(19)
(11)EP 3 164 531 B1

(12)EUROPEAN PATENT SPECIFICATION

(45)Mention of the grant of the patent:
13.05.2020 Bulletin 2020/20

(21)Application number: 15731989.8

(22)Date of filing:  01.07.2015
(51)Int. Cl.: 
C25D 3/22  (2006.01)
C25D 3/56  (2006.01)
C25D 3/24  (2006.01)
(86)International application number:
PCT/EP2015/065014
(87)International publication number:
WO 2016/001317 (07.01.2016 Gazette  2016/01)

(54)

ADDITIVE FOR ALKALINE ZINC PLATING

ADDITIV ZUR ALKALISCHEN VERZINKUNG

ADDITIF POUR ZINCAGE ALCALIN


(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR

(30)Priority: 04.07.2014 EP 14175798

(43)Date of publication of application:
10.05.2017 Bulletin 2017/19

(73)Proprietor: BASF SE
67056 Ludwigshafen am Rhein (DE)

(72)Inventors:
  • BAUER, Frederic
    67146 Deidesheim (DE)
  • URBAN, Tobias
    64625 Bensheim (DE)
  • ESKUCHEN, Rainer
    40764 Langenfeld (DE)
  • MAKSYM, Lukas
    51107 Koeln (DE)
  • SCHRIEFERS, Yvonne
    67056 Ludwigshafen (DE)

(74)Representative: Zumstein, Angela 
Maiwald Patentanwalts- und Rechtsanwaltsgesellschaft mbH Elisenhof Elisenstraße 3
80335 München
80335 München (DE)


(56)References cited: : 
CN-A- 103 122 468
US-A1- 2005 133 376
US-A1- 2011 052 896
GB-A- 1 164 874
US-A1- 2008 264 799
US-A1- 2014 107 230
  
      
    Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention).


    Description

    Field of the invention



    [0001] The present invention refers to a process for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate, an aqueous alkaline plating bath for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate and the use of a zinc plating bath additive in a process for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate for improving the optical appearance and/or the adhesion of a zinc or zinc alloy coating on a metallic substrate. Furthermore, the present specification describes a zinc or zinc alloy coated metallic substrate having a specific gloss.

    Background of the invention



    [0002] The electrolytic deposition of zinc onto metallic substrates from alkaline solution for the preparation of zinc coated metallic substrates is widely used to prevent such metallic substrates from corrosion and to impart specific optical and mechanical properties on the resulting end product. A process of such electrolytic deposition typically comprises applying a current density to a metal substrate to be zinc coated while placing said substrate in a zinc plating bath. Due to the applied current, zinc ions dissolved in the zinc plating bath deposit on the metallic substrate surface such that a zinc coating is formed thereon.

    [0003] In the art, several attempts for improving the electrolytic deposition of zinc onto metallic substrates in alkaline solution have been proposed. For instance, US 2012/0138473 A1 refers to a zinc plating bath additive enabling the rapid formation of a zinc coating having small variations in the thickness depending on the position on the surface of an object to be plated. The zinc plating bath additive contains a water soluble copolymer having, as the structural units, two amine compounds. WO 03/006360 A2 refers to an alkaline zinc-nickel electroplating bath that comprises zinc ions, nickel ions, a primary brightener, which is an N-methylpyridinium compound substituted at the 3-position of the pyridine ring with a carboxylate group or a group which is hydrolyzable to a carboxylate group, and a secondary brightener, which is an aliphatic amine. US 3,886,054 A refers to non-cyanide, alkaline electroplating baths for bright zinc plating containing quarternized polymeric condensates of alkylene polyamines and 1,3-dihalo-2-propanol as grain refiners preferably in admixture with aldehyde type brighteners and mercapto substituted heterocyclic compounds capable of producing bright, fine grained deposits over a broad current density range. US 2005/133376 A1 refers to an aqueous zinc-nickel electroplating bath, including water; nickel ion; zinc ion; at least one complexing agent; and at least one non-ionogenic, surface active polyoxyalkylene compound, wherein the bath has an alkaline pH. However, the preparation of zinc coated metallic substrates by the electrolytic deposition of zinc or zinc alloy onto the substrate is challenging. For instance, during the electrolytic deposition of zinc or a zinc alloy onto a metallic substrate, hydrogen is generated which tends to adhere on the coating surface as small gas bubbles resulting in a zinc or zinc alloy coating formed on the metallic substrate having worsened optical appearances. Such worsened optical appearances are typically visible on the surface in the form of stripes. In addition thereto, such formation of bubbles also decreases the adhesion of the zinc coating on the metallic substrate which may be detectable as small blisters on the surface and thus also decreased mechanical properties are obtained. Thus, the addition of surfactants into the plating bath would be desirable in order to support the formation of an even coating on the metallic substrate and thus to improve the optical appearance of the zinc or zinc alloy coated metallic substrate surface. In this regard, it is to be noted that surfactants considered as being suitable in zinc plating processes should be soluble in the plating bath. However, such water-soluble surfactants also tend to stabilize foams generated during the deposition process which may then interfere with the deposition of zinc or zinc alloy on the metallic substrate such that an uneven coating is formed thereon resulting again in optically deteriorated appearances. In contrast thereto, surfactants which are known as being sufficient as regards the non-stabilization of foams are typically insoluble in the aqueous zinc plating bath and are thus considered unsuitable in such baths.

    [0004] GB 1 164 874 describes electrodeposition of zinc, e.g. on Fe, Cu, Mg or alloys thereof such as steel or brass, from an aqueous alkaline pyrophosphate bath containing, as brighteners (1) at least one aldehyde or ketone, or bisulphitic combination thereof; and (2) at least one glucoside or polyoside. The zinc may be added to the bath as Zn pyrophosphate, ZnSO4, ZnCl2 or ZnO, and the pH is adjusted to 10-11 by adding KOH or pyrophosphoric acid.

    [0005] US 2014/107230 relates to a wetting agent comprising a biodegradable surfactant which is present in a solution in water, and allows foam to be formed during an electroplating application. The stability of the foam formed allows retarded release of the hydrogen that is formed during the galvanic application.

    [0006] CN 103 122 468 relates to a preparation method of flaky zinc powder. The preparation method comprises the following steps in sequence: machining metallic zinc into a flake and using the flake as a soluble anode; in an anode chamber of an electrochemical reactor, taking zinc sulfate and sulfuric acid water solution as an electrolyte, oxidizing the soluble anode into zinc ions and enabling the zinc ions to enter the anolyte; and after solid-liquid separation and anolyte impurity removal, adding an alkyl glycoside surfactant to the anolyte and adjusting the pH value to 5.0-6.5, then injecting the electrolyte into a cathode chamber of the electrochemical reactor, carrying out electrochemical reduction reaction on the cathode and separating out metallic zinc through deposition, simultaneously starting an ultrasonic generating device to strip metallic zinc on the cathode and deposit metallic zinc in the electrolyte below the cathode and then obtaining the qualified flaky zinc powder through solid-liquid separation, washing, vacuum drying, screening and packaging.

    [0007] Thus, there is a need in the art for providing a process which avoids the foregoing disadvantages and especially allows for the preparation of a zinc or zinc alloy coated metallic substrate imparting very well optical characteristics to a resulting end product while its mechanical properties are kept on a high level or even improved. In particular, it is desirable to provide a process for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate which is well balanced with regard to optical appearances resulting from the non-formation of foam and bubbles in the plating bath on the one hand and the adhesion of the zinc or zinc alloy coating on the metallic substrate on the other hand.

    [0008] Accordingly, it is an object of the present invention to provide a process for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate. Furthermore, it is an object of the present invention to provide a process in which a zinc or zinc alloy coating is formed on the metallic substrate having an even thickness. It is an even further object of the present invention to provide a process in which the optical appearance of the resulting zinc or zinc alloy coating formed on the metallic substrate is improved. Another object of the present invention is to provide a process in which the mechanical properties of the resulting zinc or zinc alloy coating formed on the metallic substrate are kept on a high level or are even improved. A still further object of the present invention is to provide a process in which a good wetting of the metallic substrate surface is obtained such that an improved release of gas bubbles is caused improving the optical appearance of the resulting zinc or zinc alloy coated metallic substrate. A further object of the present invention is to provide a process in which the obtained zinc or zinc alloy coated metallic substrate is the result of well-balanced properties with regard to the wetting behavior as well as the adhesion of the zinc or zinc alloy coating on the metallic substrate. Further objects can be gathered from the following description of the invention.

