ZINC ELECTROLYTE DEVOID OF BORIC ACID AND AMMONIUM FOR THE ELECTRODEPOSITION OF ZINC COATINGS

The invention relates to an aqueous electrolyte devoid of boric acid and ammonium for the electrodeposition of zinc coatings and to a method for producing such an electrolyte. The electrolyte comprises (a) Zn2+ in a concentration of 15 to 70 g/L; (b) Cl− in a concentration of 100 to 200 g/L; (c) K+ and/or Na+ in a total concentration of 0.75 to 6.0 mol/L; (d) acetate in a concentration of 5.0 to 45 g/L; (e) glycine and/or alanine in a total concentration of 0.5 to 30 g/L; and (f) water. The electrolyte has a pH of 4.5 to 6.5. In a preferred variant, the electrolyte contains (g) nicotinic acid and/or (h) ethoxylated thiodiglycol. The invention also relates to a method for producing a component having a zinc coating, which uses the electrolyte.

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Description
TECHNICAL FIELD

The invention relates to an aqueous electrolyte devoid of boric acid and ammonium for the electrodeposition of zinc coatings, to a method for producing such an electrolyte and to a method for producing a component having a zinc coating by way of electrodeposition from this electrolyte.

TECHNICAL BACKGROUND

Zinc coatings are used in many technical fields since they are able to provide very good cathodic corrosion protection for components, in particular those made of ferrous materials. The zinc coatings can be formed on the metal component by way of electrodeposition (electrochemical deposition) from a zinc electrolyte. The deposition preferably occurs from weakly acidic electrolytes since these generally facilitate significantly higher deposition speeds than alkaline electrolytes. A good deposition speed is important for a cost-effective coating of components, for example of accessory parts for automobile manufacturing.

Generally, a good optical impression, i.e. a homogenous and glossy surface, of the zinc coating is desirable. Moreover, for many technical applications it is also customary to anneal the zinc coatings. Annealing drives out the hydrogen absorbed during the electroplating process, in order to counteract the risk of the component subsequently becoming embrittled. Particularly in the case of high-strength steel components, there is an increased risk of hydrogen-induced brittle fracture due to the material. During the annealing process, it is desirable to avoid bubbles and other defects if at all possible, and to maintain a good optical impression.

Until now, the prior art for the deposition of zinc coatings has been to use a weakly acidic zinc electrolyte containing boric acid as a buffer. Boric acid is considered necessary to be able to work at high current densities (e.g. 6 to 8 A/dm2), which is desirable for a high deposition speed. Boric acid prevents haphazard and dendritic growth of the zinc layer at high current densities, which is important for annealability and the optical properties of the zinc coating. If boric acid is omitted, it is generally the case that disordered, powdery, black zinc layers are deposited.

Boric acid also prevents the reduction of protons to hydrogen (2H++2e−→H2), which can occur at the cathode in addition to the reduction of zinc ions to zinc (Zn2++2e−→Zn). The reduction of protons increases the pH in the surroundings of the cathode, since an excess of negatively charged hydroxide ions is generated. This leads to zinc hydroxide precipitating directly on the cathode surface and can result in coating defects.

Boric acid is also characterized by an affordable price, electrochemical stability, process compatibility and unproblematic wastewater treatment.

However, boric acid and related compounds containing boron, such as disodium tetraborate, diboron trioxide and tetraboron disodium heptaoxide, are considered Category 1B reproductive toxicants (teratogenic) and on 18 Jun. 2010 they were added to the candidate list for the REACH Regulation. After a thorough review and consideration of all known data and facts, on 1 Jul. 2015 the European Chemicals Agency (ECHA) argued in favor of the inclusion of boric acid in Annex XIV of the REACH Regulation on substances requiring authorization and submitted a corresponding recommendation to the EU Commission. Although boric acid and the other boron compounds mentioned above meet the criteria for inclusion in Annex XIV of the REACH Regulation, the EU Commission decided not to classify these compounds as requiring authorization for the time being (Regulation (EU) 2017/999, as published in the Official Journal of the European Union on 13 Jun. 2017).

Even though boric acid does not (yet) currently require authorization in the EU, it is desirable for sustainability reasons and on health and safety grounds to reduce the use of boric acid or to avoid it altogether.

Electrolytes containing zinc that are devoid of boric acid have been described in the literature in isolated cases:

In a publication by J. Heber (Galvanotechnik, 2014, Vol. 105, 2150-2156) it was proposed that boric acid be replaced by acetic acid. It was noted in this publication that the use of acetic acid limits use at high current densities. Moreover, replacing boric acid with acetic acid worsens the metal distribution on the substrate and reduces the cloud point (temperature at which the organic additives precipitate). Furthermore, a lower current yield is observed than with boric acid.

In EP 2 706 132 A1, the use of a weakly acidic zinc nickel electrolyte devoid of boric acid is described, which uses, in particular, a mixture of adipic acid, succinic acid, glutaric acid, sulfosuccinic acid and propionic acid to replace boric acid. In addition to these organic acids used as buffer substances, stronger complexing agents (such as DETA or EDA) for the nickel also have to be added to a zinc-nickel electrolyte so that nickel can be sufficiently incorporated into the zinc-nickel layer (typically up to 12 to 15% by weight). These complexing agents in turn require a complex wastewater treatment.

DE 2 251 103 A1 describes an acidic zinc electrolyte which is devoid of boric acid. In addition to zinc sulfate and zinc chloride, the electrolyte contains sodium succinate, nicotinic acid amide and large amounts of ammonium chloride. However, the use of ammonium chloride and other ammonium salts is questionable from an environmental perspective since neither ammonium nor ammonia should enter the wastewater system. One reason for this is that ammonia is a very good ligand for various heavy metals and increases the mobility thereof. The use of ammonium compounds is not desirable on account of the problematic wastewater treatment thereof.

U.S. Pat. No. 4,877,497 describes an acidic zinc electrolyte which does not contain boric acid. The electrolyte substantially contains zinc chloride, ammonium chloride and/or potassium chloride. Moreover, succinic acid, acetic acid, lactic acid, malonic acid, adipic acid, tartaric acid and citric acid or the salts thereof may be contained. The pH is preferably between 3 and 4. Ammonium chloride requires a complex process wastewater treatment, as already described above. The low pH values in these electrolytes result in a high solubility of the zinc anodes (anode solubility). The high anode solubility means that a steady increase in the zinc concentration in the electrolyte is observed when the electrolyte is not in operation.

As described above, it is desirable primarily for health reasons and in terms of occupational health and safety to reduce the use of boric acid in zinc electrolytes and in particular to avoid it altogether. Similarly, other CMR substances, i.e. substances that are carcinogenic, mutagenic or toxic to reproduction, should be avoided. Ammonium chloride should be avoided for environmental reasons owing to the wastewater problem. Thus, there is a need for a zinc electrolyte which requires neither boric acid nor ammonium chloride and which can nevertheless be economically viable and which is largely harmless from a health and environmental perspective. The electrolyte should facilitate the deposition of glossy zinc coatings even at high current densities.

