WO2012031753A1

WO2012031753A1 - Anode and use thereof in an alkaline electroplating bath

Info

Publication number: WO2012031753A1
Application number: PCT/EP2011/004511
Authority: WO
Inventors: Hartmut Trenkner; Thorsten KÜHLER
Original assignee: Coventya Gmbh
Priority date: 2010-09-07
Filing date: 2011-09-07
Publication date: 2012-03-15
Also published as: DE102010044551A1

Abstract

The invention relates to an anode and to the use thereof in an alkaline electroplating bath. In the application in electroplating, said anode is suitable for use in highly alkaline electroplating electrolytes based on sodium hydroxide or potassium hydroxide for depositing zinc and zinc alloys onto substrates of steel and die-cast zinc.

Description

Anode and their use in an alkaline

electroplating bath

The invention relates to an anode and its use in an alkaline electroplating bath. This anode is suitable in electroplating applications for use in strongly alkaline, galvanic electrolytes based on sodium hydroxide or potassium hydroxide for the deposition of zinc and zinc alloys on substrates of steel and zinc die-casting.

According to the state of the art, similar membrane anodes are used in the field of cataphoretic dip coating in order to convert interfering anions, which are formed during the electrophoretic coating process in the water-based coating bath, into a dilute acid anolyte via special anion exchange membranes and thus out of the membrane To remove paint bath. The functional principle of the anion exchange membrane when used in electroplating for metal deposition is analogous, but special features such as:

- Type of membrane

- Cross sections of the current-conducting parts

- Chemical resistance of the materials to the requirements much higher applied

Currents and lower voltages, strong

Alkalinity of the galvanic electrolytes and the

Anolyte adapted. It is also state of the art to use anodes made of steel, stainless steel or nickel-plated steel most frequently in alkaline zinc-plating electrolytes. Different geometric shapes are chosen, e.g. Plates in rectangular shape, expanded metal in rectangular or cylindrical form, round bars, tubes and others.

Strongly alkaline galvanic electrolytic electrolytes tend, depending on the electrolyte composition, to have relatively high deposits on the anode surfaces after a relatively short operating time of a few weeks. This has the disadvantage of the gradual deterioration of the cathodic current efficiency and thus the efficiency of the galvanic process and the galvanic see plant. The cost of electrical energy per coated surface area increases.

These deposits are large proportions of sodium carbonate and sodium oxalate due to oxidation at the anode surface in sodium hydroxide based electrolytes. In addition, organic change Degradation products, the output characteristics of the electro ^¬ rule electrolytes.

A regular, sometimes high cleaning effort on the anodes and the containers is necessary. Of the

Carbonate such electrolytes must often be ^¬ consumption decreased with crystallizers with additional Elektroenergiever. Alternatively, the galvanic baths are recalculated. The consumed electric lyte be disposed of and generate additional Chemi ^¬ kalien-, disposal and wastewater treatment costs and plant downtime.

The anodes described in EP 1 344 850 A1 are due to the high quality materials used

- Platinized titanium as the anode material

- perfluorinated cation exchange membrane

very cost-intensive in procurement and are therefore used exclusively for alkaline zinc-nickel electrolytes. The zinc-nickel

Electrolyte (catholyte) is separated from the anode by a cation exchange membrane. The anolyte used is dilute sulfuric acid. As disadvantages of the large-scale application of

EP 1 344 850 A1 may be mentioned: a. Dilution of the zinc-nickel electrolyte during the galvanic process due to neutralization of the sodium hydroxide by protons of the sulfuric acid anolyte, which are "transported" through the cation exchange membrane and with

Hydroxide ions react to water:

H ⁺ anolyte + OH catholyte ~> ^ ORatholyt

This process is unidirectional and leads to permanent dilution of the zinc-nickel Electrolytes.

b. Volume increase of the zinc-nickel electrolyte:

The dilution described in point a) and the resulting necessary addition of sodium hydroxide to restore the sodium hydroxide concentration in the zinc-nickel electrolyte required for the alloy deposition leads to a continuous increase in the volume of the electrolyte. Water transport from the sulfuric acid anolyte via the cation exchange membrane into the catholyte (zinc-nickel electrolyte) due to osmotic pressure differences enhances this effect.

c. From a) and b) following, an additional, considerable technical and energetic effort is operated to concentrate the permanently formed, weakly diluted electrolyte volume excess by means of a vacuum evaporator and discontinuously fed back into the process.