    Summary of the invention



    [0009] The foregoing and other objects are solved by the subject-matter of the present invention. According to a first aspect of the present invention, a process for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate is provided. The process comprises at least the steps of:
    1. a) providing an aqueous alkaline plating bath comprising
      1. i) a source of zinc ions,
      2. ii) a source of hydroxide ions, and
      3. iii) a zinc plating bath additive being at least one compound of the general formula (I),

        wherein R is C4-C6-alkyl; G1 is selected from monosaccharides with 4 to 6 carbon atoms; x is in the range of from 1 to 4 and refers to average values, and
    2. b) placing a metallic substrate in the aqueous alkaline plating bath such that a zinc or zinc alloy coating is formed on the metallic substrate.


    [0010] The present specification also describes a zinc or zinc alloy coated metallic substrate having a gloss being defined by inequation (I)

    wherein

    (GUwithout) is the gloss unit determined on a metallic substrate coated without using the at least one compound of the general formula (I) defined herein and as measured with a gloss meter at a measuring angle of 85°,

    (GUwith) is the gloss unit determined on a metallic substrate coated by using the at least one compound of the general formula (I) defined herein and as measured with a gloss meter at a measuring angle of 85°.



    [0011] According to a further aspect of the present invention, an aqueous alkaline plating bath for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate is provided, wherein the bath comprises
    1. a) a source of zinc ions as defined herein,
    2. b) a source of hydroxide ions as defined herein, and
    3. c) a zinc plating bath additive as defined herein.


    [0012] According to a still further aspect, an use of a zinc plating bath additive as defined herein for improving the optical appearance and/or the adhesion of a zinc or zinc alloy coating on a metallic substrate is provided. According to still another aspect, an use of a zinc plating bath additive for improving the optical and/or mechanical surface properties of a zinc or zinc alloy coating on a cast iron substrate is provided.

    [0013] Advantageous embodiments of the inventive process for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate are defined in the corresponding sub-claims.

    [0014] According to one embodiment, the source of zinc ions is zinc oxide and/or the zinc ions are present in the aqueous alkaline plating bath in an amount of from 2.0 to 30.0 g/L bath.

    [0015] According to another embodiment, the source of hydroxide ions is sodium hydroxide and/or the hydroxide ions are present in the aqueous alkaline plating bath in an amount of from 50.0 to 250.0 g/L bath.

    [0016] According to one embodiment, in the general formula (I) R is C4-alkyl; G1 is glucose and/or xylose and/or arabinose and x is in the range of from 1 to 1.8.

    [0017] According to another embodiment, the zinc plating bath additive is present in the aqueous alkaline plating bath in an amount of from 0.1 to 10.0 g/L bath.

    [0018] According to yet another embodiment, the aqueous alkaline plating bath has a pH of from 12.0 to 14.0.

    [0019] According to one embodiment, the aqueous alkaline plating bath further comprises at least one conventional additive selected from the group comprising brightener such as high-gloss brightener, basic brightener and mixtures thereof, water-soluble polymers, leveling agents, water softener, complexing agents, a source of cyanide ions and mixtures thereof.

    [0020] According to another embodiment, process step b) is carried out at a temperature of from 10 to 40 °C.

    [0021] According to yet another embodiment, process step b) is carried out at a current density of from 0.05 to 15.0 A/dm2.

    [0022] According to one embodiment, the zinc or zinc alloy coating formed on the metallic substrate has a thickness of from 2.0 to 30.0 µm.

    [0023] In the following, the details and preferred embodiments of the inventive process will be described in more detail. It is to be understood that these technical details and embodiments also apply to the zinc or zinc alloy coated metallic substrate obtainable by the process, the inventive aqueous alkaline plating bath for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate and its use.

    Detailed description of the invention



    [0024] According to step a) of the instant process an aqueous alkaline plating bath is provided.

    [0025] The term "aqueous" alkaline plating bath refers to a system in which the solvent comprises, preferably consists of, water. However, it is to be noted that said term does not exclude that the solvent comprises minor amounts of a water-miscible organic solvent selected from the group comprising methanol, ethanol, acetone, acetonitrile, tetrahydrofuran and mixtures thereof. If the solvent comprises a water-miscible organic solvent, the water-miscible organic solvent is present in an amount from 0.01 to 10.0 wt.-%, preferably from 0.01 to 7.5 wt.-%, more preferably from 0.01 to 5.0 wt.-% and most preferably from 0.01 to 2.5 wt.-%, based on the total weight of the solvent. For example, the solvent of the aqueous alkaline plating bath consists of water. If the solvent of the aqueous alkaline plating bath consists of water, the water to be used can be any water available such as tap water and/or deionised water, preferably deionised water.

    [0026] The term aqueous "alkaline" plating bath refers to a system having a pH of > 7. For example, the aqueous alkaline plating bath has a pH of from 12.0 to 14.0, more preferably of from 13.0 to 14.0.

    [0027] It is one requirement of the instant process that the aqueous alkaline plating bath comprises a source of zinc ions.

    [0028] It is appreciated that the aqueous alkaline plating bath may contain a source of zinc ions that is known to the skilled person as being suitable as source of zinc ions in an aqueous alkaline plating bath.

    [0029] For example, the source of zinc ions is selected from the group comprising zinc, zinc oxide, zinc sulfate, zinc carbonate, zinc sulfamate, zinc acetate and mixtures thereof. Preferably, the source of zinc ions is zinc oxide. Zinc oxide is present as zincate in the aqueous alkaline plating bath.

    [0030] The aqueous alkaline plating bath preferably contains the source of zinc ions such that the amount of zinc ions in the bath is in a range usual for such bath. Thus, the zinc ions are preferably present in the aqueous alkaline plating bath in an amount of from 2.0 to 30.0 g/L bath, preferably from 5.0 to 25.0 g/L bath and most preferably from 5.0 to 20.0 g/L bath.

    [0031] The corresponding amount of the source of zinc ions to be used in the present process is determined by appropriate calculation in order to reach the given amount of zinc ions.

    [0032] In one embodiment, the aqueous alkaline plating bath comprises in addition to the source of zinc ions a further source of metal ions such that a zinc alloy coating is formed on the metallic substrate by the instant process.

    [0033] It is appreciated that the further source of metal ions can be any source of metal ions that is known to the skilled person as being suitable as source of metal ions in an aqueous alkaline plating bath in combination with a source of zinc ions. However, the further source of metal ions preferably comprises ions of nickel, manganese, cobalt, iron and mixtures thereof.

    [0034] Preferably, the further source of metal ions may be any source of metal ions which is soluble in the aqueous alkaline plating bath. For example, the source of metal ions is selected from the group comprising nickel sulfate, manganese chloride, cobalt sulfate, iron sulfate and mixtures thereof.

    [0035] If the aqueous alkaline plating bath comprises a further source of metal ions, the bath may contain the further source of metal ions in a wide range. For example, the metal ions obtained from the further source of metal ions are present in the aqueous alkaline plating bath in an amount of from 0.1 to 100.0 g/L bath, preferably from 0.2 to 75.0 g/L bath and most preferably from 0.5 to 50.0 g/L bath.

    [0036] Accordingly, if the aqueous alkaline plating bath comprises a further source of metal ions, the bath preferably contains the zinc ions in an amount of from 2.0 to 30.0 g/L bath, preferably from 5.0 to 25.0 g/L bath and most preferably from 5.0 to 20.0 g/L bath and the metal ions obtained from the further source of metal ions in an amount of from 0.1 to 100.0 g/L bath, preferably from 0.2 to 75.0 g/L bath and most preferably from 0.5 to 50.0 g/L bath.

    [0037] The corresponding amount of the further source of metal ions to be used in the present process in order to reach the given amount of metal ions is determined by appropriate calculation.

    [0038] It is appreciated that the aqueous alkaline plating bath functions as catholyte. The anode can be any anode, such as stainless steel or platinum-coated titanium anodes or soluble zinc anodes, that is known to the skilled person as being suitable in processes for the electrolytic deposition of a zinc or zinc coating on a metallic substrate in which the zinc or zinc alloy coating is formed in an aqueous alkaline plating bath.

    [0039] As already mentioned above, the plating bath has an alkaline pH. Thus, it is a further requirement of the instant process that the aqueous alkaline plating bath comprises a source of hydroxide ions.

    [0040] It is appreciated that the aqueous alkaline plating bath comprises a source of hydroxide ions that is known to the skilled person as being suitable to adjust the pH of an aqueous alkaline plating bath to the desired alkaline pH.

    [0041] For example, the source of hydroxide ions is selected from sodium hydroxide and/or potassium hydroxide, preferably sodium hydroxide.

    [0042] The aqueous alkaline plating bath comprises the source of hydroxide ions in an amount being sufficient to provide the aqueous alkaline plating bath with the desired alkaline pH.

