EP1717353A1 - Alkaline galvanizing bath comprising a filtration membrane - Google Patents

Abstract

The alkaline electroplating bath has an anode and a cathode. The anode region and cathode region are separated by a filtration membrane. An independent claim is also included for use of filtration membrane.

Description

The invention relates to an alkaline electroplating bath for applying zinc alloys to substrates, in which the anode space and the cathode space are separated from one another by a filtration membrane. With the alkaline electroplating bath according to the invention, zinc alloys can be deposited on substrates in a consistently high quality. The electroplating bath is operated with zinc alloy baths containing organic additives such as brighteners and wetting agents and complexing agents in addition to soluble zinc salts and optionally other metal salts selected from iron, nickel, cobalt and tin salts.

In order to allow the deposition of functional layers of zinc baths, organic brighteners and wetting agents are added to the bath. Furthermore, the bath contains complexing to allow the deposition of other metals of the zinc alloy. The complexing agent serves to regulate the potential and keep the metals in solution so that the desired alloy composition is achieved. However, the use of the aforementioned organic components leads to operation of the baths to problems, as for example in the WO 00/06807 to be discribed. There is felt in particular as a disadvantage that these baths after a few hours of operation a color change of originally blue-violet after brown show. The brown color comes from decomposition products, the amount of which increases in the course of the operation of the bath. After several weeks or months, this staining intensifies. This causes considerable disruption of the coating of the workpieces, such as uneven layer thicknesses or blistering. A continuous cleaning of the bath is therefore essential. However, this is time consuming and costly (see page 2, lines 3 to 10 of WO 00/06807 ).

With phase separation and increasing content of organic contaminants, increasing decorative problems of coating occur and result in decreased productivity. To reduce the decorative problems usually increased dosages of organic bath additives are made, whereby the content of degradation products continues to increase.

Remedies are several methods that are described below:

Bath dilution reduces the concentration of impurities in proportion to the degree of dilution. A dilution is easy to carry out, but has the disadvantage that the amount of electrolyte removed from the bath has to be supplied to cost-intensive disposal. A complete new approach of the bath can be considered in this context as a special case of Badverdünnung.

Activated carbon treatment by stirring 0.5-2 g / l of activated carbon into the bath followed by filtration reduces the concentration of impurities by adsorption on the coal. Disadvantage of this method is that it is laborious is and causes only a relatively small reduction.

Alkaline Zn baths contain a factor of 5 to 10 lower proportion of organic additives as acidic baths. Accordingly, contamination by decomposition products is generally less critical. In the case of alkaline alloy baths, however, the addition of significant amounts of organic complexing agents is required to complex the alloying additive (Fe, Co, Ni, Sn). These are oxidatively degraded at the anode and the accumulated decomposition products have a negative effect on the production process.

The EP 1 369 505 A2 discloses a method for purifying a zinc-nickel electrolyte in a galvanic process in which a portion of the process bath used in the process is evaporated until phase separation into a lower phase, at least one middle phase and an upper phase occurs, and the lower one and the upper phase are separated. This process requires several stages and is disadvantageous in terms of its energy requirements from a cost point of view.

The WO 00/06807 and WO 01/96631 describe electroplating baths for applying zinc-nickel coatings. To avoid the undesirable decomposition of additives at the anode, it is proposed to separate the anode from the alkaline electrolyte through an ion exchange membrane.

However, the inventions have the disadvantage that the use of such membranes is cost-intensive and maintenance-prone.

Furthermore, from the WO 00/06807 such as WO 01/96631 operated electroplating baths with anolyte and catholyte operated which are materially different. So will in the WO 00/06807 used as the anolyte sulfuric acid solution, in WO 01/96631 a basic solution, preferably sodium hydroxide, so that a separate anolyte circuit is required.

Furthermore, the baths known in the prior art have the disadvantage that in the anodic decomposition of the nitrogen-containing complexing agent cyanide is formed and accumulates in non-negligible concentration.