SUMMARY OF THE INVENTION

The invention is based on the object of providing a zinc electrolyte for the electrodeposition of zinc coatings which does not require the addition of boric acid and ammonium compounds. In particular, the zinc electrolyte should be able to be operated at high current densities and it should use buffer substances that are largely harmless from an environmental and health perspective. The electrolyte should, in particular, facilitate the deposition of glossy zinc coatings having an advantageous optical impression. Moreover, the electrolyte should preferably also facilitate the electrodeposition of zinc coatings that can be annealed, which even after the annealing process have good mechanical properties and convey a good optical impression. Additional objects are to provide a method for producing the zinc electrolyte and a method for producing a component having a zinc coating which uses the electrolyte for electrodepositing zinc.

These objects are achieved by the zinc electrolyte, the method for the production thereof and the method for producing a component having a zinc coating as according to the independent claims.

In particular, the present invention relates to the following:

  • (1) An aqueous electrolyte devoid of boric acid and ammonium for the electrodeposition of zinc coatings, comprising:
  • (a) Zn2+ in a concentration of 15 to 70 g/L;
  • (b) Cl− in a concentration of 100 to 200 g/L;
  • (c) K+ and/or Na+ in a total concentration of 0.75 to 6.0 mol/L;
  • (d) acetate in a concentration of 5.0 to 45 g/L;
  • (e) glycine and/or alanine in a total concentration of 0.5 to 30 g/L; and
  • (f) water;
    • wherein the electrolyte has a pH of 4.5 to 6.5.
  • (2) The electrolyte according to (1), wherein the concentration of Zn2+ is 20 to 60 g/L.
  • (3) The electrolyte according to (1) or (2), wherein the concentration of Zn2+ is 25 to 50 g/L, preferably 35 g/L.
  • (4) The electrolyte according to any one of (1) to (3), wherein the concentration of Cl− is 120 to 190 g/L.
  • (5) The electrolyte according to any one of (1) to (4), wherein the concentration of Cl− is 130 to 180 g/L, preferably 160 g/L.
  • (6) The electrolyte according to any one of (1) to (5), wherein the concentration of K+ is 0.75 to 6.0 mol/L.
  • (7) The electrolyte according to any one of (1) to (6), wherein the concentration of K+ is 2.7 to 4.8 mol/L.
  • (8) The electrolyte according to any one of (1) to (7), wherein the concentration of K+ is 3.3 to 4.1 mol/L.
  • (9) The electrolyte according to any one of (1) to (8), optionally containing Na+ at 0 to 0.5 mol/L, preferably 0 to 0.2 mol/L.
  • (10) The electrolyte according to any one of (1) to (9), wherein Zn2+, K+ and Na+ account for at least 95% by weight, preferably at least 98% by weight, most preferably at least 99% by weight or 100% by weight, of all cations in the electrolyte.
  • (11) The electrolyte according to any one of (1) to (10), wherein Zn2+ and K+ account for at least 90% by weight, preferably at least 95% by weight, most preferably at least 99% by weight or 100% by weight, of all cations in the electrolyte.
  • (12) The electrolyte according to any one of (1) to (11), wherein the concentration of acetate is 7.5 to 30 g/L.
  • (13) The electrolyte according to any one of (1) to (12), wherein the concentration of acetate is 10 to 20 g/L, preferably 12 g/L.
  • (14) The electrolyte according to any one of (1) to (13), wherein the total concentration of glycine and/or alanine is in the range of 0.5 to 20 g/L.
  • (15) The electrolyte according to any one of (1) to (14), wherein the total concentration of glycine and/or alanine is in the range of 1.0 to 10 g/L.
  • (16) The electrolyte according to any one of (1) to (15), wherein the total concentration of glycine and/or alanine is in the range of 1.5 to 5 g/L, preferably 2.5 g/L.
  • (17) The electrolyte according to any one of (1) to (16), containing glycine and optionally 0 to 1.5 g/L, preferably 0 to 0.5 g/L, of alanine.
  • (18) The electrolyte according to any one of (1) to (17), containing (g) nicotinic acid in a concentration of 0.01 to 2.0 g/L.
  • (19) The electrolyte according to any one of (1) to (18), containing (g) nicotinic acid in a concentration of 0.5 to 1.0 g/L.
  • (20) The electrolyte according to any one of (1) to (19), containing (g) nicotinic acid in a concentration of 0.08 to 0.5 g/L, preferably 0.1 g/L.
  • (21) The electrolyte according to any one of (1) to (20), containing (h) ethoxylated thiodiglycol, with an average of at least 20 structural units derived from ethylene oxide, in a concentration of 0.3 to 10 g/L.
  • (22) The electrolyte according to any one of (1) to (21), containing (h) ethoxylated thiodiglycol, with an average of at least 20 structural units derived from ethylene oxide, in a concentration of 0.5 to 5 g/L.
  • (23) The electrolyte according to any one of (1) to (22), containing (h) ethoxylated thiodiglycol, with an average of at least 20 structural units derived from ethylene oxide, in a concentration of 1.0 to 3.5 g/L, preferably 2.1 g/L.
  • (24) The electrolyte according to any one of (1) to (23), wherein the pH is 4.5 to 6.0, preferably 4.8 to 5.3.
  • (25) A method for producing an electrolyte according to any one of (1) to (24), comprising the steps of:
    • A) forming an aqueous solution of
      • (a′) zinc chloride and/or zinc acetate;
      • (b′) potassium chloride and/or sodium chloride;
      • (c′) at least one of potassium acetate, sodium acetate and acetic acid;
      • (d′) at least one from the group consisting of glycine, a salt thereof, alanine and a salt thereof; and
      • (e′) optionally nicotinic acid or a salt thereof; and
      • (f′) optionally ethoxylated thiodiglycol, with an average of at least 20 structural units derived from ethylene oxide; and
    • B) optionally setting the pH to 4.5 to 6.5 by adding hydrochloric acid or by adding potassium hydroxide and/or sodium hydroxide, which can be added as solids or in the form of an aqueous solution.
  • (26) A method for producing a component having a zinc coating, comprising the electrodeposition of zinc on a metal component from an electrolyte according to any one of (1) to (24).
  • (27) The method according to (26), wherein the metal component comprises or consists of iron or an iron alloy.
  • (28) The method according to (26) or (27), wherein deposition occurs at a temperature of 20 to 50° C.
  • (29) The method according to any one of (26) to (28), wherein deposition occurs at a temperature of 25 to 40° C.
  • (30) The method according to any one of (26) to (29), wherein during deposition the current density is from 0.2 to 10 A/dm2.
  • (31) The method according to any one of (26) to (30), wherein during deposition the current density is from 0.5 to 8.0 A/dm2, preferably from 1.0 to 8.0 A/dm2.
  • (32) The method according to any one of (26) to (31), wherein during deposition the current density is from 0.5 to 6 A/dm2, preferably from 2.0 to 6 A/dm2.
  • (33) The method according to any one of (26) to (32), wherein after the electrodeposition the component having a zinc coating is subjected to a passivation treatment.
  • (34) The method according to any one of (26) to (32), wherein after the electrodeposition the component having a zinc coating is annealed, optionally before or after a passivation treatment.
  • (35) The method according to (34), wherein the temperature during annealing is 180 to 230° C., preferably 200 to 230° C.
  • (36) The method according to (34) or (36), wherein annealing is carried out for 2 to 24 hours.

DETAILED DESCRIPTION

(1) An aqueous electrolyte devoid of boric acid and ammonium is provided for the electrodeposition of zinc coatings (herein also referred to in short as “electrolyte” or “zinc electrolyte”). The electrolyte comprises:

  • (a) Zn2+ in a concentration of 15 to 70 g/L;
  • (b) Cl− in a concentration of 100 to 200 g/L;
  • (c) K+ and/or Na+ in a total concentration of 0.75 to 6.0 mol/L;
  • (d) acetate in a concentration of 5.0 to 45 g/L;
  • (e) glycine and/or alanine in a total concentration of 0.5 to 30 g/L; and
  • (f) water.