    [0043] Preferably, the aqueous alkaline plating bath comprises the source of hydroxide ions in an amount such that the aqueous alkaline plating bath has a pH of > 7, preferably from 12.0 to 14.0 and most preferably of from 13.0 to 14.0. For example, the hydroxide ions are preferably present in the aqueous alkaline plating bath in an amount of from 50.0 to 250.0 g/L bath, preferably from 50.0 to 200.0 g/L bath and most preferably from 50.0 to 150.0 g/L bath.

    [0044] The corresponding amount of the source of hydroxide ions to be used in the present process in order to reach the given amount of hydroxide ions is determined by appropriate calculation.

    [0045] The aqueous alkaline plating bath further comprises a zinc plating bath additive. The zinc plating additive is at least one compound of the general formula (I),

    wherein R is C4-C6-alkyl; G1 is selected from monosaccharides with 4 to 6 carbon atoms; x is in the range of from 1 to 4 and refers to average values.

    [0046] Said zinc plating bath additive improves the process for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate in that only a small amount of foam or no foam is formed and, if foam is formed, in that it can be easily rinsed off from the metallic substrate. This also severely reduces the amount of foam which is attached to the zinc or zinc alloy coating on the metallic substrate when it is taken out of the aqueous alkaline plating bath such that the formation of foam marks on the coated substrate surface is clearly reduced in the present process. Thus, it was surprisingly found that the addition of the instant zinc plating bath additive in a process for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate results in a zinc or zinc alloy coated metallic substrate having improved optical appearances. Furthermore, said zinc plating bath additive has the advantage that it shows a good wetting behavior such that the release of gas bubbles is improved from the metallic substrate resulting in a coated substrate surface showing less or no stripes resulting from such bubbles. Also, the adhesion of the zinc or zinc alloy coating on the metallic substrate is excellent by using said zinc plating bath additive. Accordingly, the optical properties are improved, i.e. less or no foam marks and stripes, and the mechanical properties of the resulting zinc or zinc alloy coating formed on the metallic substrate are kept on a high level or are even improved by using said zinc plating bath additive.

    [0047] The term "at least one" zinc plating bath additive means that the zinc plating bath additive comprises, preferably consists of, one or more zinc plating bath additive(s).

    [0048] In one embodiment, the at least one zinc plating bath additive(s) comprises, preferably consists of, one zinc plating bath additive. Alternatively, the at least one zinc plating bath additive(s) comprises, preferably consists of, two or more zinc plating bath additives. For example, the at least one zinc plating bath additive(s) comprises, preferably consists of, two or three zinc plating bath additives. In other words, if the at least one zinc plating bath additive(s) of general formula (I) comprises, preferably consists of, two or more zinc plating bath additives, the at least one zinc plating bath additive(s) of general formula (I) comprises, preferably consists of, a mixture of different zinc plating bath additives.

    [0049] If the at least one zinc plating bath additive(s) of general formula (I) is a mixture of different zinc plating bath additives, the mixture comprises, preferably consists of, three to twenty zinc plating bath additives of general formula (I). For example, the mixture of the zinc plating bath additives of general formula (I) comprises, preferably consists of, five to fifteen zinc plating bath additive(s) of general formula (I) or the mixture of the zinc plating bath additives of general formula (I) comprises, preferably consists of, five to ten zinc plating bath additive(s) of general formula (I).

    [0050] Preferably, the at least one zinc plating bath additive(s) comprises, more preferably consists of, one zinc plating bath additive.

    [0051] In the general formula (I), R is C4-C6-alkyl such as substituted or unsubstituted, linear or branched C4-C6-alkyl. For example, R is C4-alkyl such as substituted or unsubstituted, linear or branched C4-alkyl or R is C5-alkyl such as substituted or unsubstituted, linear or branched C5-alkyl or R is C6-alkyl such as substituted or unsubstituted, linear or branched C6-alkyl. Most preferably, R is C4-alkyl such as substituted or unsubstituted, linear or branched C4-alkyl such as substituted or unsubstituted, linear C4-alkyl.

    [0052] As used herein, the term "alkyl" is a radical of a saturated aliphatic group, including linear chain alkyl groups and branched chain alkyl groups, wherein such linear and branched chain alkyl groups may each be optionally substituted with a hydroxyl group.

    [0053] In one embodiment, R is unsubstituted linear C4-C6-alkyl. For example, R is unsubstituted linear C4-alkyl or unsubstituted linear C5-alkyl or unsubstituted linear C6-alkyl. Most preferably R is unsubstituted linear C4-alkyl.

    [0054] Alternatively, R is unsubstituted branched C5-alkyl, such as isoamyl.

    [0055] In the general formula (I), G1 is selected from monosaccharides with 4 to 6 carbon atoms. For example, G1 is selected from tetroses, pentoses, and hexoses. Examples of tetroses are erythrose, threose, and erythulose. Examples of pentoses are ribulose, xylulose, ribose, arabinose, xylose and lyxose. Examples of hexoses are galactose, mannose and glucose. Monosaccharides may be synthetic or derived or isolated from natural products, hereinafter in brief referred to as natural saccharides or natural polysaccharides, and natural saccharides natural polysaccharides being preferred. More preferred are the following natural monosaccharides: galactose, glucose, arabinose, xylose, and mixtures of the foregoing, even more preferred are glucose, arabinose and xylose, and in particular glucose. Monosaccharides can be selected from any of their enantiomers, naturally occurring enantiomers and naturally occurring mixtures of enantiomers being preferred. Naturally, in a specific molecule only whole groups of G1 can occur.

    [0056] Thus, if G1 in the general formula (I) is a tetrose, the tetrose may be selected from erythrose such as D-erythrose, L-erythrose and mixtures thereof, preferably D-erythrose, threose such as D-threose, L- threose and mixtures thereof, preferably D-threose, and erythulose such as D-erythulose, L-erythulose and mixtures thereof, preferably D-erythulose. If G1 in the general formula (I) is a pentose, the pentose may be selected from ribulose such as D-ribulose, L-ribulose and mixtures thereof, preferably D-ribulose, xylulose such as D-xylulose, L-xylulose and mixtures thereof, preferably D-xylulose, ribose such as D-ribose, L-ribose and mixtures thereof, preferably D-ribose, arabinose such as D-arabinose, L-arabinose and mixtures thereof, preferably L-arabinose, xylose such as D-xylose, L-xylose and mixtures thereof, preferably D-xylose and lyxose such as D-lyxose, L-lyxose and mixtures thereof, preferably D-lyxose. If G1 in the general formula (I) is a hexose, the hexose may be selected from galactose such as D-galactose, L-galactose and mixtures thereof, preferably D-galactose, mannose such as D-mannose, L-mannose and mixtures thereof, preferably D-mannose and glucose such as D-glucose, L-glucose and mixtures thereof, preferably D-glucose. More preferably, G1 in the general formula (I) is glucose, preferably D-glucose, galactose, preferably D-galactose, arabinose, preferably D-arabinose, xylose, preferably D-xylose, and mixtures of the foregoing, even more preferably G1 in the general formula (I) is glucose, preferably D-glucose, arabinose, preferably L-arabinose, and xylose, preferably D-xylose, and in particular glucose, preferably D-glucose.

    [0057] In one embodiment of the present invention, G1 is selected from monosaccharides with 6 carbon atoms, preferably from glucose, most preferably from D-glucose.

    [0058] In the general formula (I), x is in the range of from 1 to 4, preferably x is in the range of from 1 to 2 and most preferably x is in the range of from 1 to 1.8. In one embodiment, x is 1. In the context of the present invention, x refers to average values, and x is not necessarily a whole number. In a specific molecule only whole groups of G1 can occur. It is preferred to determine x by High Temperature Gas Chromatography (HTGC), e.g. 400°C, in accordance with K. Hill et al., Alkyl Polyglycosids, VCH Weinheim, New York, Basel, Cambrigde, Tokyo, 1997, in particular pages 28 ff..

    [0059] In one embodiment, the zinc plating additive is at least one compound of the general formula (I),

    wherein R is C4-C6-alkyl; G1 is selected from monosaccharides with 4 to 6 carbon atoms; x is in the range of from 1 to 4 and refers to average values.

    [0060] For example, the zinc plating additive is at least one compound of the general formula (I),

    wherein R is C6-alkyl; G1 is selected from monosaccharides with 4 to 6 carbon atoms; x is in the range of from 1 to 4 and refers to average values.

    [0061] Alternatively, the zinc plating additive is at least one compound of the general formula (I),

    wherein R is C5-alkyl; G1 is selected from monosaccharides with 4 to 6 carbon atoms; x is in the range of from 1 to 4 and refers to average values.

    [0062] Alternatively, the zinc plating additive is at least one compound of the general formula (I),

    wherein R is C4-alkyl; G1 is selected from monosaccharides with 4 to 6 carbon atoms; x is in the range of from 1 to 4 and refers to average values.