Proceeding from this, it is an object of the invention to provide a low-cost anode which minimizes the formation of degradation products in strongly alkaline electrolytes, regenerates used zinc electrolytes operated with conventional anodes and makes them more efficient and keeps newly added electrolytes at a consistently high level of performance.

This object is achieved by the anode with the features of claim 1. Claim 11 relates to an alkaline electroplating bath and claim 15 relates to the use of the anode. Further advantageous embodiments are contained in the dependent claims.

According to the invention, an anode with an ion exchange membrane for an alkaline electroplating bath is used for deposition. proposed by zinc and / or zinc alloys, wherein the anode is at least partially surrounded by the ion exchange membrane through which the anode of an alkaline electrolyte and the cathode is separable. The anode according to the invention is characterized in that the ion exchange membrane is an anion exchange membrane.

An anion exchange membrane separates the galvanic zinc electrolyte from the anode to prevent undesired side reactions such as oxidation of the organic additives with formation of deposits on the anode surfaces.

Aged electrolytes containing decomposition products can be regenerated when using the anode according to the invention and thus become more efficient again. Consequently, in this way, the life can be extended before a new galvanic bath is to be set.

In this case, the anion exchange membrane preferably consists of polyetheretherketone or contains this.

Polyetheretherketone belongs to the group of polyetherketones. These are characterized by the fact that they are high temperature resistant thermoplastics. The most important member of this group is the polyetheretherketone, whose melting point is 335 ° C. Polyether ketones are resistant to almost all organic and inorganic chemicals. Furthermore, they are resistant to hydrolysis up to about 280 ° C.

Preferably, the anion exchange membrane of the anode according to the invention has a thickness of 0.1 mm to 0.13 mm up.

Furthermore, the anion-exchange membrane before Trains t ^¬ a resistivity of <1 Ω / cm.

Another variant of the anion exchange membrane is a multilayer structure.

The anode can be made of steel, stainless steel, nickel and / or nickel-plated steel or contain this. Furthermore, an expanded metal from the aforementioned materials in a cylindrical shape with

Screw closure and fastening device can be used on an anode rail.

The anode may be rectangular, round, cylindrical or tubular. Further, the anode of art ^¬ material, in particular in the form of a synthetic fabric, plastic walls or a plastic base to be surrounded. Moreover, the anode can be rigid or flexi ^¬ bel.

For example, a round anode cylindermantel- form of an inner Stut cgitterrohr, a

Anion exchange membrane and a protective stocking, e.g. consists of plastic fabric or a plastic mesh, be surrounded. In this case, the protective sock, the anion exchange membrane and the inner support grid tube are preferably connected to one another in a layered manner.

In a box-shaped, preferred embodiment variant, the anolyte inlet and the anolyte outlet are arranged perpendicular to one another with the anolyte inlet arranged on the upper side of the anode box and the anolyte outlet arranged on the rear side of the anode box is. In this case, the anode may be made of plug metal.

According to a further variant, it is preferred that the anode is constructed angled. In the case of a tubular, angled variant, the anode may be designed, for example, as a spiral, which may be flexible.

Furthermore, each anode used in a galvanic electrolyte preferably has one each

Anolyte feed and anolyte return, so that all anodes have the same functional and reaction conditions.

