sufficient to bring the chromium content of the electrolyte to a desired level, which can last from a few minutes up to several hours.

Alternatively, an equilibrium is set so that chromium(III) ions are continuously supplied to the coating bath and this operates continuously. The duration of dissolution of the chromium as chromium(III) ions is advantageously selected in such a way that a constant concentration of chromium(III) ions results in the electrolyte, in particular, a stable equilibrium is maintained between the supply and consumption of chromium(III) ions.

Preferably, the chromium metal can be replenished during the process. The chromium mold of the first auxiliary electrode is preferably replenished during solubilization of the chromium(III) ions without current by the action of the electrolyte in step (G). In this state, replenishment can occur without any problems and therefore does not additionally interrupt the process. It is understood that, in case of the newly replenished chromium molds, the passivation layer must first be removed again (step (E)), as already described, before the chromium(III) ions dissolve again in the electrolyte without current (steps (F) and (G)).

The invention also relates to a method for keeping the chromium(III) content in an electrolyte constant in the process disclosed herein by comparing the weight of the chromium metal used in the first auxiliary electrode with the weight of the chromium metal used up for the chromium layer and replenishing the chromium metal in the first auxiliary electrode before the chromium(III) content in the electrolyte decreases. Controlling the chromium(III) content by measuring the weight of the chromium metal present in the first auxiliary electrode in comparison to the chromium metal used up by the coating allows the chromium(III) content in the electrolyte to be kept constant. The weight can be measured by pressure sensors, for example.

The supply of metallic chromium to the electrolytes makes it possible to keep the chromium(III) content almost constant during electrolysis taking place over a long period of time, for example, from several hours up to several months, with chromium(III) ions being supplied later to the electrolyte depleted in the chromium(III). Keeping the chromium(III) content in the electrolyte constant is understood to mean that the chromium(III) content preferably only changes by ±10%.

Furthermore, the subject matter of the invention is the use of the method for producing a chromium layer on an object.

The invention also relates to the use of the electrolysis cell for producing a chromium layer on an object.

The advantages of the invention are extremely complex:

The method or electrolysis cell according to the invention is based on a technical design that can easily be achieved by a person skilled in the art. A continuous polarity reversal using an expensive pulse rectifier is not necessary. Rather, a simple rectifier with a pole inverter can be used, whereas the polarity reversal is only used at the beginning and end of the dissolution of the chromium(III) ions.

Another major advantage is that dissolution of the chromium(III) ions, following dissolution of the passivation layer, occurs without current. For this purpose, no additional use of energy is required during the dissolution process. This is particularly important for large industrial plants. The energy costs for the process or the electrolysis cell are significantly reduced as a result.

The method according to the invention also allows keeping the chromium(III) content in the electrolyte constant, for example, by correlating the weight of the chromium metal in the first auxiliary electrode with the weight of the chromium metal used up by the coating and controlling it accordingly. This makes it possible to keep the chromium(III) content almost constant during electrolysis taking place over a long period of time of, for example, several hours up to several months, with chromium(III) ions being supplied later to the electrolyte depleted of the chromium(III). Keeping the chromium(III) content in the electrolyte constant is understood to mean that the chromium(III) content preferably only changes by ±10%.

In the method according to the invention, electrolytic deposition can also advantageously occur without the use of a semi-permeable membrane. Previously, semi-permeable membranes have been used to separate the anode from the cathode so that no chromium(VI) is formed. This is not required in the procedure according to the invention. In the method according to the invention and the electrolysis cell provided, the formation of chromium(VI) during and after the electrolytic deposition of chromium is generally avoided. Chromium(VI) cannot be detected in the process according to the invention.

Chromium coating can thus be carried out easily, quickly and cost-efficiently, even for longer periods of time.

According to one embodiment, it can be preferable when the electrolyte does not contain a nitrogen-containing compound, so that the chromium layer formed does not contain a nitrogen-containing compound either. This results in a coating with particularly advantageous properties.

The chromium layer can be applied for decorative or technical reasons by an electrolytic deposition of chromium. Examples of objects for which chromium plating is used for technical reasons are rotationally symmetrical objects such as rods, pistons and cylinders, in particular gravure cylinders. Gravure cylinders or gravure rollers refer to the printing cylinder for gravure printing. The base cylinder is generally a tubular steel core, which is first coated with copper in an electrolytic bath and then with chromium after the image data has been applied. This process is carried out by galvanic coating of the gravure cylinder with chromium.