    [0063] Preferably, the zinc plating additive is at least one compound of the general formula (I),

    wherein R is C4-C6-alkyl; G1 is selected from monosaccharides with 5 or 6 carbon atoms; and x is in the range of from 1 to 2 and refers to average values.

    [0064] For example, the zinc plating additive is at least one compound of the general formula (I),

    wherein R is C6-alkyl; G1 is selected from monosaccharides with 5 or 6 carbon atoms; and x is in the range of from 1 to 2 and refers to average values.

    [0065] Alternatively, the zinc plating additive is at least one compound of the general formula (I),

    wherein R is C5-alkyl; G1 is selected from monosaccharides with 5 or 6 carbon atoms; and x is in the range of from 1 to 2 and refers to average values.

    [0066] Alternatively, the zinc plating additive is at least one compound of the general formula (I),

    wherein R is C4-alkyl; G1 is selected from monosaccharides with 5 or 6 carbon atoms; and x is in the range of from 1 to 2 and refers to average values.

    [0067] In one embodiment, the zinc plating additive is at least one compound of the general formula (I),

    wherein R is C4-C6-alkyl; G1 is glucose and/or xylose and/or arabinose and x is in the range of from 1 to 1.8 and refers to average values.

    [0068] For example, the zinc plating additive is at least one compound of the general formula (I),

    wherein R is C6-alkyl; G1 is glucose and/or xylose and/or arabinose and x is in the range of from 1 to 1.8 and refers to average values.

    [0069] Alternatively, the zinc plating additive is at least one compound of the general formula (I),

    wherein R is C5-alkyl; G1 is glucose and/or xylose and/or arabinose and x is in the range of from 1 to 1.8 and refers to average values.

    [0070] More preferably, the zinc plating additive is at least one compound of the general formula (I),

    wherein R is C4-alkyl; G1 is glucose and/or xylose and/or arabinose and x is in the range of from 1 to 1.8 and refers to average values.

    [0071] In an alternative embodiment, the zinc plating additive is at least one compound of the general formula (I),

    wherein R is C4-C6-alkyl; G1 is glucose and x is in the range of from 1 to 1.8 and refers to average values.

    [0072] For example, the zinc plating additive is at least one compound of the general formula (I),

    wherein R is C6-alkyl; G1 is glucose and x is in the range of from 1 to 1.8 and refers to average values.

    [0073] Alternatively, the zinc plating additive is at least one compound of the general formula (I),

    wherein R is C5-alkyl; G1 is glucose and x is in the range of from 1 to 1.8 and refers to average values.

    [0074] Most preferably, the zinc plating additive is at least one compound of the general formula (I),

    wherein R is C4-alkyl; G1 is glucose and x is in the range of from 1 to 1.8 and refers to average values.

    [0075] If the at least one zinc plating bath additive(s) of general formula (I) comprises, preferably consists of, two or more zinc plating bath additives, the two or more zinc plating bath additives present in the aqueous alkaline plating bath differ in at least one of the groups R, G1 and x in the general formula (I). That is to say, the groups R, G1 and/or x can be independently selected from each other.

    [0076] For example, if the at least one zinc plating bath additive(s) of general formula (I) comprises, preferably consists of, two or more zinc plating bath additives, R may be independently selected from C4-C6-alkyl such as substituted or unsubstituted, linear or branched C4-C6-alkyl and most preferably from C4-alkyl such as substituted or unsubstituted, linear or branched C4-alkyl or C5-alkyl such as substituted or unsubstituted, linear or branched C5-alkyl or C6-alkyl such as substituted or unsubstituted, linear or branched C6-alkyl, for each zinc plating bath additive, while G1 and x in the general formula (I) are the same for each zinc plating bath additive. Alternatively, x may be independently selected from the range of from 1 to 4, preferably from the range of from 1 to 2 and most preferably from the range of from 1 to 1.8, while R and G1 in the general formula (I) are the same for each zinc plating bath additive. Alternatively, G1 may be independently selected from monosaccharides with 4 to 6 carbon atoms, preferably from monosaccharides with 5 or 6 carbon atoms and more preferably from glucose and/or xylose and/or arabinose for each zinc plating bath additive, while R and x in the general formula (I) are the same for each zinc plating bath additive. For example, if the at least one zinc plating bath additive(s) of general formula (I) comprises, preferably consists of, two or more zinc plating bath additives, preferably two zinc plating bath additives, G1 is glucose for one zinc plating bath additive and G1 is xylose for another zinc plating bath additive, while R and x in the general formula (I) are the same for each zinc plating bath additive. Alternatively, if the at least one zinc plating bath additive(s) of general formula (I) comprises, preferably consists of, two or more zinc plating bath additives, preferably two zinc plating bath additives, G1 is arabinose for one zinc plating bath additive and G1 is xylose for another zinc plating bath additive, while R and x in the general formula (I) are the same for each zinc plating bath additive. Alternatively, if the at least one zinc plating bath additive(s) of general formula (I) comprises, preferably consists of, two or more zinc plating bath additives, preferably three zinc plating bath additives, G1 is glucose for one zinc plating bath additive and G1 is xylose for another zinc plating bath additive and G1 is arabinose for another zinc plating bath additive, while R and x in the general formula (I) are the same for each zinc plating bath additive. Further examples of advantageous mixtures of monosaccharides G1 are described in the example section of, for example, DE 695 04 158 T2 and DE 697 12 602 T2.

    [0077] Examples of advantageous mixtures of monosaccharides G1 with artificially prepared monosaccharides are also described in, for example, DE 695 04 158 T2 and DE 697 12 602 T2.

    [0078] In one embodiment, the at least one zinc plating bath additive(s) of general formula (I) is an alkyl glycosid.

    [0079] It is appreciated that the term "glycosid" refers to (G1)x in the general formula (I) as defined above. Preferably, the term "glycosid" refers to (G1)x in the general formula (I) in which x is above 1. Thus, the term "glycosid" preferably refers to (G1)x being an oligosaccharide, more preferably a dissacharide, wherein the at least two monosaccharides G1 are selected from xylose, glucose, galactose and arabinose. For example, the term "glycosid" refers to (G1)x being a disaccharide composed of xylose and glucose or xylose and galactose or xylose and arabinose or glucose and galactose or glucose and arabinose or galactose and arabinose, more preferably xylose and glucose.

    [0080] For example, the at least one zinc plating bath additive(s) of general formula (I) is an alkyl glycosid, wherein the alkyl group is C4-C6-alkyl such as substituted or unsubstituted, linear or branched C4-C6-alkyl and most preferably C4-alkyl such as substituted or unsubstituted, linear or branched C4-alkyl or C5-alkyl such as substituted or unsubstituted, linear or branched C5-alkyl or C6-alkyl such as substituted or unsubstituted, linear or branched C6-alkyl.

    [0081] Preferably, the at least one zinc plating bath additive(s) of general formula (I) is an alkyl glycosid selected from the group comprising hexyl glycosid, isoamyl glycosid, butyl glycosid and mixtures thereof. More preferably, the at least one zinc plating bath additive(s) of general formula (I) is an alkyl glycosid selected from isoamyl glycosid, butyl glycosid and mixtures thereof.

    [0082] In one embodiment, the at least one zinc plating bath additive(s) of general formula (I) is a mixture of different zinc plating bath additives, wherein the mixture preferably comprises, more preferably consists of, butyl glucosid and a further zinc plating bath additive selected from the group comprising isoamyl glucosid, isoamyl xylosid, isoamyl glycosid and mixtures thereof. For example, the mixture of different zinc plating bath additives comprises, preferably consists of, butyl glucosid and isoamyl glucosid or isoamyl xylosid or isoamyl glycosid. Alternatively, the mixture of different zinc plating bath additives comprises, preferably consists of, butyl xylosid and a further zinc plating bath additive selected from the group comprising isoamyl glucosid, isoamyl xylosid, isoamyl glycosid and mixtures thereof. For example, the mixture of different zinc plating bath additives comprises, preferably consists of, butyl xylosid and isoamyl glucosid or isoamyl xylosid or isoamyl glycosid. Alternatively, the mixture of different zinc plating bath additives comprises, preferably consists of, butyl glycosid and a further zinc plating bath additive selected from the group comprising butyl xylosid, isoamyl glucosid, isoamyl xylosid, isoamyl glycosid and mixtures thereof. For example, the mixture of different zinc plating bath additives comprises, preferably consists of, butyl glycosid and isoamyl glucosid or isoamyl xylosid or isoamyl glycosid. Alternatively, the mixture of different zinc plating bath additives comprises, preferably consists of, hexyl glycosid and a further zinc plating bath additive selected from the group comprising butyl glucosid, butyl xylosid, butyl glycosid, isoamyl glucosid, isoamyl xylosid, isoamyl glycosid and mixtures thereof. For example, the mixture of different zinc plating bath additives comprises, preferably consists of, hexyl glucosid and butyl glucosid or butyl xylosid or butyl glycosid or isoamyl glucosid or isoamyl xylosid or isoamyl glycosid. Alternatively, the mixture of different zinc plating bath additives comprises, preferably consists of, hexyl xylosid and a further zinc plating bath additive selected from the group comprising butyl glucosid, butyl xylosid, butyl glycosid, isoamyl glucosid, isoamyl xylosid, isoamyl glycosid and mixtures thereof. For example, the mixture of different zinc plating bath additives comprises, preferably consists of, hexyl xylosid and butyl glucosid or butyl xylosid or butyl glycosid or isoamyl glucosid or isoamyl xylosid or isoamyl glycosid. Alternatively, the mixture of different zinc plating bath additives comprises, preferably consists of, hexyl glycosid and a further zinc plating bath additive selected from the group comprising butyl glucosid, butyl xylosid, butyl glycosid, isoamyl glucosid, isoamyl xylosid, isoamyl glycosid and mixtures thereof. For example, the mixture of different zinc plating bath additives comprises, preferably consists of, hexyl glycosid and butyl glucosid or butyl xylosid or butyl glycosid or isoamyl glucosid or isoamyl xylosid or isoamyl glycosid.