The electrolyte has a pH of 4.5 to 6.5. At this pH, acetate may be present in protonated and/or deprotonated form. Similarly, glycine and/or alanine may also be present in protonated and/or deprotonated form.

Surprisingly, it has been found that with the specific combination of acetate with glycine and/or alanine in the aforementioned concentrations, neither the addition of boric acid nor the addition of ammonium is necessary to provide a versatile zinc electrolyte for the electrodeposition of zinc coatings. Acetate and glycine or alanine serve, inter alia, as buffer substances, for keeping the pH in the electrolyte constant. Of course, from both a health perspective and an environmental perspective, acetate and the amino acids glycine and alanine are completely harmless and can be disposed of via the wastewater system without any problems. Moreover, acetate, glycine and alanine are inexpensive and there is an almost limitless supply thereof, and therefore they constitute a particularly advantageous replacement for the use of boric acid or related boron compounds as well as ammonium compounds such as ammonium chloride.

With the electrolyte according to the invention, zinc coatings can be deposited over a practicable current density range such as 0.2 to 8 A/dm2. Particularly at high current densities, i.e. in the range of 6 to 8 A/dm2, significant improvements can be achieved in the quality of the deposited zinc coatings in terms of the optical impression, i.e. homogenous gloss and reduced burn marks compared with zinc electrolytes without these substances. “Burn marks” are generally to be understood to be discolored, dark, amorphous or coarsely crystalline, mostly powdery regions, as a result of which the zinc coating may be unusable for many applications.

The electrolyte is suitable for coating metal components (substrates) with dense, homogenous zinc coatings that provide good, adhesive protection against corrosion, for example on iron or iron alloys. Furthermore, with the zinc electrolyte it was possible to produce zinc coatings that can be annealed. With the buffer substances acetate, glycine and/or alanine, it was possible to reduce hydrogen development during the electrodeposition, in particular at high current densities, as a result of which pH changes in the electrolyte and the precipitation of zinc hydroxide at the cathode were reduced or avoided.

A further advantage resulting from the combination of acetate and glycine is that a versatile process for electrodeposition is facilitated. For example, zinc salts and conducting salts (potassium chloride and/or sodium chloride) can be used in wide concentration ranges and the temperature can be varied over a wide range such as 15 to 50° C. The electrolyte is also suitable for conventional deposition devices such as drum or rack devices.

A further advantage of these buffer compounds is that they only weakly bind to most foreign metals such as iron. Iron can therefore easily be precipitated and removed in the form of iron(III) hydroxide (Fe(OH)3) in the presence of these buffer compounds. Furthermore, the buffer compounds do not increase anode solubility (generally zinc anodes) if the electrolyte is not in operation. This is advantageous since the concentrations in the electrolyte that is not in operation largely remain constant.

(2) The zinc salts naturally serve to provide the metal to be deposited with zinc. Since the zinc salts are dissolved in the electrolyte, it is often the case that it can no longer be determined which counterion was present in the zinc salt. For this reason, the essential Zn2+ concentration in the electrolyte is specified herein. In principle, all water-soluble zinc salts such as zinc sulfate, zinc methane sulfonate, zinc acetate and zinc chloride, for example, can be used. It goes without saying that all types of salts, such as anhydrous salts or salts with water of crystallization, can be used. Preferably, zinc acetate and/or zinc chloride are used, since the anions acetate and chloride are advantageous for the buffer effect and improving the conductivity of the electrolyte and do not adversely affect the electrodeposition of zinc. Particularly preferably, zinc chloride is used, on account of its better conductivity, its good solubility and for reasons of economy.

As already mentioned above, the zinc concentration can be varied over a wide range and the electrolyte can therefore be specifically adapted to different requirements. The concentration of Zn2+ in the electrolyte is 15 to 70 g/L. The concentration of Zn2+ is, in particular, 20 to 60 g/L, preferably 25 to 50 g/L, more preferably 30 to 40 g/L and most preferably 35 g/L. If the zinc concentration is too low, the deposition speed reduces. When zinc concentrations are too high, solubility problems can arise. With the zinc concentrations according to the invention, the deposition speed, the stability of the zinc electrolyte and the quality of the zinc coatings are all particularly good.

(3) In addition to zinc, which, of course, is required to form the zinc coating, the electrolyte also contains one or more conducting salts. Conducting salts increase the conductivity of the electrolyte, which is important in order to be able to coat with zinc at high current densities and at a good deposition rate. In particular, potassium chloride and/or sodium chloride are used as conducting salts. Preferably, potassium chloride is used, since it has a higher conductivity than sodium chloride.

The conducting salt is of course present in the electrolyte in a dissolved state, which is why the concentrations of the respective ions are specified herein. The concentration of Cl− in the electrolyte is 100 to 200 g/L. The concentration of Cl− can, in particular, be 120 to 190 g/L, preferably 130 to 180 g/L and more preferably 160 g/L. With these chloride concentrations and the corresponding conducting salt concentrations, the electrolyte can be operated well.

(4) The cations K+ and/or Na+ can enter the electrolyte from various reagents. On the one hand, K+ and/or Na+ originate from the conducting salt described above. Furthermore, these ions can also be introduced into the electrolyte as a component of other salts. For instance, acetate, glycine and/or alanine and other weak acids, also in the form of the corresponding potassium salt and/or sodium salt, can be used for forming the electrolyte.

Potassium and sodium are, of course, harmless both from a health perspective and from an environmental perspective and do not interfere with the operation of the electrolyte. The electrolyte may contain further cations in addition to Zn2+ and K+ and/or Na+, but it does not have to. Preferably, Zn2+, K+ and Na+ account for at least 95% by weight, more preferably at least 98% by weight, most preferably at least 99% by weight or 100% by weight, of all cations in the electrolyte. Due to the advantages of potassium chloride as described above, the electrolyte preferably contains Zn2+ and Kt Zn2+ and K+ can account for at least 90% by weight, more preferably at least 95% by weight, most preferably at least 99% by weight or 100% by weight, of all cations in the electrolyte.

The total concentration of K+ and/or Na+ in the electrolyte is in the range of 0.75 to 6.0 mol/L. Due to the difference in mass between K+ and Na+, the molar concentration is specified herein. Preferably, the electrolyte contains K+ and optionally Na+, wherein the concentration of K+ is 0.75 to 6.0 mol/L, in particular 2.7 to 4.8 mol/L, preferably 3.3 to 4.1 mol/L. Preferably, the concentration of Na+ is 0.5 mol/L or less and more preferably it is 0.2 mol/L or less, which includes 0 mol/L (i.e. 0 to 0.5 mol/L and 0 to 0.2 mol/L respectively).

(5) As already mentioned above, the electrolyte contains acetate as a buffer substance. With the pH in the electrolyte of 4.5 to 6.5, the acetate (AcO−) is naturally in equilibrium with the conjugate acid (AcOH=acetic acid). AcO− and AcOH form a conjugate acid base pair, or the deprotonated and protonated form of the acetate. With low pH values, the proportion of conjugate acids (protonated form) is higher than with higher pH values. At least one of potassium acetate, sodium acetate and acetic acid, preferably potassium acetate, can be used as the acetate reagent, i.e. as the source for the acetate in the electrolyte.