In the application of zinc-nickel alloy electrolytes, the use of special anodes based on EP 1 344 850 A1 has proven to be successful in part because the formation of degradation products by anodic oxidation with direct contact of the electrolyte consisting of sodium hydroxide , Nickel sulfate solution, amines as complexing agents and other organic compounds, is greatly reduced and the cathodic current efficiency of the galvanic process of 30 - 50% without application of anodes depending on the electrolyte composition and applied current, can be increased to 60 - 90%.

Unlike the application of cation exchange membrane anodes, as described in EP 1 344 850 A1, when using the anode according to the invention the

Use of investment- and cost-intensive, technical peripherals such as vacuum evaporator to restrict the volume of catholyte volume without meaning, since the effects described on page 3, line 25 to page 4, line 14 does not occur. According to the invention, an alkaline electroplating bath is provided for depositing zinc and / or zinc alloy with an anode and a cathode, wherein the anode is separated from an alkaline electrolyte by an ion exchange membrane. According to the invention, the ion exchange membrane is an anion exchange membrane.

The electroplating bath preferably contains an alkaline solution as anolyte. This has the advantage that, in the case of embrand defects, no acid neutralizes the alkaline zinc or zinc alloy electrolyte and makes it unusable in extreme cases.

As the membrane, an alkali-resistant anion exchange membrane is preferably used for this invention.

Preferably, the alkaline solution in the electroplating bath is a sodium hydroxide solution and / or a potassium hydroxide solution.

It is preferred that the alkaline solution has a pH in the range of 10 to> 14. If in the anode according to the invention the anion exchange membrane is replaced by a cation exchange membrane and sulfuric acid is used as the anolyte, the regeneration function can not be fulfilled for previously conventionally used zinc-nickel electrolytes, since the contact of protons from the anolyte

(Sulfuric acid) with cyanide ions from the catholyte (zinc-nickel electrolyte) on the membrane cathode side the risk of the formation of highly toxic hydrogen cyanide is given. According to the invention is also the use of beschrie ^¬ surrounded anode in an electroplating bath.

The power consumed in the galvanic etallabscheidung at the Ka ^¬ Thode negative charge amount corresponding to the equivalent amount of positive charge that will be consumed in the form of positively charged hydroxide ions at the anode, thereby forming water and oxygen.

Anode:

4 OH ^" - 4 e -> 2H ₂ 0 + 0 ₂ (I) Cathode:

Zn ²⁺ + 2e -> 2 Zn (metal deposition) (II) 2 H ⁺ + 2e -> H ₂ (hydrogen deposition) (III)

The reaction (III) as co-reaction to the metal deposition (II) is greatly slowed down when using membrane anodes.

It follows that a uniform consumption of hydroxide ions takes place in the zinc or zinc alloy electrolyte and in the anolyte, which is accompanied by volume reduction. The volume reduction is partially compensated in the galvanic practice by the necessary supplemental chemicals and depending on volume carryover with the galvanized goods and their goods carriers is additionally filled with water if required.

Thus, no continuous increase in volume in the galvanic electrolyte due to dilution effects is produced.

Osmosis effects due to dehydration from the anolyte during system downtimes are prevented by the anolyte being upgraded to a higher sodium hydroxide concentration. concentration is set as the catholyte (zinc ^¬ electrolyte).

From the laboratory and pilot experiments, a sodium hydroxide concentration of 150-160 g / l in the anolyte was found to be favorable at a sodium hydroxide content of 120 g / l in the zinc electrolyte.

At too high a concentration of sodium hydroxide in the anolyte in the currentless state osmosis is performed in ent ^¬ opposite direction, that is, dilution of the anolyte by water from the catholyte to the anolyte volume increase · up to compensate for the ion activities of anolyte and catholyte.

This condition is uncritical, since the volume increase in the anolyte can be traced back to the catholyte (in practice, however, the anolyte can "overflow" from the membrane anodes).

The volume balance of anolyte and catholyte remains balanced.