    [0083] In one embodiment, the at least one zinc plating bath additive(s) of general formula (I) is selected from alkyl glucosid, alkyl xylosid and mixtures thereof. For example, the at least one zinc plating bath additive(s) of general formula (I) is an alkyl glucosid and/or alkyl xylosid, wherein the alkyl group is C4-C6-alkyl such as substituted or unsubstituted, linear or branched C4-C6-alkyl and most preferably C4-alkyl such as substituted or unsubstituted, linear or branched C4-alkyl or C5-alkyl such as substituted or unsubstituted, linear or branched C5-alkyl or C6-alkyl such as substituted or unsubstituted, linear or branched C6-alkyl.

    [0084] Preferably, the at least one zinc plating bath additive(s) of general formula (I) is preferably selected from the group comprising butyl glycosid, isoamyl glycosid, , isoamyl xylosid, hexyl glycosid, and mixtures thereof. More preferably, the at least one zinc plating bath additive(s) of general formula (I) is selected from the group comprising butyl glycosid, isoamyl glycosid, glycosid and mixtures thereof. Even more preferably, the at least one zinc plating bath additive(s) of general formula (I) is selected from butyl glycosid, isoamyl glycosid and mixtures thereof. Most preferably, the at least one zinc plating bath additive(s) of general formula (I) is butyl glycosid.

    [0085] In one embodiment, the at least one zinc plating bath additive(s) of general formula (I) is preferably selected from the group comprising butyl glucosid, isoamyl glucosid, isoamyl xylosid, hexyl glucosid, and mixtures thereof. More preferably, the at least one zinc plating bath additive(s) of general formula (I) is selected from the group comprising butyl glucosid, isoamyl glucosid, hexyl glucosid and mixtures thereof. Even more preferably, the at least one zinc plating bath additive(s) of general formula (I) is selected from butyl glucosid, hexyl glucosid and mixtures thereof. Most preferably, the at least one zinc plating bath additive(s) of general formula (I) is butyl glucosid.

    [0086] It is appreciated that the compounds of the general formula (I) can be present in the alpha and/or beta conformation. For example, the at least one zinc plating bath additive(s) of general formula (I) is in the alpha or beta conformation, preferably beta conformation. Alternatively, the at least one zinc plating bath additive(s) of general formula (I) is in the alpha and beta conformation.

    [0087] If the at least one zinc plating bath additive(s) of general formula (I) is in the alpha and beta conformation, the at least one zinc plating bath additive(s) of general formula (I) comprise the alpha and beta conformation preferably in a ratio (α/β) from 10:1 to 1:10, more preferably from 5:1 to 1:10, even more preferably from 4:1 to 1:10 and most preferably from 3:1 to 1:10.

    [0088] It is appreciated that compounds of the general formula (I) are well known in the art and can be prepared by methods well known to the skilled person.

    [0089] In one embodiment of the present invention, the compound of the general formula (I) is present in the bleached form or the unbleached form, preferably the bleached form.

    [0090] The aqueous alkaline plating bath preferably contains the at least one zinc plating bath additive(s) of general formula (I) in an amount of from 0.1 to 10.0 g/L bath, preferably from 0.1 to 7.5 g/L bath and most preferably from 0.1 to 5.0 g/L bath.

    [0091] The corresponding amount of the at least one zinc plating bath additive(s) of general formula (I) to be used in the present process is based on the active amount of the at least one zinc plating bath additive(s) of general formula (I).

    [0092] The aqueous alkaline plating bath may further comprise at least one conventional additive selected from the group comprising brightener, water-soluble polymers, leveling agents, water softener, complexing agents, a source of cyanide ions and mixtures thereof.

    [0093] For example, the aqueous alkaline plating bath may comprise known brightener, which can be classified as basic brightener and high-gloss brightener. Examples of advantageous basic brighteners are polyethyleneimines or their derivatives and/or reaction products of epichlorohydrin with heterocyclic nitrogen compounds such as imidazole, 1,2,4-triazole or their derivatives as described in, for example, US patent number 4,166,778. Preferably, the basic brightener is a reaction product of epichlorohydrin with heterocyclic nitrogen compounds such as imidazole, 1,2,4-triazole or their derivatives as described in, for example, US patent number 4,166,778.

    [0094] The aqueous alkaline plating bath preferably comprises a basic brightener in a total amount of from 0.1 to 15.0 g/L bath, and preferably from 1.0 to 10.0 g/L bath.

    [0095] In general, the high-gloss brightener includes substances from a large variety of classes such as for example brightener selected from the group comprising aldehydes, ketones, amines, polyvinyl alcohol, polyvinyl pyrrolidone, sulfur compounds, polyamines or heterocyclic nitrogen compounds and mixtures thereof as described in, for example, US patent number 6,652,728 B1 and US patent number 4,496,439 and WO 2007/147603 A2.

    [0096] Preferably, the high-gloss brightener is n-benzylnicotinat.

    [0097] The aqueous alkaline plating bath preferably comprises the high-gloss brightener in a total amount of from 0.01 to 2.0 g/L bath, preferably from 0.01 to 0.5 g/L bath.

    [0098] Additionally or alternatively, the aqueous alkaline plating bath comprises known water-soluble polymers as polarization reagents such as cationic polymers, anionic polymers, amphoteric polymers and mixtures thereof, preferably cationic polymers. Examples of advantageous polarization reagents are the reaction products of N,N'-bis[3-(dialkylamino)alkyl]ureas with 1,ω-dihalogen alkanes as described in, for example, US patent number 6,652,728 B1.

    [0099] The instant aqueous alkaline plating bath preferably comprises the water-soluble polymer in a total amount of from 0.1 to 15.0 g/L bath, preferably from 1.0 to 10.0 g/L bath.

    [0100] Additionally or alternatively, the aqueous alkaline plating bath comprises known leveling agents such as 3-mercapto-1,2,4-triazole and/or thiourea, preferably thiourea. The instant aqueous alkaline plating bath preferably comprises the leveling agent in a total amount of from 0.1 to 2.0 g/L bath, preferably from 0.1 to 1.0 g/L bath.

    [0101] Additionally or alternatively, the aqueous alkaline plating bath comprises known water softener such as EDTA, sodium silicates, tartaric acid and mixtures thereof. The instant aqueous alkaline plating bath preferably comprises the water softener in a total amount of from 0.1 to 2.0 g/L bath, preferably from 0.1 to 1.0 g/L bath.

    [0102] Additionally or alternatively, the aqueous alkaline plating bath comprises known complexing agents such as sodium gluconate, diethanolamine, triethanolamine, polyethylenediamine, EDTA, aminotris(methylenephosphonic acid), sorbitol, sucrose and mixtures thereof. The instant aqueous alkaline plating bath preferably comprises the complexing agent in a total amount of from 0.1 to 100.0 g/L bath, preferably from 0.1 to 50.0 g/L bath.

    [0103] Additionally or alternatively, the aqueous alkaline plating bath comprises known sources of cyanide ions such as so sodium cyanide, potassium cyanide and mixtures thereof. The instant aqueous alkaline plating bath preferably comprises the source of cyanide ions in a total amount of from 25.0 to 150.0 g/L bath, preferably from 50.0 to 100.0 g/L bath and most preferable about 75 g/L bath.

    [0104] According to step b) of the instant process, a metallic substrate is placed in the aqueous alkaline plating bath such that a zinc or zinc alloy coating is formed on the metallic substrate.