In accordance with the invention, chromium coatings of a particularly excellent quality are obtained, which surprisingly also meet the high requirements placed on gravure cylinders. Smooth, uniform surfaces are obtained which essentially have no pores, pocks or craters. The layer thickness of the chromium layers obtained can be thicker than the layers usually obtained in the art. Layer thicknesses of 100 μm and more can be obtained. Furthermore, chromium layers of high hardness can be produced, in particular, of more than 900 HV. The chromium layers obtained are corrosion-resistant, wear-resistant, have favorable frictional properties and are thermally and chemically resistant; they are bright and highly reflective and are therefore also suitable for decorative purposes.

In the following text, embodiments of the present invention are described by way of example with reference to the accompanying figures, which are drawn schematically and not to scale, so that no assumption can be made as to exact geometric values with respect to the original size. The figures of the present disclosure are part of the description and represent a part of it and illustrate embodiments of the invention, without being restricted to the specific embodiments described. The drawings, together with the description, serve to illustrate the present disclosure.

FIGS. 2a, 2b and 2c depict an embodiment for the sequence of the method according to the invention or the states of an electrolysis cell for the controlled supply of chromium(III) ions by starting, continuing and ending the dissolution of the chromium(III) ions in connection with an electrolytic process for the electrolytic deposition of chromium layers. FIGS. 2a, 2b and 2c therefore show the various states of an electrolysis cell, which illustrate the individual steps of the method according to the invention in accordance with one embodiment:

FIG. 2a shows an electrolysis cell 10 in the form of a bath device at a time during which a chromium layer is produced by an electrolytic deposition of chromium from an electrolyte 25 by means of direct current, using an anode 44 and a cathode 48. In the example shown, the cathode is a gravure cylinder 48, which has been inserted into the bath device, for example, by means of a crane not depicted. The gravure cylinder 48 is held by bearing bridges 30 belonging to a bearing device. The lateral surface of the gravure cylinder 48 is to be coated with chromium. Naturally, another object, in particular, a rotationally symmetrical object, could also be coated instead of the gravure cylinder 48 depicted.

The electrolysis cell 10 includes a trough 15 in which there is a liquid electrolyte 25 comprising water as a solvent, which electrolyte contains at least one Cr(III) salt: In the example depicted, the electrolyte 25 has a pH value in the range from 2.0 to 3.5. According to one embodiment, the electrolyte has the following composition:

• • (a) one or more chromium(III) salts, as already explained in detail;
  • (b) a compound of the formula (I)

wherein R is selected from NH₂, OH or SO₃H, and/or their salts, in particular salts with monovalent cations, such as Na⁺ and/or K⁺ or bivalent cations, wherein n represents an integer from 1 to 3;

(c) formic acid and/or its salts, in particular salts with monovalent cations, such as Na⁺ and/or K⁺, or bivalent cations, and

(d) optionally, one or more additives, as already described in detail.

Other electrolyte compositions are also possible.

Furthermore, a vertically movable anode device is provided in the trough 15, which essentially consists of an anode rail 42 and an anode basket 44 which is electrically and mechanically coupled to the anode rail 42 and serves as a metal holding device. The anode basket 44 can also consist of several combined anode baskets or grids. The anode 44 represents an insoluble anode or inert electrode and, for example, can comprise or consist of the following materials: platinized titanium, carbon materials such as graphite, titanium coated with indium and/or tantalum, and mixed metal oxides such as iridium-ruthenium mixed oxide, iridium-ruthenium-titanium mixed oxide or iridium-tantalum mixed oxide; mixed metal oxides, where titanium serves as the anode base material coated with platinum, iridium, tantalum and/or palladium oxide; titanium, niobium or tantalum sheet coated with mixed metal oxides; titanium, tantalum or niobium coated with iridium transition metal mixed oxide; or an expanded metal made of titanium, or an expanded metal made of titanium coated with a mixed oxide or coated with graphite, as well as combinations of these materials.