The specification of the concentration of acetate refers to the mass of the acetate ion (AcO−), i.e. the mass of the deprotonated form, irrespective of the size of the equilibrium proportion of the conjugate acid (AcOH), i.e. the protonated form. It can be calculated from the added amount of the acetate reagent. In the electrolyte, the concentration of acetate is 5.0 to 45 g/L. In particular, it can be 7.5 to 30 g/L, preferably 10 to 20 g/L, most preferably 12 g/L.

As the buffer substance, the acetate serves firstly to keep the pH of the electrolyte constant. The acetate also helps to create glossy zinc coatings that can be annealed. Acetate proved to be essential for the operation of the electrolyte, particularly at high current densities. If the acetate concentration (which includes the conjugate acid, as described above) is too low, the desired effects are only achieved to an unsatisfactory degree. With higher concentrations, solubility problems can arise. The cost-effectiveness of the electrolyte and the quality of the zinc coatings are best in the preferred ranges.

(6) As already stated above, in addition to acetate, the electrolyte also contains glycine and/or alanine, which also serve as buffer substances. Glycine and/or a water-soluble salt thereof can be used as the source of the glycine. Alanine and/or a water-soluble salt thereof can be used as the source of the alanine. Preferably, at least one of glycine, potassium salt of glycine (potassium glycinate), sodium salt of glycine (sodium glycinate), alanine, potassium salt of alanine (potassium alaninate) and sodium salt of alanine (sodium alaninate) is used. Particularly preferably, just glycine and/or alanine is used. Most preferably, glycine is used.

Glycine has two pKs-values, such that depending on the pH of 4.5 to 6.5, glycine can exist in equilibrium in several species in the electrolyte. Glycine can in principle be fully protonated (H3N—CH2—CO2H+, protonated form), neutral to the outside (H2N—CH2—CO2H or H3N+—CH2—CO2−) and fully deprotonated (H2N—CH2—CO2−, deprotonated form) in the electrolyte. The species each form conjugate acid base pairs. The same applies to alanine, which can also be fully protonated (H3N—CHCH3—CO2H+, protonated form), neutral to the outside (H2N—CHCH3—CO2H or H3N+—CHCH3—CO2−) and fully deprotonated (H2N—CHCH3—CO2−, deprotonated form) in the electrolyte.

The specification that acetate, glycine and/or alanine can also be present in protonated and/or deprotonated form shows that acetate, glycine and/or alanine can be present in all protonated, deprotonated or outwardly neutral species that occur at the pH values of 4.5 to 6.5. At the corresponding pH, the species form in equilibrium in the electrolyte on their own, as is typical of weak acids and as is known to persons skilled in the art.

The concentration of the glycine and/or alanine is calculated in relation to the mass of glycine (H2N—CH2—CO2H) and/or alanine (H2N—CHCH3—CO2H) as such, irrespective of which of the species is formed or is present in equilibrium. In the electrolyte, the total concentration of glycine and/or alanine is 0.5 to 30 g/L. The concentration of glycine and/or alanine can, in particular, be 0.5 to 20 g/L, preferably 1.0 to 10 g/L, more preferably 1.5 to 5 g/L and most preferably 2.5 g/L. If the electrolyte contains too little glycine and/or alanine, the desired effects are only achieved to an unsatisfactory degree. If the amounts are too large, solubility problems can arise.

Preferably, the electrolyte contains glycine and optionally alanine. In this embodiment, the electrolyte preferably contains glycine in a concentration of 0.5 to 20 g/L, more preferably 1.0 to 10 g/L, even more preferably 1.5 to 5 g/L and most preferably 2.5 g/L, and alanine in a concentration of less than 1.5 g/L and more preferably 0.5 g/L or less, which includes 0 g/L (i.e. 0 to 1.5 g/L and 0 to 0.5 g/L respectively).

As already mentioned above, the pH of the electrolyte is 4.5 to 6.5. Preferably, it can be 4.5 to 6.0 and more preferably 4.8 to 5.3. At these pH values, good deposition rates and low anode solubilities are observed. The pH of the electrolyte can result from the composition of the components alone. It is also possible to set the pH, as will be described below.

According to a further embodiment, the electrolyte also contains one or more further buffer substances. The further buffer substance is, in particular, at least one buffer substance selected from the group consisting of succinic acid, adipic acid, malic acid, 2-(N-morpholino)ethanesulfonic acid, tris(hydroxymethyl)aminomethane, triethanolamine, taurine, β-alanine, glutamic acid, glycylglycine, threonine, bicine, tricine, ascorbic acid, citric acid and salts thereof. In particular, the salts are potassium and/or sodium salts.

The further buffer substance is therefore optional and can be used in a concentration of 0.1 to 40 g/L.

(7) It goes without saying that the various embodiments of the components present in the electrolyte can be combined as desired. According to a preferred variant, the electrolyte contains:

  • (a) Zn2+ in a concentration of 20 to 60 g/L;
  • (b) Cl− in a concentration of 120 to 190 g/L;
  • (c) K+ in a concentration of 2.7 to 4.8 mol/L and 0 to 0.5 mol/L Na+;
  • (d) acetate in a concentration of 7.5 to 30 g/L; and
  • (e) glycine and/or alanine in a total concentration of 0.5 to 20 g/L.

According to a preferred development of this variant, the electrolyte contains:

  • (a) Zn2+ in a concentration of 20 to 60 g/L;
  • (b) Cl− in a concentration of 120 to 190 g/L;
  • (c) K+ in a concentration of 2.7 to 4.8 mol/L and 0 to 0.5 mol/L Na+;
  • (d) acetate in a concentration of 10 to 20 g/L; and
  • (e) glycine and/or alanine in a concentration of 1.0 to 10 g/L.

According to a further preferred development of this variant, the electrolyte contains:

  • (a) Zn2+ in a concentration of 20 to 60 g/L;
  • (b) Cl− in a concentration of 120 to 190 g/L;
  • (c) K+ in a concentration of 2.7 to 4.8 mol/L and 0 to 0.2 mol/L Na+;
  • (d) acetate in a concentration of 10 to 20 g/L; and
  • (g) glycine in a concentration of 1.5 to 5 g/L and 0 to 0.5 g/L alanine.

(8) According to a further embodiment, the electrolyte additionally contains (g) nicotinic acid and/or (h) ethoxylated thiodiglycol. Particularly preferably, the electrolyte contains nicotinic acid and ethoxylated thiodiglycol.

Nicotinic acid as such or water-soluble salts thereof, in particular the potassium and/or sodium salt thereof, can be used as the source of the nicotinic acid. Preferably, just nicotinic acid is used.

Since nicotinic acid is a weak acid, at the pH of 4.5 to 6.5 it can exist in the electrolyte in an equilibrium of protonated and deprotonated forms, i.e. as a conjugate acid base pair. Thus, analogously to acetate and glycine and/or alanine, nicotinic acid can be present in the form of the conjugate base and/or the conjugate acid.

The concentration is calculated in relation to the mass of the nicotinic acid (C6H5NO2) as such. The concentration of nicotinic acid in the electrolyte is, in particular, 0.01 to 2.0 g/L, preferably 0.05 to 1.0 g/L, more preferably 0.08 to 0.5 g/L, most preferably 0.1 g/L.