    [0105] It is appreciated that the aqueous alkaline plating bath of the invention can be used for all kinds of metallic substrates. Examples of useful metallic substrates include steel, stainless steel, chrome-molybdenum steel, copper, copper-zinc alloys, cast iron and the like.

    [0106] In one embodiment, the metallic substrate is selected from steel, stainless steel, chrome-molybdenum steel, copper, copper-zinc alloys and the like. In an alternative embodiment, the metallic substrate is cast iron.

    [0107] Preferably, the electrolytic deposition of the zinc or zinc alloy coating on the metallic substrate such that a zinc or zinc alloy coating is formed thereon in process step b) is carried out at a temperature of from 10 to 40 °C, preferably from 15 to 35 °C and most preferably from 15 to 30 °C such as of about room temperature.

    [0108] Additionally or alternatively, the electrolytic deposition of the zinc or zinc alloy coating on the metallic substrate such that a zinc or zinc alloy coating is formed thereon in process step b) is carried out at a current density of from 0.05 to 15.0 A/dm2, preferably from 0.1 to 7.0 A/dm2 and most preferably from 0.1 to 5.0 A/dm2.

    [0109] In one embodiment, process step b) is carried out at a temperature of from 10 to 40 °C, preferably from 15 to 35 °C and most preferably from 15 to 30 °C such as of about room temperature and at a current density of from 0.05 to 15.0 A/dm2, preferably from 0.1 to 7.0 A/dm2 and most preferably from 0.1 to 5.0 A/dm2.

    [0110] The zinc or zinc alloy coating formed on the metallic substrate by the instant process preferably has a thickness of from 2.0 to 30.0 µm, more preferably from 2.0 to 25.0 µm and most preferably from 5.0 to 25.0 µm.

    [0111] It is appreciated that the zinc or zinc alloy coated metallic substrate obtained by the instant process has very well optical and mechanical characteristics. For example, the zinc or zinc alloy coated metallic substrate surface has high gloss at low amount of optical deteriorations such as stripes and/or foam marks generated on the zinc or zinc alloy coated metallic substrate during the instant process. In one embodiment, the zinc or zinc alloy coated metallic substrate obtained by the instant process has high gloss and is free of optical deteriorations such as stripes and/or foam marks generated on the zinc or zinc alloy coated metallic substrate. Furthermore, the zinc or zinc alloy coated metallic substrate provides an excellent adhesion of the zinc or zinc alloy coating on the metallic substrate. Accordingly, the zinc or zinc alloy coated metallic substrate obtained by the instant process has an improved optical appearance and/or adhesion of the zinc or zinc alloy coating on the metallic substrate.

    [0112] In view of the advantages obtained, the present specification further describes a zinc or zinc alloy coated metallic substrate having a gloss being defined by inequation (I)

    wherein

    (GUwithout) is the gloss unit determined on a metallic substrate coated without using the at least one compound of the general formula (I) defined herein and as measured with a gloss meter at a measuring angle of 85°,

    (GUwith) is the gloss unit determined on a metallic substrate coated by using the at least one compound of the general formula (I) defined herein and as measured with a gloss meter at a measuring angle of 85°.



    [0113] Preferably, the zinc or zinc alloy coated metallic substrate has a gloss being defined by inequation (Ia)

    wherein

    (GUwithout) is the gloss unit determined on a metallic substrate coated without using the at least one compound of the general formula (I) defined herein and as measured with a gloss meter at a measuring angle of 85°,

    (GUwith) is the gloss unit determined on a metallic substrate coated by using the at least one compound of the general formula (I) defined herein and as measured with a gloss meter at a measuring angle of 85°.



    [0114] More preferably, the zinc or zinc alloy coated metallic substrate has a gloss being defined by inequation (lb)

    wherein

    (GUwithout) is the gloss unit determined on a metallic substrate coated without using the at least one compound of the general formula (I) defined herein and as measured with a gloss meter at a measuring angle of 85°,

    (GUwith) is the gloss unit determined on a metallic substrate coated by using the at least one compound of the general formula (I) defined herein and as measured with a gloss meter at a measuring angle of 85°.



    [0115] Preferably, the zinc or zinc alloy coated metallic substrate has a gloss being defined by inequation (Ic)

    wherein

    (GUwithout) is the gloss unit determined on a metallic substrate coated without using the at least one compound of the general formula (I) defined herein and as measured with a gloss meter at a measuring angle of 85°,

    (GUwith) is the gloss unit determined on a metallic substrate coated by using the at least one compound of the general formula (I) defined herein and as measured with a gloss meter at a measuring angle of 85°.



    [0116] For example, the zinc or zinc alloy coated metallic substrate has a gloss being defined by inequation (Id)

    wherein

    (GUwithout) is the gloss unit determined on a metallic substrate coated without using the at least one compound of the general formula (I) defined herein and as measured with a gloss meter at a measuring angle of 85°,

    (GUwith) is the gloss unit determined on a metallic substrate coated by using the at least one compound of the general formula (I) defined herein and as measured with a gloss meter at a measuring angle of 85°.



    [0117] It is appreciated that the gloss unit is measured with the gloss meter Micro-Tri-Gloss of BYK Gardner, Germany, and is the average of ten measurements.

    [0118] In one embodiment, the zinc or zinc alloy coated metallic substrate is obtainable by the process for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate as defined herein.

    [0119] The instant specification further describes a zinc or zinc alloy coated metallic substrate obtainable by the process of the instant invention.

    [0120] Furthermore, the present invention is directed to an aqueous alkaline plating bath as defined herein for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate. Furthermore, the present invention is directed to the use of a zinc plating bath additive as defined herein for improving the optical appearance and/or adhesion of a zinc or zinc alloy coating on a metallic substrate. The metallic substrate is preferably selected from steel, stainless steel, chrome-molybdenum steel, copper, copper-zinc alloys and the like.

    [0121] The present invention is also directed to an aqueous alkaline plating bath as defined herein for the electrolytic deposition of a zinc or zinc alloy coating on a cast iron substrate. Furthermore, the present invention is directed to the use of a zinc plating bath additive as defined herein for improving the optical appearance and/or adhesion of a zinc or zinc alloy coating on a cast iron substrate.

    [0122] The scope and interest of the invention will be better understood based on the following examples which are intended to illustrate certain embodiments of the invention and are non-limitative.

    EXAMPLES


    Example 1



    [0123] The properties of the present zinc plating bath additives on the formation of foam were demonstrated in aqueous alkaline plating baths for which an electrolyte composition as outlined in table 1 below was prepared.
    Table 1: Electrolyte composition of the aqueous alkaline plating bath
    IngredientAmount based on bath
    Zinc oxide 14.94 g/L
    Sodium hydroxide 130.0 g/L
    Sodium carbonate 40.0 g/L


    [0124] To the electrolyte composition of table 1 further additives as outlined in table 2 below were added.
    Table 2: Further additives of the aqueous alkaline plating bath
    Further additive#Amount* based on bath
    Polarization reagent 4.8 g/L
    Basic brigthener 4.4 g/L
    High-gloss brigthener 50.0 mg/L
    #: the polarization reagent is a commercially available cationic reaction product of N,N'-bis[3-(dialkylamino)alkyl]ureas with 1,ω-dihalogen alkane having an active content of ∼ 62 wt.-%; the basic brightener is a commercially available copolymer of imidazole and epichlorohydrin having an active content of ∼ 45 wt.-% and the high-gloss brightener is a commercially available n-benzylnicotinate having an active content of ∼ 48 wt.-%.
    *: amount of ingredient is base on the amount of the active material.


    [0125] To the aqueous alkaline plating bath obtained from the ingredients and the further additives described in tables 1 and 2, a zinc plating bath additive as outlined in table 3 below was added in an amount of 1.0 g/L bath, based on the active material. Examples marked with (+) serve for comparison.

    [0126] The electrodeposition of the zinc coating on the substrate was carried out in a hull cell in accordance with DIN 50 957. Each bath was added to a 250 mL hull cell in which a steel panel was plated at 1 A for 30 min. The steel panels (steel number 1.0330 according to EN 10027-2) had the dimensions 70x100x0.3 mm. Before the steel panel was placed in the hull cell, the panel was acid cleaned by using hydrochloric acid (15%), subjected to an electrolytic degreasing and rinsed with water. A stainless steel anode served as anode. The bath was operated at room temperature (about 20 °C ± 1 °C).