For simplification and ease of representation, only one of the bearing bridges 30 is depicted in FIG. 2a. For example, the two bearing bridges 30 can be moved on rails (not shown) in the axial direction of the gravure cylinder 48 by means of spindles or other suitable adjustment mechanisms, so that they clamp the gravure cylinder 48 between them and hold it so that it can rotate.

As can be seen in FIG. 2a, a portion of the trough 15 remains freely accessible at the top due to the bearing bridges 30 supporting one side, so that the anode rail 42 extending there parallel to the axial direction of the gravure cylinder 48 can be freely moved vertically. The vertical movement of the anode rail 42 with the anode basket 48 is known to a skilled person from the art, so that a detailed description and illustration is not necessary.

FIG. 2a shows the electrolysis cell 10 in the galvanization phase, in which the gravure cylinder 48 is immersed almost completely. In particular, immersion depths of more than 65% can be achieved with large cylinders (circumference of 1500 mm) and up to approximately 80% with smaller cylinders (circumference of 800 mm).

For galvanization, i.e., applying the chromium layer to the gravure cylinder 48, the anode basket 44 has already been pulled up to the side so that its large basket surface surrounds the immersed gravure cylinder 48.

The anode 44 and cathode 48 therefore form a first electric circuit (not depicted).

A first auxiliary electrode 54 and a second auxiliary electrode 56 are connected to each other in a second electric circuit, which is connected independently and separately from the first electric circuit so that there is no connection between the two electric circuits.

The first auxiliary electrode 54 includes or consists of chromium metal and can therefore also be referred to as a ‘chromium electrode’. This electrode, for example, is composed of chromium molds 54a, which are held in a holder, such as a framework or basket. The molds can have a regular or irregular shape and can be smooth or porous. For this purpose, nuggets, chunks, lumps, platelets, bars, wires and grids, for example, but not powders, are suitable. The holder is a material resistant to the acidic electrolyte and may or may not conduct a current. Conductive materials, for example, are metals such as titanium. Non-conductive materials, for example, are plastics such as polypropylene and polyvinyl chloride. In the exemplary embodiment depicted, chromium metal pieces 54a, which, in particular, are also referred to as chromium nuggets, are accommodated in a plastic framework, for example, a polypropylene basket.

The shape of the first auxiliary electrode 54 is not further restricted, provided that it is suitable for the intended purpose. Suitable shapes are known to a person skilled in the art.

The selected shape of the chromium metal molds determines their surface area, with a larger surface area causing a higher dissolution rate in the electrolyte. A person skilled in the art can therefore select a suitable shape.

The second auxiliary electrode 56 is an inert electrode, which is composed of one or more electrically conductive materials and is insoluble in the electrolyte. The material for the inert electrode is not further restricted, provided it has the properties described. For example, the same material can be used as for the anode 44. The shape of the second auxiliary electrode 56 can be selected by a person skilled in the art in accordance with the structural conditions. The second auxiliary electrode 56, for example, can be a flat material, plate material, sintered material or an expanded material.

In FIG. 2a, the electrolysis cell 10 is depicted at a time when a cathode voltage using direct current is applied to the first auxiliary electrode 54 so that the passivation layer of the chromium metal on the first auxiliary electrode 54 dissolves. This represents step (E) of the method according to the invention. The first auxiliary electrode 54 is thus the cathode, and the second auxiliary electrode 56 is the anode. A power source 58, which is provided with a rectifier and a pole inverter (not depicted), is used. The cathode voltage has a reducing effect on the chromium metal nuggets 54a, and the passivation layer that has formed on the surface of the chromium metal nuggets 54a of the first auxiliary electrode 54 begins to degrade and dissolve.

The applied cathode voltage, for example, can lie in the range from 1.0 to 10.0 volts, preferably 2.0 to 9.0 volts, more preferably 2.5 to 8.5 volts, even more preferably 2.5 to 8.0 volts, most preferably 3.0 to 7.0 volts. The current density lies preferably in the range from 2.5 to 4 A/dm², particularly preferably at 3.4 A/dm². The passivation layer is degraded after 5 to 60 seconds, preferably 5 to 45 seconds, particularly preferably 5 to 30 seconds. Depending on the selected electrolysis conditions, in particular, the pH value, the temperature, the selected voltage, the current density, the thickness of the chromium metal layer to be deposited and the type of auxiliary electrodes used, the duration can also be shorter or longer.