The invention has for its object to provide an alkaline electroplating bath, which does not have the aforementioned disadvantages. In particular, the life of the bath is to be increased, the anodic decomposition of organic constituents of the bath to be minimized and, when it is used, a layer thickness of consistently high quality to be obtained on the coated substrate.

The invention relates to an alkaline electroplating bath for applying zinc alloys on substrates having a cathode and an anode, which has a filtration membrane which separates the anode space and the cathode space of the bath from each other.

In the bath according to the invention known filtration membranes are used. The size of the pores of these filtration membranes is generally in a range from 0.0001 to 1.0 μm and from 0.001 to 1.0 μm, depending on the type of membrane (nano- or ultrafiltration membrane). Preferably, in the alkaline electroplating bath, filtration membranes having a pore size in the range of 0.05 to 0.5 microns used. Particularly preferably, the pore size is in a range of 0.1 to 0.3 microns.

The filtration membrane contained in the alkaline electroplating bath according to the invention may consist of various organic or inorganic, alkali-resistant materials. These materials are, for example, ceramics, polytetrafluoroethylene (PTFE), polysulfones and polypropylene.

Particularly preferred is the use of filtration membranes made of polypropylene.

In general, the filtration membrane in the alkaline electroplating bath according to the invention is designed as a flat membrane. However, the alkaline electroplating bath according to the invention can also be realized with other membrane forms, examples being hoses, capillaries and hollow fibers.

• 80-250 g/l NaOH bzw. KOH
• 5-20 g/l Zink in Form des löslichen Zinksalzes
• 0,02-10 g/l des Legierungsmetalls Ni, Fe, Co, Sn in Form der löslichen Metallsalze
• 2-200 g/l Komplexbildner ausgewählt aus Polyalkenylaminen, Alkanolaminen, Polyhydroxycarboxylaten
• 0,1-5 g/l aromatischer bzw. heteroaromatischer Glanzbildner

In the alkaline electroplating bath according to the invention, customary zinc alloy baths can be used. These are usually composed as follows:

• 80-250 g / l NaOH or KOH
• 5-20 g / l zinc in the form of the soluble zinc salt
• 0.02-10 g / l of the alloying metal Ni, Fe, Co, Sn in the form of the soluble metal salts
• 2-200 g / l complexing agent selected from polyalkenylamines, alkanolamines, polyhydroxycarboxylates
• 0.1-5 g / l aromatic or heteroaromatic brightener

Such baths are for example in US 5,417,840 . US 4,421,611 . US 4,877,496 or US 6,652,728 described.

The alkaline electroplating bath according to the invention has the advantage that in it also baths for the deposition of zinc alloys can be used, which are suitable for use in the from WO 00/06807 and WO 01/96631 known alkaline zinc-nickel bath with an ion exchange membrane are not suitable. In this context, for example, sold by the applicant bath "Protedur Ni-75" to call, which is characterized by a particularly high efficiency.

With a commonly used ion exchange membrane and an anolyte of 100 g / l sulfuric acid solution could be deposited from a fresh approach of the bath Protedur Ni-75 no functional layers. An already 50 Ah / l powered approach could no longer operate after another 10 Ah / l. The process obviously requires a certain amount of anodically produced degradation products, which are prevented by the use of ion exchange membranes.

In experiments with filtration membrane was found that from a pore size of 0.2 microns even with this type of bath enough decomposition products are formed to allow trouble-free operation. The efficiency was even higher than without filtration membrane and the consumption of organic additives significantly lower. Compare Table 1. <b><u> Table 1: </ u></b> Protedur Ni-75 without filtration membrane with filtration membrane efficiency: 64% 73% Consumption supplementary solution 4.5 l / 10,000 Ah 2.8 l / 10,000 Ah Consumption Brilliant addition 3.0 l / 10,000 Ah 1.7 l / 10,000 Ah Consumption depth shaker 1.1 l / 10,000 Ah 0.8 l / 10,000 Ah

In the alkaline electroplating bath according to the invention, the previously used anodes can be used further. These are mostly nickel anodes. The use of these anodes is less expensive than that from the WO 00/06807 known electroplating bath, in which special platinum-plated titanium anodes must be used in addition.