    [0127] The optical appearance of the obtained zinc coated metallic substrate and the foam development during the process are summarized in table 3 below.
    Table 3: The zinc plating bath additive, optical appearance of the obtained zinc coated substrate and foam development
    TestZinc plating bath additiveAppearanceFoam development
    1(+) -- matt finish, streaky; HCD: matt, rare stripes rare foam development, foam remains in the bath
    2 Butyl glucosid matt finish, streaky; HCD, MCD: improved to test 1 rare foam development, foam remains in the bath
    3 Isoamyl xylosid matt finish, streaky; MCD: improved to test 1 rare foam development, foam remains in the bath
    4 Hexyl glucosid matt finish, streaky; MCD: improved to test 1 rare foam development, foam remains in the bath


    [0128] From table 3, it can be gathered that a zinc coated metallic substrate prepared by using the zinc plating bath additive of the instant invention shows improved optical characteristic compared to a zinc coated metallic substrate prepared without using the zinc plating bath additive of the instant invention.

    Example 2



    [0129] The properties of the present zinc plating bath additives on the gloss of a coated substrate were determined in aqueous alkaline plating baths for which an electrolyte/additive composition as outlined in table 4 below was prepared.
    Table 4: Electrolyte composition of the aqueous alkaline plating bath
    IngredientAmount based on bath
    Zinc oxide 9.34 g/L
    Sodium hydroxide 97.0 g/L
    Sodium carbonate 35.0 g/L
    Polarization reagent#1 7.7 g/L
    Basic brigthener#2 7.0 g/L
    High-gloss brigthener#3 50.0 mg/L
    Butyl glucosid 1.0 g/L
    #1: the polarization reagent is a commercially available cationic reaction product of N,N'-bis[3-(dialkylamino)alkyl]ureas with 1 ,ω-dihalogen alkane having an active content of ∼ 62 wt.-%; #2: the basic brightener is a commercially available copolymer of imidazole and epichlorohydrin having an active content of ∼ 45 wt.-%; #3: and the high-gloss brightener is a commercially available n-benzylnicotinate having an active content of ∼ 48 wt.-%.


    [0130] The electrodeposition of the zinc coating on the substrate was carried out in a hull cell in accordance with DIN 50 957. The bath was added to a 250 mL hull cell in which a steel panel was plated at 1 A for 40 min. The steel panels (steel number 1.0330 according to EN 10027-2) had the dimensions 70x100x0.3 mm. Before the steel panel was placed in the hull cell, the panel was acid cleaned by using hydrochloric acid (15%), subjected to an electrolytic degreasing and rinsed with water. A stainless steel anode served as anode. The bath was operated at room temperature (about 20 °C ± 1 °C).

    [0131] The optical appearance of the obtained zinc coated metallic substrate and of a reference sample being coated in the absence of butyl glucosid are summarized in table 5 below. Furthermore, the gloss unit determined by using the gloss meter Micro-Tri-Gloss of BYK Gardner, Germany (serial number: 9 014 327) at a measuring angle of 85° for the metallic substrate coated with a zinc plating bath additive in accordance with the present invention as well as for the reference sample, i.e. the metallic substrate is coated in the absence of the zinc plating bath additive of the present application, are also outlined in table 5 below. The set-up is carried out in accordance with the operating instruction manual of the gloss meter Micro-Tri-Gloss. The gloss unit values are the average of ten measurements. The standard deviation of the gloss unit is ± 2 GU (GU = gloss unit).
    Table 5: Optical appearance
    Zinc plating bath additiveAppearanceGloss Unit
    -- Glossy finish with rare stripes 71.4
    Butyl glucosid Glossy finish with rare stripes, less stripes than test 1 113.9


    [0132] From table 5, it can be gathered that a zinc coated metallic substrate prepared by using the zinc plating bath additive of the instant invention shows improved gloss compared to a zinc coated metallic substrate prepared without using the zinc plating bath additive of the instant invention.

    Example 3



    [0133] The properties of the present zinc plating bath additives on the adhesion of the coating determined by the formation of blisters were demonstrated in aqueous alkaline plating baths for which an electrolyte composition as outlined in table 6 below was prepared.
    Table 6: Electrolyte composition of the aqueous alkaline plating bath
    IngredientAmount based on bath
    Zinc oxide 14.94 g/L
    Sodium hydroxide 130.0 g/L
    Sodium carbonate 40.0 g/L


    [0134] To the electrolyte composition of table 6 further additives as outlined in table 7 below were added.
    Table 7: Further additives of the aqueous alkaline plating bath
    Further additive#Amount* based on bath
    Polarization reagent 3.0 g/L
    Basic brigthener 1.1 g/L
    High-gloss brigthener 50.0 mg/L
    #: the polarization reagent is a commercially available cationic reaction product of N,N'-bis[3-(dialkylamino)alkyl]ureas with 1 ,ω-dihalogen alkane having an active content of ∼ 62 wt.-%; the basic brightener is a commercially available copolymer of imidazole and epichlorohydrin having an active content of ∼ 45 wt.-% and the high-gloss brightener is a commercially available n-benzylnicotinate having an active content of ∼ 48 wt.-%.
    *: amount of ingredient is base on the amount of the active material.


    [0135] To the aqueous alkaline plating bath obtained from the ingredients and the further additives described in tables 6 and 7, a zinc plating bath additive as outlined in table 8 below was added in an amount of 1.0 g/L bath, based on the active material. Examples marked with (+) serve for comparison.

    [0136] Each bath was added to a parallel cell in which punched steel panels were plated at a current of 1 A/dm2 for 50 min, 0.5 A/dm2 for 75 min or 3 A/dm2 for 25 min on both sides. A soluble zinc anode served as anode. The bath was operated at room temperature (about 20 °C ± 1 °C). The steel panels (steel number 1.0330 according to EN 10027-2) had the dimensions 70x100x0.3 mm. For each zinc plating bath additive three tests were carried out under the same conditions. Before the steel panels were placed in the parallel cell, each steel panel was acid cleaned by using hydrochloric acid (15%), and rinsed with water. Then, each steel panel was subjected to an alkaline degreasing by using an aqueous degreasing solution as outlined in table 8. After the alkaline degreasing, each steel panel was rinsed with water, dried until moisture is no longer visible and weighed.
    Table 8: Composition of the aqueous degreasing solution
    IngredientAmount
    NaOH (pA) 20 g/L
    Na2CO3 (LPW quality) 22 g/L
    Na3PO4 x 12 H2O (pure) 16 g/L
    Trilon® powder #1 1 g/L
    Lutensol® AP 10 #2 0.5 g/L
    #1 is the chelating agent tetrasodium salt of ethylenediaminetetraacetic acid and is commercially available from BASF, Germany.
    #2 is a non-ionic surfactant and is commercially available from BASF, Germany.


    [0137] The aqueous degreasing solution was prepared by dissolving and mixing the single ingredients in distilled water such that a clear solution is obtained.

    [0138] After the coating, the steel panels were rinsed with water, dried until moisture is no longer visible and weighed. Then, the steel panels were wrapped in a foil and stored 3 months at room temperature (about 20 °C ± 1 °C). Subsequently, the steel panel surfaces were evaluated with regard to the formation of pits and blisters. For this, a pressure-sensitive adhesive tape having a width of at least 50 mm and a bonding strength of 6-10 N/25 mm width was attached on the surface of each coated steel panel. The adhesive tapes were evenly pressed on the steel panel surfaces by hand (the even adhesion can be controlled by the color of the steel panel surfaces through the tape) and then quickly removed from the surfaces. The tape removal was carried out by removing the tapes from the steel panel surfaces within 0.5-1 s in an angle of about 60 °. The removal of the tapes was carried out within 5 min after their application on the steel panel surfaces. The tests were carried out at a temperature of about 23 °C ± 2 °C and a humidity of about 50 % ± 5 %. The evaluation of the steel panel surfaces was carried under good illumination from all sides with the naked eye.

    [0139] The adhesion of the coating determined by the formation of pits and blisters observed on the obtained zinc coated substrates are summarized in table 9 below.
    Table 9: The zinc plating bath additive and the coating adhesion of the obtained zinc coated substrate (test 3 not according to the invention)
    TestZinc plating bath additiveCoating adhesion after 3 months
    1(+) -- formation of pits
    2 Butyl glucosid no blisters, good adhesion
    3 2-Ethylhexyl glucosid no blisters, good adhesion
    4 Isoamyl xylosid no blisters, good adhesion
    5 n-Butyl-glucosid-xylosid no blisters, good adhesion


    [0140] From table 9, it can be gathered that a zinc coated metallic substrate prepared by using the zinc plating bath additive of the instant invention shows improved behavior as regards the formation of pits and blisters compared to a zinc coated metallic substrate prepared without using the zinc plating bath additive of the instant invention. Thus, it can be concluded that a zinc coated metallic substrate prepared by using the zinc plating bath additive of the instant invention has improved coating adhesion compared to a zinc coated metallic substrate prepared without using the zinc plating bath additive of the instant invention.