In FIG. 2b, the passivation layer on the first auxiliary electrode 54 has already degraded, with the appearance of bubbles 55 (hydrogen formation due to chromium(III) ions going into solution) over the entire surface of the chromium metal on the first auxiliary electrode 54 indicating that the passivation layer has dissolved.

In FIG. 2b, the passivation layer is therefore already dissolved, and the current supply to the first auxiliary electrode 54 and to the second auxiliary electrode 56 is interrupted (step (F)). The voltage is switched off. This is represented schematically in FIG. 2b by means of the non-closed electrical switch 59. Solubilization of the chromium metal from the first auxiliary electrode 54 in the form of chromium(III) ions without current occurs by the action of the electrolyte 25 (step (G)), which here has a pH value in the range from 2.0 to 3.5. The chromium metal nuggets 54a are therefore attacked by the acidic electrolyte and dissolved chemically. This takes place without current. The chromium(III) ions formed migrate in the electrolyte 25 to the surface of the gravure cylinder 48 connected as a cathode, where they deposit themselves in the form of a chromium coating.

By dissolving the chromium(III) ions in the electrolyte, the chromium metal in the first auxiliary electrode (54) therefore decreases. The chromium metal in the first auxiliary electrode 54 can now be replenished during step (G), represented by FIG. 2b. Subsequently, steps (E), (F) and (G) of the method according to the invention are repeated. This can be continued as often as required. A quasi-continuous process is therefore established, represented by: FIG. 2a->FIG. 2b->replenishing->FIG. 2a->FIG. 2b->replenishing->etc.

In FIG. 2c, dissolution of the chromium(III) ions in the electrolyte is discontinued by applying an anode voltage to the first auxiliary electrode 54, as a result of which the passivation layer of the chromium metal forms again on the first auxiliary electrode 54 and dissolution of the chromium(III) ions in the electrolyte 25 is thereby terminated. The first auxiliary electrode 54 depicted then becomes the anode, and the second auxiliary electrode 56 or inert electrode becomes the cathode. Direct current flows again, but with reverse polarity.

For example, the polarity reversal of FIG. 2a (first auxiliary electrode 54 is a cathode) to FIG. 2c (first auxiliary electrode 54 is an anode) can be achieved by a rectifier with a pole inverter, which is connected to a power source 58.

In the example depicted in FIG. 2c, the passivation layer has already fully formed again, the bubbles 55 on the chromium metal surface, i.e., the chromium molds 54a, have disappeared completely, as no more chromium(III) ions go into solution into the electrolyte.

The applied anode voltage, for example, can lie in the range from 1.0 to 10.0 volts, preferably 2.0 to 9.0 volts, more preferably 2.5 to 8.5 volts, even more preferably 2.5 to 8.0 volts, most preferably 3.0 to 7.0 volts. The current density, for example, lies in the range from 2.5 to 4 A/dm², preferably at 3.4 A/dm². The passivation layer is built up after about 5 to 60 seconds, preferably about 5 to 45 seconds, particularly preferably about 5 to 30 seconds. Depending on the selected electrolysis conditions, in particular, the pH value, the temperature, the selected voltage, the current density, the thickness of the chromium metal layer to be deposited and the type of auxiliary electrodes used, the duration can also be shorter or longer.

Alternatively, the process can be terminated by pulling the first auxiliary electrode 54 out of the electrolyte 25 or by pumping the electrolyte 25 out of the electrolysis cell 10 (not shown) during step (G), so that the first auxiliary electrode 54, in particular, the chromium molds 54a, comes into contact with ambient air and thus builds up a passivation layer. Alternatively, the chromium metal of the first auxiliary electrode 54 may no longer be replenished, so that solubilization of the chromium metal from the first auxiliary electrode 54 in the form of chromium(III) ions without current by the action of the electrolyte ends as soon as the chromium metal present has completely gone into solution.

The formation of chromium(VI) ions could not be detected during the process.