The use of membrane anodes with anion exchange membrane in used zinc-nickel electrolytes allows regeneration thereof by cyanide ions formed by conventional zinc-nickel baths (without membrane technology) by chemical reaction at the anode and reduce the cathodic current efficiency, get into the anolyte due to their negative electrical charge and can be disposed of with this targeted.

Furthermore, carbonate and sulfate ions, which also slow the galvanic deposition process, are or zinc alloy electrolytes are transferred via the anion exchange membrane in the anolyte and can be removed systematically.

Reference to the following figures 1 to 5 and with reference to Example 1, the application according to the object will be explained in more detail, without limiting this to these variants.

Figure 1 shows an anode structure with a cylindrical shape.

FIG. 2 shows an anode construction with a membrane body in the form of a box.

FIG. 3 shows an angled anode construction

Tubular form.

FIG. 4 shows several anodes in a galvanic bath.

FIG. 5 shows the schematic structure of a galvanic bath.

Figure la shows an embodiment of the anode 1 in a schematic representation. This has a cylindrical shape. At the top of the Anolytzu- run 2 is shown in the center and the Anolytrücklauf 4 of the sodium hydroxide as anolyte 3 side. This embodiment has a screw cap 5. The anode 1 is surrounded by an inner support grid tube 8 and the anion exchange membrane 7. As materials for the anion exchange membrane 7, for example, fumasep® FAB or else fumasep® FAA from Fumatech can be used. The protective sock 6 made of plastic fabric, for example, polypropylene be made. Furthermore, an outer Schutzgit ^¬ terrohr optionally be present. The anode 1 as tube anode can optionally be made of steel, stainless steel, nickel or expanded metal of the aforementioned material in cylindrical form with screw cap 5 and fastening device on the anode rail. The Anolytrücklauf 4 ensures degassing of the anolyte 3 of oxygen, which is formed during the galvanic process at the anode 1. As anolyte 3, for example, sodium hydroxide is used (150 to

180 g / l sodium hydroxide). At the anode attachment 9 in addition, the power supply takes place. Furthermore, below the anode 1, the flow of the anolyte 19 is shown.

In Figure lb is a side view of an anode 1 with screw 5 is shown. On the screw 5, the anode attachment 9 is arranged. Furthermore, the Anolytzulauf 2 and the anolyte return 4 are shown.

Figure lc shows a rotated by 90 ° view of Figure lc constructed of an anode 1 with screw 5 and arranged thereon Anodebefestigung 9. Furthermore, the Anolytzulauf 2 and the Anolytrücklauf 4 are shown.

FIG. 2 shows a box-shaped variant of the anode 1 according to the invention. The anode 1 here has a rectangular shape. It is surrounded by the anode box 11. By the Anolytzulauf 2 and the Anolytablauf 4 ', the liquid level of the anolyte 10 in the anode box 11 is regulated. The Anolytzulauf 2 is disposed at the top of the structure. The Anolytablauf 4 'is located at the back of the anode box 11. The anode 1 may consist of stretched consist of metal.

3 shows an angled pipe shape made of synthetic material ^¬ is shown. The anode 1 is made in this exemplary form of flexible anode material, example ^¬ as stainless steel, and is designed as a spiral. In this embodiment, the anode 1 has a tubular, angled casing 20.

4 shows a galvanic bath with Galvanisier ^¬ drum 21 and anode rails 23 for mounting and power transmission of the anodes 1 for the operation of multiple anodes 1 is shown. The anolyte circuit runs via a circulating pump 13, a filter 14 and a respective flow meter 15 per anode. Each membrane anode is connected to the anolyte circuit via feed outlets 24 and return lines 25. This ensures that all anodes 1 can obtain the same reaction conditions. Furthermore, the structure has an anolyte storage container 12. By this construction, it is possible at any time corrections of the sodium hydroxide content, sodium carbonate content and the

Anolyte volume perform. Furthermore, separated via the anion exchange membrane 7, the galvanic process inhibiting anions, such as cyanide, carbonate or sulfate, be removed.