Quite surprisingly, it has been found that by adding small amounts of nicotinic acid, the quality of the zinc coatings is significantly improved further still, even at high current densities such as 6 to 8 A/dm2. In addition to the optical impression (gloss) of the deposited zinc coatings, the ability thereof to anneal, in particular, is also further improved. Even at high current densities, it is possible to produce zinc coatings which can be annealed and which after annealing have no or very few defects such as bubbles and create a particularly advantageous optical impression, i.e. a homogenous gloss without cloudiness or fog. If the concentration of nicotinic acid is too high, the formation of fog or cloudiness in the zinc coating can increase again.

(9) Ethoxylated thiodiglycol is an oligomeric or polymeric compound obtained from the addition reaction of thiodiglycol (HO—CH2—CH2—S—CH2—CH2—OH) with ethylene oxide (oxirane, C2H4O). A suitable ethoxylated thiodiglycol has an average of at least 20 structural units derived from ethylene oxide. Preferably, it has an average of 20 to 100 structural units derived from ethylene oxide. With fewer structural units derived from ethylene oxide, the advantageous effects could not be observed to the same degree.

Ethoxylated thiodiglycol can be added to the electrolyte in the form of an aqueous solution. Ethoxylated thiodiglycol is commercially available as a 70% aqueous solution, for example, such as CHE ED 7127 70% (from the company Erbslöh) or Aduxol TDG-027 70% (from Schärer & Schläpfer).

The concentration of ethoxylated thiodiglycol with an average of at least 20 structural units derived from ethylene oxide in the electrolyte can be 0.3 to 10 g/L, preferably 0.5 to 5 g/L, more preferably 1.0 to 3.5 g/L and most preferably 2.1 g/L.

Quite surprisingly, it has been found that also with the ethoxylated thiodiglycol described above the quality of the zinc coatings is significantly improved further still, even at high current densities such as 6 to 8 A/dm2. In addition to the optical impression (gloss) of the deposited zinc coatings, the ability thereof to anneal, in particular, is also further improved. Even at high current densities, it is possible to produce zinc coatings which can be annealed and which after annealing have no or very few defects such as bubbles and have an advantageous optical impression. With concentrations that are too high, the optical impression of the zinc coatings worsens again.

(10) According to a further variant of the electrolyte, it contains

  • (g) nicotinic acid in a concentration of 0.05 to 1.0 g/L, and/or
  • (h) ethoxylated thiodiglycol, with an average of at least 20 structural units derived from ethylene oxide, in a concentration of 0.5 to 5 g/L.

The advantageous effects of nicotinic acid and ethoxylated thiodiglycol are very similar, as already stated above, even though the substances are structured quite differently. It has also surprisingly been found that by using a combination of nicotinic acid and ethoxylated thiodiglycol, the advantageous properties of the zinc coatings can be improved further still. Preferably, the electrolyte therefore contains nicotinic acid and ethoxylated thiodiglycol, in particular in the concentrations specified above.

In addition to the aforementioned components, the electrolyte can also contain one or more common additives for zinc electrolytes in appropriate quantities, as is already known to persons skilled in the art. The electrolyte can contain, for example, at least one additive from the group consisting of surfactants (e.g. alkoxylated (3-naphthol), basic brighteners (e.g. sodium benzoate), solubilizers or hydrotropes (e.g. sodium cumene sulfonate) and brighteners (e.g. benzalacetone and/or o-chlorobenzaldehyde). Surfactants serve to reduce the surface tension of the electrolyte and ensure a good wetting of the surface to be coated. Solubilizers and hydrotropes ensure a sufficient solubility of organic substances. Basic brighteners and brighteners are used in combination to further control the degree of gloss and brilliance of the zinc coating.

Suitable additives are commercially available. For example, SLOTANIT BSF 1668 (an additive for zinc electrolytes which contains surfactants and basic brighteners) and SLOTANIT BSF 1662 (an additive for zinc electrolytes which contains solubilizers and brighteners) can be obtained from Schlötter. These additives can be used in the usual quantities according to the corresponding instructions for use of the manufacturer.

(11) As a further aspect of the invention, a method for producing the electrolyte according to any one of the embodiments described above is provided. The method comprises the steps of

  • A) forming an aqueous solution of
    • (a′) zinc chloride and/or zinc acetate;
    • (b′) potassium chloride and/or sodium chloride;
    • (c′) at least one of potassium acetate, sodium acetate and acetic acid;
    • (d′) at least one from the group consisting of glycine, a salt thereof, alanine and a salt thereof; and
    • (e′) optionally nicotinic acid or a salt thereof; and
    • (f′) optionally ethoxylated thiodiglycol, with an average of at least 20 structural units derived from ethylene oxide; and
  • B) optionally setting the pH to 4.5 to 6.5 by adding hydrochloric acid or by adding potassium hydroxide and/or sodium hydroxide, which can be added as solids or in the form of an aqueous solution.

Thus, in step A), an aqueous solution of the components is formed. The components can be added in the specified order or in any other order. Further buffer substances and additives, as described above, can also be contained in the aqueous solution. In particular, (g′) common surfactants, basic brighteners, solubilizers and brighteners can be added to the aqueous solution, such as the additives SLOTANIT BSF 1668 and SLOTANIT BSF 1662 from Schlötter. With regard to the components and the respective embodiments of the electrolyte, reference is made to the statements made above.

In order to accelerate step A), in particular steps (a′) to (c′), the mixture can be mixed, in particular stirred. Furthermore, heating, for example to 60° C., can also be used for acceleration. Once the aqueous solution has formed, it can be cooled down again to a lower temperature.

The pH of the electrolyte of 4.5 to 6.5 can result from the composition of the components alone. It is also possible to set the pH in an optional step B). This can be done by adding hydrochloric acid, to reduce the pH. The hydrochloric acid can, for example, be diluted (1 N) or be made more concentrated. Concentrated hydrochloric acid (ca. 12 N) can also be used. 5 or 6 N (semi-concentrated) hydrochloric acid is particularly suitable since it can be handled safely and does not lead to the electrolyte being significantly diluted.

By adding potassium hydroxide and/or sodium hydroxide, the pH can be reduced. These bases can be added as solids or in the form of an aqueous solution. For example, 50% potassium hydroxide solution (KOH) can be used.

Hydrochloric acid, potassium hydroxide and sodium hydroxide are particularly suitable since they do not introduce additional ions into the electrolyte.

(12) As a further aspect of the invention, a method for producing a component having a zinc coating is provided. The method comprises the electrodeposition of zinc on a metal component from an electrolyte according to any one of the embodiments described above.

(13) In principle, any metal substrate that is suitable for the electrodeposition of zinc can be used as the component. Preferably, the component comprises iron or an iron alloy or it consists wholly thereof. Accessory parts for the automotive industry, for example, can be provided with zinc coatings. The method can also be particularly effectively used, for example, for the electrodeposition of zinc coatings on components made of cast materials such as wrought iron or gray cast iron.

In general, the component is pre-treated prior to the electrodeposition, in order to clean it, in particular to degrease it. Appropriate measures are known to persons skilled in the art and suitable reagents are commercially available. For example, the component, such as a steel sheet, can first be 1) degreased in a hot cleaning solution, 2) pickled in acid (e.g. semi-concentrated hydrochloric acid), 3) electrolytically degreased and then 4) pickled with diluted hydrochloric acid. After each of steps 1 to 4), it is rinsed with water.