FIGS. 3a to 3c represent schematically another embodiment of the method according to the invention, where a third auxiliary electrode is present in addition to the first and second auxiliary electrodes. Similar to the second auxiliary electrode, the third auxiliary electrode is an inert electrode. There is an interconnected unit of auxiliary electrodes, which, for example, can replace the circuit consisting of the first auxiliary electrode 54 and the second auxiliary electrode 56 in FIG. 2a. FIG. 3a shows 2 inert electrodes 56.1 and 56.2, which are also referred to herein as the second and third auxiliary electrodes. These virtually surround the chromium electrode 54, which is also referred to herein as the first auxiliary electrode. In the embodiment depicted, an anode voltage is applied to the inert electrodes 56.1 and 56.2, and a cathode voltage is applied to the chromium electrode 54. This is step (E), in which the passivation layer is dissolved.

FIG. 3b depicts the embodiment according to FIG. 3a, however, no voltage is applied to the auxiliary electrodes (step (F)), and in FIG. 3c an anode voltage is applied to the first auxiliary electrode (termination of the process during step (G)).

According to other variations of the present invention, the embodiment with a third auxiliary electrode (FIGS. 3a, 3b and 3c) could also replace the second circuit in FIGS. 2a, 2b and 2c respectively.

FIGS. 4a, 4b and 4c represent schematically another embodiment of the method according to the invention, where a series connection of the electrodes is to be illustrated. Inert electrodes 56.1, 56.2, 56.3 and 56.4 are depicted, each alternating with chromium electrodes 54.1, 54.2 and 54.3. The inert electrodes 56.1 and 56.4 each represent edge electrodes, as they are located around the edge of the electrodes. There are interconnected units of auxiliary electrodes, which, for example, could replace the second circuit consisting of the first auxiliary electrode 54 and the second auxiliary electrode 56 in FIGS. 2a, 2b and 2c respectively.

In the embodiment depicted in FIG. 4a, an anode voltage is applied to each of the inert electrodes 56.1, 56.2, 56.3 and 56.4, and a cathode voltage is applied to each of the chromium electrodes 54.1, 54.2 and 54.3. This is step (E), in which the passivation layer is dissolved.

FIG. 4b depicts the embodiment according to FIG. 4a, however, no voltage is applied to the auxiliary electrodes (step (F)), and in FIG. 4c an anode voltage is applied to the first auxiliary electrode in each case (termination of the process during step (G)).

FIGS. 5a, 5b and 5c depict another embodiment of the invention. FIGS. 5a, 5b and 5c depict another embodiment for the sequence of the method according to the invention or the states of an electrolysis cell for the controlled supply of chromium(III) ions by starting, continuing and ending the dissolution of the chromium(III) ions in connection with an electrolytic process for the electrolytic deposition of chromium layers.

In FIG. 5a, the trough of the electrolysis cell 100 in the form of a bath device is divided into an upper trough 110 and a lower trough 120 arranged below it. A liquid electrolyte 125 is located in the upper trough 110 and in the lower trough 120, which is pumped from the lower trough 120 into the upper trough 110 by means of a pump 160 and flows back again into the lower trough 120 via an overflow 127, which can be moved vertically in at least two positions. Alternatively, it is also possible to arrange two alternately openable overflows at different height levels.

As already explained with regard to FIGS. 2a, 2b and 2c, a vertically movable anode device is arranged in the upper trough 110, which essentially consists of an anode rail 142 and an anode basket 144 which is electrically and mechanically coupled to the anode rail 142 and serves as a metal holding device. The anode basket 144 can also consist of several combined anode baskets or grids. The anode basket 144 is part of an insoluble anode.

The cathode 148, here a gravure cylinder, is held by two bearing bridges 130 (only one is shown) so that it can move on rails in the axial direction of the gravure cylinder 148 by means of appropriate adjustment mechanisms, so that the gravure cylinder 148 is clamped between these and held so that it can rotate. The upper half of the upper trough 110 is therefore freely accessible, so that the anode rail 142 can be moved vertically.

The filling level of the electrolyte 125 in the upper trough 110, i.e., the height level of the electrolyte 125, can be adjusted suitably with the aid of the vertically movable overflow 127.

FIG. 5a shows the electrolysis cell 100 in the galvanization phase, in which the gravure cylinder 148 is immersed almost completely. For galvanization, i.e., applying the chromium layer to the gravure cylinder 148, the anode basket 144 has already been pulled up to the side so that its large basket surface surrounds the immersed gravure cylinder 148.