FIG. 5 shows a variant of the electroplating bath. The anodes 1 are surrounded by the anolyte 3 and separated from the zinc-nickel electrolyte 18 by a respective anion exchange membrane 7. The anodes 1 and the cathode 17 are located in an electrolyte container 16. As the anolyte 3, sodium hydroxide solution surrounds the anodes 1. Example 1:

Technical requirements: Experimental arrangement: see FIG. 5

Laboratory membrane anode:

0 V2A tube anode: 1/2 "(2.07 cm)

Effective length of tubular steel anode: 12 cm Geometric area Steel tube anode: 0.8 dm ²

Anolyte cell with anion exchange membrane:

0 membrane tube: 65 mm

Length membrane tube: 11 cm surface membrane tube: 2.5 dm ² volume membrane tube: max. 330 ml

Volume membrane tube effective: 250 ml

Number of anolyte cells: 2 pieces anolyte: caustic soda

NaOH 1st day 120 g / 1

2nd day 150 g / 1

3rd day 180 g / 1 cathode: steel sheet 21x15 cm

Geometric area: 2 sides 6.4 dm ²

Zinc-nickel electrolyte: Performa 285

Condition: used Color: brown

Electrolyte container: plastic

Volume: 10 liters Electrolyte composition:

Zn

Ni

NaOH

Na ₂ CO ₃

Na ₂ S0 ₄

cyanide

Electroplating data:

Amperage:

Cathodic current density

Anodic current density:

Exposure time per plating cycle: 2 h Experimental procedure: a) The zinc-nickel electrolyte was loaded with 30 Ah / l, i. three days approx. 8 hours each with the described current load.

b) The chemical composition with regard to sodium hydroxide (NaOH), nickel (Ni), zinc (Zn) was regularly checked analytically and kept constant by adding nickel solution, sodium hydroxide and dissolving zinc metal.

c) The cathode sheets were in the cycle of a

Hour and measured the layer thickness of the zinc-nickel layer.

Test results: a) At the beginning of the experiments, an average deposited layer thickness of 9 μm was measured on the cathode plate. This corresponds to a cathodic current efficiency of approx. 30%.

b) After a current load of the electrolyte of approx.

20 Ah / 1, a color change from brown to wine red observed. This indicated qualitatively that the plating process did not lead to any new decomposition products due to anodic oxidation.

For comparison: A newly prepared zinc-nickel electrolyte is of violet color and shows a burgundy color after a low current load. With the color change, a significant increase in the cathodic current efficiency could be measured: After a coating time of one hour, an average layer thickness of 15 μm was measured. This corresponds to a cathodic

Current efficiency of approx. 50%.

After the daily downtimes without current load (about 12-14 hours) with the retention of the anolyte-filled membrane tubes in the zinc-nickel bath, it was found after the first day that the anolyte volume in both membrane tubes had fallen by about 25% and below Zinc-nickel electrolyte levels.

The sodium hydroxide concentration of the remaining anolyte had increased to 150 g / l. The cause can be explained by osmotic pressure differences between anolyte and catholyte (zinc-nickel electrolyte).

An increase of the sodium hydroxide concentration to 150 g / l and filling of the anolyte volume in the membrane tubes with demineralized water led after the second day to no dehydration in the anolyte.

Increasing the sodium hydroxide concentration to 180 g / l in the anolyte on the third day after the downtime showed an increase in the volume of the anolyte in the membrane tubes beyond the electrolyte level of the zinc-nickel bath to overflow.