(14) Customary conditions can be used for the electrodeposition. As stated above, with the electrolyte according to the invention it is possible to vary the temperature over a wide range. The temperature for the electrodeposition can, in particular, be 20 to 50° C., preferably 25 to 40° C.

Moreover, practicable current densities, such as 0.2 to 10 A/dm2, in particular 0.5 to 8.0 A/dm2, preferably 1.0 to 8.0 A/dm2 or 0.5 to 6 A/dm2, more preferably 2.0 to 6 A/dm2, can be applied. This allows the method to be operated with good economic efficiency.

If desired or required, the pH can be adjusted during the procedure. The procedure can be analogous to step B) described above. Iron can optionally be precipitated and removed as iron(III) hydroxide.

All conventional devices, such as drum devices or rack devices, are suitable for the procedure. A plurality of cathodes and/or anodes can be used. Normally, the electrolyte is mixed during the electrodeposition. For this, one or more of stirring devices, circulation pumps (also in combination with Venturi nozzles) or air injection systems can be used, for example.

For the electrodeposition of zinc, the metal component is connected as the cathode and divalent zinc ions are reduced to metallic zinc on the surface thereof, as a result of which the zinc coating is formed. Normally, zinc is used as the anode.

(15) According to a further embodiment of the method, after the electrodeposition the component having a zinc coating is subjected to a passivation treatment. For this, the component having a zinc coating is treated with an appropriate passivation agent, as is known to persons skilled in the art. For this, the coated components can, for example, be treated with a thin-film or thick-film passivation. Such passivation processes typically contain chromium(III) and cobalt ions as corrosion inhibitors, as well as further film-forming substances (such as fluorides, sulfates and nitrates). One commercially available passivation solution for zinc coatings is, for example, the thin-film passivation SLOTOPAS Z 20 Blau, or the passivation concentrate SLOTOPAS Z 21 Blau, which are available from Schlötter. The corresponding instructions for use of the manufacturer can be followed.

The passivation treatment takes place after the electrodeposition. Normally, the components having a zinc coating are firstly rinsed with water. If the components having a zinc coating are annealed, the passivation treatment can in principle be carried out before or after annealing.

(16) According to a further embodiment of the method, after the electrodeposition the component having a zinc coating is annealed. Annealing drives out the hydrogen absorbed during the electroplating process, in order to counteract the risk of the component subsequently becoming embrittled by the hydrogen. Particularly in the case of high-strength steel components, there is an increased risk of hydrogen-induced brittle fracture due to the material.

For annealing, it is common to heat to a temperature of 180 to 230° C., preferably 200 to 230° C. Normally, annealing is carried out over a long period of time, for example 2 to 24 hours.

Prior to annealing, the component is usually rinsed with water and dried.

In principle, the annealing is optional and can take place irrespective of whether or not a passivation treatment is carried out. If a passivation treatment is carried out, this can happen both before and after the annealing.

To clarify the invention, reference is made to the drawings and to the examples below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic test set-up for the electrodeposition of zinc on an angular steel sheet.

FIG. 2 shows a schematic test set-up for the electrodeposition of zinc on a straight steel sheet.

 EXAMPLES Production of Electrolytes E1 to E6 Procedure:

The electrolytes were prepared at room temperature by adding deionized water to the other components and stirring. The dissolution of the solids was accelerated by heating them to 60° C. Once the solution had formed, it was cooled to 25° C.

Where necessary, the pH was set to 5.2 by adding 5 N hydrochloric acid or 50% potassium hydroxide solution (756 g/L KOH).

Reagents Used:

Technical grade potassium chloride was used.

Zinc chloride HP from Schlötter was used as the zinc chloride.

The additives SLOTANIT BSF 1668 (basic additive) and SLOTANIT BSF 1662 (brightener additive), which are both available from Schlötter, were used.

“Purest” quality potassium acetate was used.

“Purest” quality glycine was used.

Technical grade nicotinic acid was used.

A 70% solution, namely CHE ED 7127 70%, which is available from the company Erbsloh, or Aduxol TDG-027 70%, which is available from Schärer & Schläpfer, was used as the ethoxylated thiodiglycol.

Components for Electrolyte E1 (Base Electrolyte):

73 g/L ZnCl2 (corresponds to 35 g/L Zn2+ and 38 g/L Cl−) 260 g/L KCl (corresponds to 136 g/L K+ and 124 g/L Cl−) 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANIT BSF 1662

Components for Electrolyte E2:

73 g/L ZnCl2 260 g/L KCl 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANIT BSF 1662 20 g/L Potassium acetate (corresponds to 8 g/L K+ and 12 g/L AcO−)

Components for Electrolyte E3:

73 g/L ZnCl2 260 g/L KCl 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANIT BSF 1662 20 g/L Potassium acetate 2.5 g/L Glycine

Components for Electrolyte E4:

73 g/L ZnCl2 260 g/L KCl 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANIT BSF 1662 20 g/L Potassium acetate 2.5 g/L Glycine 2.11 g/L CHE ED 7127 70% or Aduxol TDG-027 70%

Components for Electrolyte E5:

73 g/L ZnCl2 260 g/L KCl 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANIT BSF 1662 20 g/L Potassium acetate 2.5 g/L Glycine 0.113 g/L Nicotinic acid

Components for Electrolyte E6:

73 g/L ZnCl2 260 g/L KCl 35.0 ml/L SLOTANIT BSF 1668 0.3 ml/L SLOTANIT BSF 1662 20 g/L Potassium acetate 2.5 g/L Glycine 2.11 g/L CHE ED 7127 70% or Aduxol TDG-027 70% 0.113 g/L Nicotinic acid

Pre-Treatment of the Steel Sheets

  • 1) Decoction degreasing with SLOTOCLEAN 160 (from Schlötter) for 15 minutes at 65° C. and subsequent rinsing with water.
  • 2) Hydrochloric acid pickling with 19% hydrochloric acid and 40 mL/L pickling degreaser SLOTOCLEAN BEF 30 (from Schlötter) for 7 minutes at 25° C. and subsequent rinsing with water.
  • 3) Electrolytic degreasing by means of anodic degreasing with SLOTOCLEAN DCG (from Schlötter) at a cathodic current density of 3 to 6 A/dm2 for 2 minutes at 25° C. and subsequent rinsing with water.
  • 4) Pickling with diluted hydrochloric acid (1:10 dilution of concentrated hydrochloric acid) for 1 minute at 25° C. and subsequent rinsing with water.

Electrodeposition Deposition Parameters in the Electroplated Zinc Electrolyte:

Electrolyte volumes: 3.0 L.

pH of the electrolyte: 5.2.

Electrolyte temperature: 25° C.

Stirring speed set on the magnetic stirrer: 300 rpm.

General Procedure:

The test set-up for electrodeposition with an angular sheet of steel is illustrated in FIG. 1. The test set-up for electrodeposition with a straight sheet of steel is illustrated in FIG. 2.

The electrolyte (1) is present in a beaker (10) and was kept permanently in motion by means of a magnetic stirrer (15) and a magnetic stirrer bar (length: 40 mm, diameter: 8 mm) (16). A Schott Duran beaker (3.0 L, low form) was used as the beaker. The temperature was controlled and held constant by means of a contact thermometer (17) which is connected to the heating relay of the magnetic stirrer. In the Examples, a magnetic stirrer with a heatable plate, the IKA RET basic model from the company IKA, was used. A stabilizer (rectifier) (20) from the company Gossen Metrawatt, the SLP 240-40 model, served as the power source. A current strength of 3.0 A and a voltage of 2.5 V are illustrated by way of example.