The anode 144 and cathode in the form of a gravure cylinder 148 therefore form a first electric circuit (not depicted). In a second electric circuit, which operates independently from the first electric circuit, a first auxiliary electrode 154 and a second auxiliary electrode 156 are connected to each other in the lower trough 120.

The first auxiliary electrode 154 includes or consists of chromium metal and is also referred to herein as a ‘chromium electrode’. The structure of the chromium electrode 154 and inert electrode 156 is as already explained in FIG. 2a (there: chromium electrode 54 and inert electrode 56).

In the electrolysis cell 100 depicted in FIG. 5a, a cathode voltage is now applied initially to the first auxiliary electrode 154 using direct current so that the passivation layer of the chromium metal on the first auxiliary electrode 154 dissolves (step (E)). The first auxiliary electrode 154 is thus the cathode, and the second auxiliary electrode 156 is the anode. For example, a rectifier with a pole inverter (not shown) connected to the power source 158 is used. The cathode voltage has a reducing effect on the chromium metal nuggets 154a, and the passivation layer that has formed on the surface of the chromium metal nuggets 154a of the first auxiliary electrode 154 begins to degrade.

The applied cathode voltage, for example, can lie in the range from 1.0 to 10.0 volts, preferably 2.0 to 9.0 volts, more preferably 2.5 to 8.5 volts, even more preferably 2.5 to 8.0 volts, most preferably 3.0 to 7.0 volts. The current density, for example, lies in the range from 2.5 to 4 A/dm², preferably at 3.4 A/dm². The passivation layer is degraded after about 5 to 60 seconds, preferably about 5 to 45 seconds, particularly preferably about 5 to 30 seconds. Depending on the selected electrolysis conditions, in particular, the pH value, the temperature, the selected voltage, the current density, the thickness of the chromium metal layer to be deposited and the type of auxiliary electrodes used, the duration can also be shorter or longer.

In FIG. 5b, the disappearance of the passivation layer on the first auxiliary electrode 154 is indicated by the appearance of bubbles 155, caused by the formation of hydrogen due to chromium(III) ions going into solution. The bubbles 155 are present here over the entire surface of the chromium metal on the first auxiliary electrode 154 when the passivation layer has completely dissolved.

As soon as the passivation layer has dissolved, the current supply to the first auxiliary electrode 154 and the second auxiliary electrode 156 is interrupted (step (F)). This is represented schematically in FIG. 5b by means of the electrical switch 159 that interrupts the current.

In the following, solubilization of the chromium metal from the first auxiliary electrode 154 in the form of chromium(III) ions occurs without current by the action of the electrolyte 125 in the lower trough 120 (step (G)), with the electrolyte 125 having a pH value in the range from 2.0 to 3.5, for example. The chromium metal nuggets 154a are attacked by the acidic electrolyte and dissolved chemically. This takes place without supplying current to the auxiliary electrodes 154, 156. The chromium(III) ions formed are distributed in the electrolyte 125, which is pumped out of the lower trough 120 into the upper trough 110 by means of the pump 160 and flows back again into the lower trough 120 via an overflow 127, which can be moved vertically in at least two positions. The chromium(III) ions migrate via the electrolyte 125 to the surface of the gravure cylinder 144 connected as a cathode and form a chromium coating on it.

To reduce the amount of electrolytes in the upper trough 110, the upper trough 110 is tapered in the lower area, for example. The tapering can be achieved with the aid of additionally used sheet plates 133 or by adapting the walls of the upper trough 110 accordingly. Blocks or boxes can also be used to displace volume. Limiting or reducing the volume of the upper trough 110 has the advantage that no excessive amount of electrolyte 125 has to be pumped upwards from the lower trough 120. Accordingly, there is no risk of the lower trough 120 being emptied completely and the pump 160 running dry.

If desired, the chromium metal of the first auxiliary electrode 154 can be replenished during the performance of step (G)—according to FIG. 5b—and thus the process can be continued accordingly and steps (E), (F) and (G) can be repeated, so that a sequence of the process of: FIG. 5a->FIG. 5b->replenishing->FIG. 5a->FIG. 5b->replenishing-> . . . is given.