• Figur 1 zeigt schematisch das erfindungsgemäße Galvanikbad. Hierin bedeutet (1) das Bad, (2) die Anoden und (3) die Kathode bzw. das zu beschichtende Werkstück. Weiter dargestellt sind der die Anode umgebende Anolyt (4) und der die Kathode umgebende Katholyt (5). Anolyt und Katholyt sind durch eine Filtrationsmembran (6) voneinander getrennt. Die Filtrationsmembran ermöglicht den Betrieb des Bades, begrenzt aber gleichzeitig die Zersetzung der in dem Katholyten befindlichen organischen Bestandteile, insbesondere des Komplexbildners, durch Wanderung an die Anode bzw. in den Anodenraum. Die Komplexbildner können nur vermindert an der Anode reagieren, d.h. sie werden begrenzt zu Carbonaten, Oxalaten, Nitrilen bzw. Cyaniden umgesetzt. Daher beobachtet man bei Betrieb des erfindungsgemäßen Galvanikbads auch keine Phasentrennung. Eine kontinuierliche Reinigung des Bades ist mithin nicht erforderlich.

The invention is explained in more detail by the figures attached as an annex:

• FIG. 1 shows schematically the electroplating bath according to the invention. Here, (1) means the bath, (2) the anodes, and (3) the cathode or the workpiece to be coated. Also shown are the anolyte (4) surrounding the anode and the catholyte (5) surrounding the cathode. Anolyte and catholyte are separated by a filtration membrane (6). The filtration membrane allows the operation of the bath, but at the same time limits the decomposition of the organic constituents present in the catholyte, in particular of the complexing agent, by migration to the anode or into the anode space. The complexing agents can react only reduced at the anode, ie they are limited to carbonates, oxalates, nitriles or cyanides implemented. Therefore observed when operating the electroplating bath according to the invention also no phase separation. A continuous cleaning of the bath is therefore not required.

In the case of the bath according to the invention, the anode space is preferably made smaller than the cathode space, since the essential processes take place there.

The invention will be explained in more detail by the following embodiments.

Examples

Initially, a bath of zinc-nickel alloy composition having the composition below was operated at a rate of 5 Ah / l, so that the initially higher consumption stabilized after the start of the operation of the bath. As a result, unwanted deposition processes are avoided. This bath is hereafter referred to as a "new approach".

• Zink 10,4 g/l (als lösliches ZnO)
• Nickel 1,2 g/l (als Nickelsulfat)
• NaOH 120 g/l
• Quadrol 35 g/l
• Pyridinium-N-propan-3-sulfonsäure 1,25 g/l
• Polyethylenimin 5 g/l

It consists of the following components:

• Zinc 10.4 g / l (as soluble ZnO)
• Nickel 1.2 g / l (as nickel sulphate)
• NaOH 120 g / l
• Quadrol 35 g / l
• Pyridinium N-propane-3-sulfonic acid 1.25 g / l
• Polyethyleneimine 5 g / l

Furthermore, a bath of the same type was used, which had been operated for a long time, ie a throughput of> 1000 Ah / l. This bath is hereafter referred to as "old approach".

Both baths were operated with and without filtration membrane in 5-liter tanks. The filtration membrane used was the Abwa-Tec polymer membrane P150F, which has a pore size of 0.12 μm. The membrane was placed in the anode to cathode bath with the anolyte and catholyte being identical, ie, no special anolyte was added. Subsequently, iron sheets (7 × 10 cm), which are usually used for Hull cell tests, were used as workpieces to be coated and coated at a current density of 2 A / dm ² . The baths were operated in serial connection. The movement of the iron sheets was mechanical, at a speed of 1.4 m / min.