Two anodes (2, 2′) were used for the electrodeposition. High-grade zinc anodes (99.99% Zn) in accordance with DIN EN 1179 were used as the anode material (length: 10 cm, width: 5 cm, thickness: 1 cm). The anodes were immersed 9 cm deep in the electrolyte.

Before the cathode sheets were introduced, they were subjected to the pre-treatment for steel sheets described above.

The cathode sheet (3) was arranged in the middle of the two anodes in the beaker. The distance to the anodes was 6 cm both from the front side and the rear side of the cathode sheet. The cathode sheet is immersed so deeply in the electrolyte that the total immersed area (front and rear sides) is one square decimeter.

Where necessary, the pH was set to 5.2 by adding 5 N hydrochloric acid or 50% potassium hydroxide solution (756 g/L KOH) before the start of the deposition.

In each case, the electrodeposition was performed from the respective electrolyte at the current density specified in the Examples.

Passivation:

After the electrodeposition of zinc, the coated sheets were thoroughly rinsed with water and subsequently brightened in diluted hydrochloric acid (15 mL 25% HCl in 1 L water). Following another rinse with water, passivation was carried out in a thin-film passivation SLOTOPAS Z 20 blau (prepared with the passivation concentrate SLOTOPAS Z 21 BLAU from Schlötter, 35 mL/L) at 25° C., pH=1.9, and 60 seconds immersion time. The coated sheets were subsequently rinsed with water and dried at 80° C. for 15 minutes in a convection oven. Prior to assessment or further treatment of the sheets, they were cooled to room temperature.

Test A): Burn Marks on the Angular Sheet Examples 1 to 4, Comparative Examples 1 and 2

Angular sheets having type 2 geometries according to DIN 50957-2 were used. The material of the angular sheets was cold-rolled steel according to DIN EN 10139/10140 (quality: DC03 LC MA RL). The angular sheets were pre-treated in accordance with the procedure described above and then coated. The electrolytes E1 to E6 as specified in Table 1 were used for Comparative Examples 1 and 2 (CE1 and CE2) as well as for Examples 1 to 4. The current density was varied and was 0.25, 1.0, 2.0, 4.0, 6.0 or 8.0 A/dm2. The deposited layer thicknesses were 10 μm. The sheets were then passivated as described above.

The appearance of the deposited zinc coating in the edge region of the angular sheets and the number of burn marks (discolored, dark, amorphous or coarsely crystalline, mostly powdery regions in the coating) which can be caused by increased local current densities were assessed by a visual inspection.

Key:

Δ no burn marks
∘ mild burn marks
● moderate burn marks
▴ major burn marks

The results are summarized in Table 1.

TABLE 1 Burn Marks on the Angular Sheet Current Density in A/dm2 Example/Electrolyte 0.25 1.0 2.0 4.0 6.0 8.0 CE1 E1 Δ Δ Δ ◯ ● ▴ CE2 E2 Δ Δ Δ ◯ ◯ ● 1 E3 Δ Δ Δ ◯ ◯ ◯ 2 E4 Δ Δ Δ Δ Δ ◯ 3 E5 Δ Δ Δ Δ Δ ◯ 4 E6 Δ Δ Δ Δ Δ Δ

At low and average current densities of up to 2 A/dm2, a usable zinc coating was deposited from all of the electrolytes E1 to E6. At high current densities of 4 or 8 A/dm2, considerable differences between the individual electrolytes were shown.

In Comparative Example 1 (CE1), electrolyte E1, which is devoid of boric acid and ammonium, resulted without the buffer substances acetate and glycine in unfit zinc coatings which have massive burn marks on the edge regions thereof. The addition of acetate allowed initial improvements to be achieved in Comparative Example 2 (CE2) with electrolyte E2, but too many burn marks were still observed at 8 A/dm2.

In contrast, high-quality zinc coatings were obtained in Example 1 with electrolyte E3 even at high current densities, which coatings exhibited only minor burn marks even at 8 A/dm2. Thus, electrolyte E3 according to the invention constitutes a veritable boric acid-free and ammonium-free substitute for conventional electrolytes containing boric acid.

In Examples 2 to 4, a further considerable improvement was then achieved with the additives ethoxylated thiodiglycol and nicotinic acid.

Test B): Annealability Examples 5 to 8, Comparative Examples 3 and 4

Straight steel sheets with the dimensions 0.3 mm thickness, 50 mm width and 130 mm length were used. The material of the straight steel sheets was cold-rolled steel according to DIN EN 10139/10140 (quality: DC03 LC MA RL). The steel sheets were pre-treated in accordance with the procedure described above and then coated. The electrolytes E1 to E6 were used for Comparative Examples 3 and 4 (CE3 and CE4) as well as for Examples 5 to 8 (see Tables 2 and 3). The current density was varied and was 0.25, 1.0, 2.0, 4.0, 6.0 or 8.0 A/dm2. The deposited layer thicknesses were 10 μm.

After the electrodeposition of zinc, the sheets were then passivated, as described above, stored for 48 hours at room temperature and subsequently annealed for 24 hours at 210° C.

Then, the bubble formation caused by annealing was assessed at room temperature.

Key:

Δ no bubble formation
∘ mild bubble formation
● moderate bubble formation
▴ major bubble formation

The results are summarized in Table 2.

TABLE 2 Bubble Formation After Annealing Current Density in A/dm2 Example/Electrolyte 0.25 1.0 2.0 4.0 6.0 8.0 CE3 E1 Δ Δ Δ ◯ ▴ ▴ CE4 E2 Δ Δ Δ ◯ ▴ ▴ 5 E3 Δ Δ Δ ● ▴ ▴ 6 E4 Δ Δ Δ Δ ◯ ▴ 7 E5 Δ Δ Δ Δ ◯ ◯ 8 E6 Δ Δ Δ Δ Δ Δ

In Example 5, zinc coatings with very good annealability which did not exhibit any bubble formation at all were produced with electrolyte E3, which contains potassium acetate and glycine, at conventional current densities between 0.25 and 2 A/dm2.

A further improvement in the annealability of zinc coatings deposited at higher current densities was observed for electrolytes E4 and E5 in Examples 6 and 7, which contained ethoxylated thiodiglycol (E4) and nicotinic acid (E5) respectively. By combining both additives in electrolyte E6 a completely bubble-free annealed zinc coating could still be achieved even at a current density of 8 A/dm2 (see Example 8).

Moreover, the optical impression (gloss) was assessed at room temperature.

Key:

Δ no milky appearance
∘ slight milky appearance
● moderate milky appearance
▴ strong milky appearance

The results are summarized in Table 3.

TABLE 3 Gloss After Annealing Current Density in A/dm2 Example/Electrolyte 0.25 1.0 2.0 4.0 6.0 8.0 CE3 E1 ▴ ▴ ▴ ▴ ▴ ▴ CE4 E2 ▴ ▴ ▴ ▴ ▴ ▴ 5 E3 ▴ ● ● ● ● ● 6 E4 Δ Δ Δ ◯ ◯ ◯ 7 E5 Δ Δ Δ Δ Δ Δ 8 E6 Δ Δ Δ Δ Δ Δ

In Comparative Examples 3 and 4, electrolytes 1 and 2, which are devoid of boric acid and ammonium, resulted in a very milky appearance with numerous streaks. With electrolyte E3, which contains acetate and glycine, the gloss was improved and the milky appearance was slightly reduced.