If the supply of chromium(III) ions to the electrolyte is to be discontinued, an anode voltage is applied to the first auxiliary electrode 154, as a result of which the passivation layer of the chromium metal forms again on the first auxiliary electrode 154 and dissolution of the chromium(III) ions in the electrolyte is terminated. This is illustrated in FIG. 5c. The first auxiliary electrode 154 becomes the anode, and the second auxiliary electrode 156 or inert electrode becomes the cathode. Direct current flows again, but with reverse polarity.

The applied anode voltage, for example, can lie in the range from 1.0 to 10.0 volts, preferably 2.0 to 9.0 volts, more preferably 2.5 to 8.5 volts, even more preferably 2.5 to 8.0 volts, most preferably 3.0 to 7.0 volts. The current density, for example, lies again in the range from 2.5 to 4 A/dm², preferably at 3.4 A/dm². The passivation layer is built up again after about 5 to 60 seconds, preferably about 5 to 45 seconds, particularly preferably about 5 to 30 seconds. Depending on the selected electrolysis conditions, in particular, the pH value, the temperature, the selected voltage, the current density, the thickness of the chromium metal layer to be deposited and the type of auxiliary electrodes used, the duration can also be shorter or longer.

For example, the polarity reversal (first auxiliary electrode 154 as the cathode, then as the anode) can be achieved by a rectifier with a pole inverter, which is connected to a power source 158.

If the passivation layer is fully formed again, no more bubbles 155 can be observed on the chromium metal surface of the first auxiliary electrode 154, as no more hydrogen gas is formed and no more chromium(III) ions go into solution into the electrolyte 125.

Alternatively, in order to terminate the process, the chromium metal in the first auxiliary electrode 154 may not be replenished, so that solubilization of the chromium metal from the first auxiliary electrode 154 in the form of chromium(III) ions without current ends by the action of the electrolyte 125 as soon as the chromium metal present has completely gone into solution. Or the first auxiliary electrode 154 is pulled out of the electrolyte 125, or the electrolyte 125 is pumped out of the electrolysis cell 100 (not shown).

The formation of chromium(VI) ions was not detectable during the entire procedure.

After completion of the galvanization phase, the anode rail 142 with the anode basket 144 is moved downwards into the upper trough 110. At the same time or with a time delay, the overflow 127 is lowered so that the electrolyte 125 flows down to a suitable height level into the lower trough 120. In this way, a state can be achieved in which the anode basket 144 is still completely covered by the electrolyte 125, while the gravure cylinder 148 stands completely free above the liquid level of the electrolyte 125 and can be easily lifted out there with a crane not depicted.

FIGS. 6a, 6b and 6c represent another embodiment of the invention in schematic form. In contrast to the embodiment of FIGS. 5a, 5b and 5c, no lower and upper trough are provided, but rather a first trough 210 and a second trough 220, which are arranged next to one another. Both troughs are connected to each other by a line with a pump 260, so that the electrolyte 225 can be pumped from the first trough 210 into the second trough 220 by means of the pump 260 and can flow back again into the first trough via an overflow (not shown). Otherwise, the mode of operation corresponds to FIGS. 5a, 5b and 5c, so that no repetition of the statements made about them is necessary here.

Alternatively, in FIGS. 5a-5c and 6a-6c, a third auxiliary electrode (not shown) could additionally be provided as an inert electrode, with the second auxiliary electrode (56, 156, 256) and the third auxiliary electrode, as depicted in FIGS. 3a to 3c, being arranged in such a way that the chromium electrode (54, 154, 254) is located between them.

According to another embodiment, several of these units consisting of an inert electrode—a chromium electrode—an inert electrode could also be connected in series—as depicted in FIGS. 4a to 4c—and could each replace the second circuit in FIGS. 5a-5c and 6a-6c.

The method and electrolysis cell according to the invention can therefore be used to obtain chromium layers with the desired properties, particularly on gravure cylinders, in a particularly advantageous manner.

The following examples are intended to further illustrate the invention. They should in no way be understood as restricting the invention.