Subsequently, the baths were analyzed and supplemented regularly. The replenishment of the baths was carried out according to the results of Hull cell tests after about 5 Ah / l. A carry-over of 12 1 bath / 10,000 Ah, which was common in production baths, was also taken into account and the bath components were supplemented accordingly.

Table 2 shows the Hull cell layer thickness in a new approach and old approach depending on the throughput with and without filtration membrane. The layer thickness measurements were made after adjustment of the baths.

It was measured at both high and low current density points. The dots lie on the Hullzellenblechen 3 cm from the bottom and 2.5 cm from the left and right lateral edge. On the left side is the high current density (point A) and on the right the low current density (point B). <b><u> Table 2: </ u></b> Hull cell: New batch without filtration membrane New batch with filtration membrane Old batch without filtration membrane Old batch with filtration membrane 1Ax10min point a Point B point a Point B point a Point B point a Point B 0-sample 3.00 1.00 3.00 1.00 2.00 0.80 2.00 0.80 5 Ah / l 2.65 1.10 3.20 1.25 2.10 0.95 2.20 0.95 10 Ah / l 2.55 1.05 3.25 1.20 2.30 0.90 2.40 0.95 15 Ah / l 2.50 1.00 3.20 1.15 2.40 0.90 2.60 0.95 20 Ah / l 2.60 0.95 3.30 1.20 2.30 0.85 2.60 0.95 25 Ah / l 2.65 0.90 3.45 1.10 2.25 0.80 2.55 0.90 30 Ah / l 2.55 1.00 3.40 1.20 2.25 0.85 2.65 0.95 35 Ah / l 2.50 1.05 3.35 1.20 2.30 0.90 2.75 1.00 40 Ah / l 2.30 0.95 3.50 1.15 2.20 0.85 2.85 1.05 45 Ah / l 2.20 0.90 3.65 1.10 2.00 0.80 2.95 1.00 Average: 2.50 0.99 3.37 1.17 2.23 0.87 2.62 0.97 increase 35% 19% 17% 12%

Surprisingly, it was found that the layer thickness decreases in the new batch without filtration membrane, while it steadily increases in the old batch with filtration membrane.

Thus, when using a filtration membrane, the average layer thickness is about 35% higher in a new batch in the high current density range and about 19% higher in the low current density range as if one had not used a filtration membrane. In the old batch, it is on average 17% and 12% higher than without filtration membrane.

Surprisingly, in the case of an old batch, a filtration membrane is produced after a throughput of> 1000 Ah / l introduced after a short time comparable to a new approach current efficiency.

Table 3 shows the average consumption (1 / 10,000 Ah) of the electrolyte in the bath for filtration membrane electroplating baths according to the invention and baths which do not have this membrane. By using the filtration membrane, the organic consumption was reduced between 12 and 29% depending on the additive. <b><u> Table 3: </ u></b> Reflectalloy ZNA: complexing brightener Without filtration membrane 4.1 2.8 With filtration membrane 3.6 2.0 Difference: -12% -29% Complexing agent: quadrol, polyethylenimine
Brilliant addition: pyridine-N-propane-3-sulfonic acid

The composition of the aforementioned baths were analyzed according to the tests described above. Of particular interest was their cyanide content. This was much lower when using the baths according to the invention with a filtration membrane as baths without membrane. As shown in Table 4 below, a bath without the membrane had a cyanide content of 680 mg / L (new batch) and 790 mg / L (bath of> 1000 Ah / L), respectively, while the corresponding membrane baths contained a cyanide Content of 96 mg / L or 190 mg / L.