By adding ethoxylated thiodiglycol to electrolyte E4 and in particular by adding nicotinic acid to electrolytes 5 and 6, the gloss of the annealed zinc coatings was improved further still (see Examples 6 to 8). Regardless of the current density used, the zinc coatings only had a slight milky appearance, or no milkiness, cloudiness or streaks at all.

Test C): Thermal Shock Test Examples 9 to 12, Comparative Examples 5 and 6

Straight steel sheets with the dimensions 0.3 mm thickness, 50 mm width and 130 mm length were used. The material of the straight steel sheets was cold-rolled steel according to DIN EN 10139/10140 (quality: DC03 LC MA RL). These were pre-treated in accordance with the procedure described above and then coated at a current density of 3.0 A/dm2. The electrolytes E1 to E6 were used for Comparative Examples 5 and 6 (CE5 and CE6) as well as for Examples 9 to 12 (see Table 4). The deposited layer thicknesses were 10 μm.

After the electrodeposition of zinc, the sheets were passivated, as described above, stored for 48 hours at room temperature and subsequently stored for 30 minutes at 220° C. and immediately placed in filtered tap water annealed to 20° C.

First, the tap water was examined and assessed for any flaking.

After the thermal shock test the sheets were dried for 15 minutes at 80° C. in a convection oven and after cooling to room temperature the adhesiveness was visually inspected again. For this, 4 cm long, 19 mm wide adhesive strips (Tesafilm® Crystal Clear from Tesa SE) were stuck on the sheets and removed again after 60 seconds. The zinc coatings were then inspected for damage.

The two adhesion tests were deemed to have been passed if no flakes or other particles, chips, bubbles or damage were observed.

Key:

Δ Passed (no flaking and good adhesion)

The results are summarized in Table 4.

TABLE 4 Adhesion After Thermal Shock Test Example/ Current Density Electrolyte 3.0 A/dm2 CE5 E1 Δ CE6 E2 Δ  9 E3 Δ 10 E4 Δ 11 E5 Δ 12 E6 Δ

No flaking was observed with any of the electrolytes, and a good adhesion of the zinc coating on the steel sheet was exhibited. Thus, it is possible to produce adhesion-resistant zinc coatings with the electrolyte devoid of boric acid and ammonium.

Claims

1. An aqueous electrolyte devoid of boric acid and ammonium for the electrodeposition of zinc coatings, comprising:

(a) Zn2+ in a concentration of 15 to 70 g/L;
(b) Cl− in a concentration of 100 to 200 g/L;
(c) K+ and/or Na+ in a total concentration of 0.75 to 6.0 mol/L;
(d) acetate in a concentration of 5.0 to 45 g/L;
(e) glycine and/or alanine in a total concentration of 0.5 to 30 g/L; and
(f) water;
wherein the electrolyte has a pH of 4.5 to 6.5.

2. The electrolyte according to claim 1, wherein the concentration of Zn2+ in the electrolyte is 20 to 60 g/L, preferably 25 to 50 g/L, more preferably 30 to 40 g/L and most preferably 35 g/L.

3. The electrolyte according to claim 1, wherein the concentration of Cl− in the electrolyte is 120 to 190 g/L, preferably 130 to 180 g/L and more preferably 160 g/L.

4. The electrolyte according to claim 1, wherein the electrolyte contains K+ and the concentration of K+ is 0.75 to 6.0 mol/L, preferably 2.7 to 4.8 mol/L and more preferably 3.3 to 4.1 mol/L.

5. The electrolyte according to claim 1, wherein the concentration of acetate in the electrolyte is 7.5 to 30 g/L, preferably 10 to 20 g/L, most preferably 12 g/L.

6. The electrolyte according to claim 1, wherein the total concentration of glycine and/or alanine in the electrolyte is in the range of 0.5 to 20 g/L, preferably 1.0 to 10 g/L, more preferably 1.5 to 5 g/L and most preferably 2.5 g/L.

7. The electrolyte according to claim 1, wherein the electrolyte contains

(a) Zn2+ in a concentration of 20 to 60 g/L;
(b) Cl− in a concentration of 120 to 190 g/L;
(c) K+ in a concentration of 2.7 to 4.8 mol/L and 0 to 0.5 mol/L Nat;
(d) acetate in a concentration of 7.5 to 30 g/L; and
(e) glycine and/or alanine in a total concentration of 0.5 to 20 g/L.

8. The electrolyte according to claim 1, wherein the electrolyte additionally contains (g) nicotinic acid in a concentration of 0.01 to 2.0 g/L, preferably 0.05 to 1.0 g/L, more preferably 0.08 to 0.5 g/L, most preferably 0.1 g/L.

9. The electrolyte according to claim 1, wherein the electrolyte additionally contains (h) ethoxylated thiodiglycol, with an average of at least 20 structural units derived from ethylene oxide, in a concentration of 0.3 to 10 g/L, preferably 0.5 to 5 g/L, more preferably 1.0 to 3.5 g/L and most preferably 2.1 g/L.

10. The electrolyte according to claim 1, wherein the electrolyte contains

(g) nicotinic acid in a concentration of 0.05 to 1.0 g/L, and/or
(h) ethoxylated thiodiglycol, with an average of at least 20 structural units derived from ethylene oxide, in a concentration of 0.5 to 5 g/L.

11. A method for producing an aqueous electrolyte according to claim 1, comprising the steps of:

A) forming an aqueous solution of (a′) zinc chloride and/or zinc acetate; (b′) potassium chloride and/or sodium chloride; (c′) at least one of potassium acetate, sodium acetate and acetic acid; (d′) at least one from the group consisting of glycine, a salt thereof, alanine and a salt thereof; and (e′) optionally nicotinic acid or a salt thereof; and (f′) optionally ethoxylated thiodiglycol, with an average of at least 20 structural units derived from ethylene oxide; and
B) optionally setting the pH to 4.5 to 6.5 by adding hydrochloric acid or by adding potassium hydroxide and/or sodium hydroxide, which can be added as solids or in the form of an aqueous solution.

12. A method for producing a component having a zinc coating, comprising the electrodeposition of zinc on a metal component from an electrolyte according to claim 1.

13. The method according to claim 12, wherein the metal component comprises or consists of iron or an iron alloy.

14. The method according to claim 12, wherein deposition occurs at a temperature of 20 to 50° C., preferably 25 to 40° C., and

a current density of 0.2 to 10 A/dm2, preferably 0.5 to 6 A/dm2, is used for the deposition.

15. The method according to claim 12, wherein after the electrodeposition the component having a zinc coating is subjected to a passivation treatment.

16. The method according to claim 12, wherein after the electrodeposition the component having a zinc coating is annealed, optionally before or after a passivation treatment.

Patent History
Publication number: 20220064814
Type: Application
Filed: Dec 9, 2019
Publication Date: Mar 3, 2022
Applicant: Dr.-lng. Max Schlötter GmbH & Co. KG (Geislingen/Steige)
Inventors: Ralph Krauss (Böbingen), Vera Lipp (Essingen), Ingo Messerschmid (Donzdorf)
Application Number: 17/312,812
Classifications
International Classification: C25D 21/14 (20060101); C25D 3/22 (20060101);