By dehydration (osmosis) from the zinc-nickel Electrolytes had set a sodium hydroxide concentration of 155 g / l in the anolyte, g) After a current load of the zinc-nickel electrolyte of 30 Ah / 1 (third day), a further increase in performance of the zinc-nickel electrolyte could be detected average measured layer thickness after one hour of coating time was now 19 pm. This corresponds to a cathodic current efficiency of approx. 60%.

h) The following analysis values were determined after 30 Ah / 1 current load (loss due to overflowing anolyte due to osmosis not taken into account): zinc-nickel electrolyte:

Zn 6.5 g / 1

Ni 0, 9 g / 1

NaOH 125.0 g / l

Na ₂ CO ₃ 86, 0 g / 1

Na ₂ S0 ₄ 12, 0 g / 1

Cyanide 11 mg / 1

Anolyte:

Zn 0, 1 g / 1

Ni 10 mg / l

NaOH 155, 0 g / l

Na ₂ CO ₃ 5 g / l

Na ₂ SO ₄ 4 g / 1

Cyanide 5 mg / 1

Experimental evaluation: a) The experiments showed a significant decrease in the cyanide concentration in the zinc-nickel electrolyte due to removal via the anion exchange membrane, whereby the absolute amount of cyanide in the anolyte was not found again. This may be due to oxidative decomposition of cyanide in the anolyte.

b) Since the new formation of cyanide was prevented by the anion exchange membrane, the concentration of cyanide in the zinc-nickel electrolyte was continuously reduced.

c) As a result of decreasing cyanide concentration, a color change of the zinc-nickel electro-lyten and a clearly measurable increase in the cathodic current yield took place.

d) Carbonate and sulfate ions have been slightly removed from the zinc-nickel electrolyte. This also contributes to the increase in the cathodic current efficiency.

e) For precipitations and deposits of calcium or:

To avoid magnesium carbonate and sulphate, as these salts can severely impair membrane function or lead to total failure, the anolyte must be prepared and supplemented with demineralized water.

The test of an anode shown in Fig. La) in the pilot experiment has confirmed the operation in accordance with the laboratory experiments set out.

Claims

claims

Anode ion exchange membrane for a alka ^¬ metallic electroplating bath for the deposition of zinc and / or zinc alloys, wherein the anode is at least partially surrounded by the ion exchange membrane through which the anode of an alkaline electrolyte and the cathode is separable,

characterized in that the ion exchange membrane is an anion exchange membrane

Anode according to the preceding claim, characterized in that the anion exchange membrane consists of or contains polyetheretherketone.

Anode according to one of the preceding claims, characterized in that the anion exchange membrane has a thickness of 0.1 mm to 0.13 mm.

Anode according to one of the preceding claims, characterized in that the anion exchange membrane has a specific resistance of <1 Ω / cm.

Anode according to one of the preceding claims, characterized in that the anion exchange membrane has a multilayer structure.

6. Anode according to one of the preceding claims, characterized in that the anode

Steel, stainless steel, nickel and / or nickel-plated steel or contains this.

Anode according to one of the preceding claims, characterized in that the anode is rectangular, round, cylindrical or tubular 8. Anode according to one of the preceding claims, characterized in that the anode is rigid or flexible.

Anode according to one of the preceding claims, characterized in that the anode of

Plastic, in particular in the form of a plastic fabric and / or synthetic material grid, plastic walls and a plastic bottom is surrounded.

Anode according to one of the preceding claims, characterized in that the anode is constructed at an angle. 11. An alkaline electroplating bath for the deposition of zinc and / or zinc alloys with an anode and a cathode, the anode being separated from an alkaline electrolyte by an ion exchange membrane,

characterized in that the ion exchange membrane is an anion exchange membrane.

12. Electroplating bath according to the preceding claim, characterized in that it contains an alkaline solution as anolyte.

13. electroplating bath according to the preceding claim, characterized in that the alkaline solution is a sodium hydroxide solution Lö ^¬ and / or a potassium hydroxide solution.

14. Electroplating bath according to one of claims 13 to 15, characterized in that the alkaline solution has a pH in the range of 10 to> 14

^' has.

15. Use of the anode according to one of claims 1 to 10 in a galvanic bath according to one of claims 11 to 14.