EXAMPLES

Example 1

One exemplary embodiment of the method according to the invention comprising steps (A) to (G) was carried out as follows:

An electrolyte with the following composition was provided:

chromium ( III ) ⁢ sulfate ⁢ ( density = 1.26 g / mL ; 3 ⁢ % ⁢ Cr ( III ) -> 37.8 g / L -> 0.727 M ) 18.6 L Na ⁢ sulfoacetate ⁢ ( 8.3 wt . % -> 104.6 g / L -> 0.568 M ) 6.27 kg Na ⁢ formate ⁢ ( 8 ⁢ wt ⁢ % -> 100.8 g / L -> 1.482 M ) 6.05 kg Na - sulfate , 1.7 wt ⁢ % -> 21.4 g / L -> 17 ⁢ mM ) 1.29 kg

One liter of electrolyte was heated to 40° C. in a beaker with constant stirring and a pH value of 2.6 was set. Thereafter, 2 electrodes were inserted into the beaker parallel to each other and opposite each other at a distance of 10 cm and connected to a direct current source. One electrode was a mixed-oxide-coated (MMO) titanium expanded metal that was used as the anode. The other electrode was a chromium plate, which was used as the cathode. The anode and cathode surfaces were selected in such a way that the working current density was approximately 4 A/dm² at a current of 3 amperes. The anode surface was selected to be the same size as the cathode surface.

A cathode voltage was applied to dissolve the passivation layer on the chromium electrode (in accordance with the invention: 1^st auxiliary electrode). The other electrode was the inert electrode, which acted as the anode.

In the present example, after a short period of about 20 s, it could be observed that gas was generated on the chromium electrode and gas bubbles rose to the surface. This meant a dissolution of metallic chromium in the form of chromium(III) ions. After observing the start of the dissolution process, the current flow was switched off, in this case after 60 seconds, so that the anode and cathode were no longer subjected to direct current. In this process, it was noted that dissolution did not stop, but continued in the same way.

A 1.0 mL sample was taken at each of the time points 0, 2, 4 and 6 hours during the experiment, and the chromium content was determined. It was found that the amount of Cr(III) had increased linearly during the observation period. This was additionally confirmed by a gravimetric analysis of the chromium electrode. The dissolved chromium species was exclusively trivalent chromium. Hexavalent chromium (Cr(VI)) could not be detected at any time.

Currentless dissolution was subsequently stopped by removing the chromium electrode from the electrolyte. Alternatively, the electrolyte could also be removed from the beaker. This exposes the chromium electrode to surface oxidation in ambient air and forms the passivation layer again. Following the build-up of the passivation layer, re-inserting the chromium electrode into the electrolyte—for example, after a dwell time of 30 seconds in ambient air—no longer led to a start of the dissolution process.

Instead of removing the chromium electrode from the electrolyte, the polarity was changed to stop the currentless dissolution, i.e., the chromium electrode was positively charged for a short time until the formation of hydrogen on the chromium electrode stopped. The formation of hydrogen was stopped after 60 seconds. After the visible gas generation had stopped, the current was switched off so that dissolution of the chromium was stopped permanently.

Example 2

Variation of the pH Value

Example 1 was carried out again, but the pH value was modified. In detail, the pH value was gradually lowered from 3.1 to 2.8, 2.6 and 2.4 with the same set-up as in example 1. In another experiment, the pH value was increased from 3.1 to 3.3 and 3.5. In both cases, the same results were obtained as described in example 1. However, the chromium dissolution rate was lower at the higher pH value.

LIST OF REFERENCE NUMERALS

• • 10, 100, 200 electrolysis cell
  • 15 trough
  • 25, 125, 225 electrolyte
  • 30, 130, 230 bearing bridge
  • 42, 142, 242 anode rail
  • 44,, 144, 244 anode, anode basket
  • 48, 148, 248 cathode, gravure cylinder
  • 54, 54.1, 54.2, 54.3, 154, 254 first auxiliary electrode, chromium electrode
  • 54a, 154a, 254a chromium mold
  • 55, 55.1, 55.2, 55.3, 155, 255 hydrogen bubbles
  • 56, 56.1, 156, 256 second auxiliary electrode
  • 56.2 third auxiliary electrode
  • 56.3, 56.4 additional auxiliary electrodes
  • 58, 158, 258 power source
  • 59, 159, 259 switch
  • 110 upper trough
  • 120 lower trough
  • 127 overflow
  • 160, 260 pump
  • 210 first trough
  • 220 second trough