Surprisingly, it was found that the cyanide content of an old batch, ie a bath with> 1000 Ah / l can be reduced when it is provided with a filtration membrane and operated. In such a bath, for example, the cyanide content was reduced from 670 mg / l to 190 mg / l. <b><u> Table 4: </ u></b> Total cyanide: start value after 50 Ah / l with filtration membrane after 50 Ah / l without filtration membrane New approach (after 5 Ah / l) 33 mg / l 96 mg / l 680 mg / l Old approach (> 10,000 Ah / l) 670 mg / l 190 mg / l 790 mg / l

In carrying out the above tests, the color of the baths was also evaluated. It was found that the color of a freshly prepared bath without membrane changed from initially violet-orange to brown within 15 Ah / l, whereby it remained purple or violet-orange over the entire time when using a filtration membrane. The old batch remained brown without using a membrane and the color changed to orange-brown after 15 Ah / l using a filtration membrane. Violet is also the color of freshly applied baths, which then turn to orange (after a few Ah / l) and at high throughput in brown.

Finally, the voltage between anode and cathode was measured. It was about 3 V and was only about 50-100 mV higher in both approaches using a filtration membrane. Is used instead of the filtration membrane an ion exchange membrane, as in the WO 00/06807 described is, the voltage is higher by at least 500 mV. This again shows the advantage of using a filtration membrane instead of an ion exchange membrane.

In summary, the use of filtration membranes over the use of ion exchange membranes offers many advantages. Thus, the coating process carried out therewith is more cost-effective, since no platinized anodes have to be used, catholyte and anolyte can have the same composition, and thus no circulation for the anolyte is required.

Compared to the operation of a plating bath without a membrane, the current efficiency is higher and the consumption is lower. Finally, decomposition products and in particular cyanides can be reduced or their concentration lowered and the quality of the layers deposited from the bath can be improved.

LIST OF REFERENCE NUMBERS

(1)

Alkaline electroplating bath

(2)

anode

(3)

cathode

(4)

anolyte

(5)

catholyte

(6)

filtration membrane

Claims (11)

Alkaline electroplating bath for applying zinc alloys on substrates having an anode and a cathode, characterized in that the anode space and the cathode space are separated from each other by a filtration membrane. Alkaline electroplating bath according to claim 1, characterized in that the size of the pores of the filtration membrane in the range of 0.0001 to 1.0 microns. Alkaline electroplating bath according to claim 2, characterized in that the size of the pores of the filtration membrane is preferably in the range of 0.1 to 0.3 microns. Alkaline electroplating bath according to claim 1, characterized in that the filtration membrane consists of a material selected from ceramic, PTFE, polysulfones or polypropylene. Alkaline electroplating bath according to claim 1, characterized in that the filtration membrane is designed as a flat membrane. Alkaline electroplating bath according to claim 1, characterized in that the anolyte located in the anode compartment has the same composition as the catholyte located in the cathode compartment. Use of a filtration membrane for separating an alkaline electroplating bath with an anode and a cathode in an anode and cathode space to increase the life of the bath, to avoid the anodic decomposition of organic components of the bath and to obtain layers of consistently high quality. Method of applying zinc alloys to substrates, characterized in that the substrate is introduced as a cathode into an alkaline electroplating bath according to claims 1 to 7 and the substrate is galvanically coated with the zinc alloy. A method according to claim 8, characterized in that the electrolyte used is a solution comprising the following components: • 80-250 g / l NaOH or KOH • 5-20 g / l zinc in the form of the soluble zinc salt 0.02-10 g / l of the alloying metal Ni, Fe, Co, Sn in the form of the soluble metal salts • 2-200 g / l complexing agent selected from polyalkenylamines, alkanolamines, polyhydroxycarboxylates • 0.1-5 g / l aromatic or heteroaromatic brightener A method according to claim 8, characterized in that one carries out the coating at a temperature of 10 to 60 ° C, preferably 20 ° to 30 ° C. A method according to claim 8, characterized in that the bath at a current density of 0.25 to 10 A / dm ² , preferably 1 to 3 A / dm ² operates.

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