    Example 4



    [0141] The properties of the present zinc plating bath additives on the gloss of a coated cast iron were determined in aqueous alkaline plating baths for which an electrolyte/additive composition as outlined in table 10 below was prepared.
    Table 10: Electrolyte composition of the aqueous alkaline plating bath
    IngredientAmount based on bath
    Zinc oxide 9.34 g/L
    Sodium hydroxide 97.0 g/L
    Sodium carbonate 35.0 g/L
    High-gloss brigthener#1 50.0 mg/L
    Butyl glucosid 2.0 g/L
    #1: the high-gloss brightener is a commercially available n-benzylnicotinate having an active content of ∼ 48 wt.-%.


    [0142] The electrodeposition of the zinc coating on the cast iron was carried out in a hull cell in accordance with DIN 50 957. The bath was added to a 250 mL hull cell in which a cast iron panel was plated at 3 A for 60 min. The cast iron panels were obtained from a cast iron grade according to ASTM A536 and had the dimensions 48x102x4.5mm. Before the cast iron panel was placed in the hull cell, the panel was acid cleaned by using hydrochloric acid (15%), subjected to an electrolytic degreasing and rinsed with water. A stainless steel anode served as anode. The bath was operated at room temperature (about 20 °C ± 1 °C).

    [0143] The gloss unit determined by using the gloss meter Micro-Tri-Gloss of BYK Gardner, Germany (serial number: 9 014 327) at a measuring angle of 60° and 85° for the cast iron substrate coated with a zinc plating bath additive in accordance with the present invention as well as for the reference sample, i.e. the cast iron substrate is coated in the absence of the zinc plating bath additive of the present application, are outlined in table 11 below. The set-up is carried out in accordance with the operating instruction manual of the gloss meter Micro-Tri-Gloss. The gloss unit values are the average of ten measurements. The standard deviation of the gloss unit is ± 2 GU (GU = gloss unit).
    Table 11: Optical appearance
    Zinc plating bath additiveMeasuring angleGloss Unit
    -- 60° 0.4
    -- 85° 0.1
    Butyl glucosid 60° 1.9
    Butyl glucosid 85° 0.7


    [0144] From table 11, it can be gathered that a zinc coated cast iron substrate prepared by using the zinc plating bath additive of the instant invention shows improved gloss compared to a zinc coated cast iron substrate prepared without using the zinc plating bath additive of the instant invention.


    Claims

    1. Process for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate, the process comprises at least the steps of:

    a) providing an aqueous alkaline plating bath comprising

    i) a source of zinc ions,

    ii) a source of hydroxide ions, and

    iii) a zinc plating bath additive being at least one compound of the general formula (I),

    wherein R is C4-C6-alkyl; G1 is selected from monosaccharides with 4 to 6 carbon atoms; x is in the range of from 1 to 4 and refers to average values, and

    b) placing a metallic substrate in the aqueous alkaline plating bath such that a zinc or zinc alloy coating is formed on the metallic substrate.


     
    2. The process according to claim 1, wherein the source of zinc ions is zinc oxide and/or the zinc ions are present in the aqueous alkaline plating bath in an amount of from 2.0 to 30.0 g/L bath.
     
    3. The process according to claim 1 or 2, wherein the source of hydroxide ions is sodium hydroxide and/or the hydroxide ions are present in the aqueous alkaline plating bath in an amount of from 50.0 to 250.0 g/L bath.
     
    4. The process according to any one of claims 1 to 3, wherein in the general formula (I) R is C4-alkyl; G1 is glucose and/or xylose and/or arabinose and x is in the range of from 1 to 1.8.
     
    5. The process according to any one of claims 1 to 4, wherein the zinc plating bath additive is present in the aqueous alkaline plating bath in an amount of from 0.1 to 10.0 g/L bath.
     
    6. The process according to any one of claims 1 to 5, wherein the aqueous alkaline plating bath has a pH of from 12.0 to 14.0.
     
    7. The process according to any one of claims 1 to 6, wherein the aqueous alkaline plating bath further comprises at least one conventional additive selected from the group comprising brightener, water-soluble polymers, leveling agents, water softener, complexing agents, a source of cyanide ions and mixtures thereof.
     
    8. The process according to any one of claims 1 to 7, wherein process step b) is carried out at a temperature of from 10 to 40 °C.
     
    9. The process according to any one of claims 1 to 8, wherein process step b) is carried out at a current density of from 0.05 to 15.0 A/dm2.
     
    10. The process according to any one of claims 1 to 9, wherein the zinc or zinc alloy coating formed on the metallic substrate has a thickness of from 2.0 to 30.0 µm.
     
    11. An aqueous alkaline plating bath for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate, wherein the bath comprises

    a) a source of zinc ions as defined in any one of claims 1 or 2,

    b) a source of hydroxide ions as defined in any one of claims 1 or 3, and

    c) a zinc plating bath additive being at least one compound of the general formula (I),



    wherein R is C4-C6-alkyl; G1 is selected from monosaccharides with 4 to 6 carbon atoms; x is in the range of from 1 to 4 and refers to average values.
     
    12. Use of a zinc plating bath additive for improving the optical and/or mechanical surface properties of a zinc or zinc alloy coating on a metallic substrate, wherein the zinc plating bath additive is at least one compound of the general formula (I),

    wherein R is C4-C6-alkyl; G1 is selected from monosaccharides with 4 to 6 carbon atoms; x is in the range of from 1 to 4 and refers to average values .
     
    13. The use of a zinc plating bath additive according to claim 12 on a cast iron substrate.
     


    Ansprüche

    1. Verfahren zur elektrolytischen Abscheidung eines Zink- oder Zinklegierungsüberzugs auf einem Metallsubstrat, wobei das Verfahren mindestens die folgenden Schritte umfasst:

    a) Bereitstellen eines wässrigen alkalischen Abscheidungsbads, umfassend

    i) eine Quelle von Zinkionen,

    ii) eine Quelle von Hydroxidionen und

    iii) ein Zinkabscheidungsbad-Additiv, bei dem es sich mindestens um eine Verbindung der allgemeinen Formel (I) handelt,



    wobei R für C4-C6-Alkyl steht; G1 aus Monosacchariden mit 4 bis 6 Kohlenstoffatomen ausgewählt ist; x im Bereich von 1 bis 4 liegt und sich auf auf Durchschnittswerte bezieht, und

    b) Einbringen eines Metallsubstrats in das wässrige alkalische Abscheidungsbad derart, dass sich auf dem Metallsubstrat ein Zink- oder Zinklegierungsüberzug bildet.


     
    2. Verfahren nach Anspruch 1, wobei es sich bei der Quelle von Zinkionen um Zinkoxid handelt und/oder die Zinkionen in dem wässrigen alkalischen Abscheidungsbadin einer Menge von 2,0 bis 30,0 g/L Bad vorliegen.
     
    3. Verfahren nach Anspruch 1 oder 2, wobei es sich bei der Quelle von Hydroxidionen um Natriumhydroxid handelt und/oder die Hydroxidionen in dem wässrigen alkalischen Abscheidungsbad in einer Menge von 50, 0 bis 250, 0 g/L Bad vorliegen.
     
    4. Verfahren nach einem der Ansprüche 1 bis 3, wobei in der allgemeinen Formel (I) R für C4-Alkyl steht; G1 für Glucose und/oder Xylose und/oder Arabinose steht und x im Bereich von 1 bis 1,8 liegt.
     
    5. Verfahren nach einem der Ansprüche 1 bis 4, wobei das Zinkabscheidungsbad-Additiv in dem wässrigen alkalischen Abscheidungsbad in einer Menge von 0,1 bis 10,0 g/L Bad vorliegt.
     
    6. Verfahren nach einem der Ansprüche 1 bis 5, wobei das wässrige alkalische Abscheidungsbad einen pH-Wert von 12,0 bis 14,0 aufweist.
     
    7. Verfahren nach einem der Ansprüche 1 bis 6, wobei das wässrige alkalische Abscheidungsbad ferner mindestens ein herkömmliches Additiv umfasst, das aus der Aufheller, wasserlösliche Polymere, Verlaufsmittel, Wasserenthärter, Komplexbildner, eine Quelle von Cyanidionen und Mischungen davon umfassenden Gruppe ausgewählt ist.
     
    8. Verfahren nach einem der Ansprüche 1 bis 7, wobei Verfahrensschritt b) bei einer Temperatur von 10 bis 40 °C durchgeführt wird.
     
    9. Verfahren nach einem der Ansprüche 1 bis 8, wobei Verfahrensschritt b) bei einer elektrischen Stromdichte von 0,05 bis 15,0 A/dm2 durchgeführt wird.
     
    10. Verfahren nach einem der Ansprüche 1 bis 9, wobei der auf dem Metallsubstrat gebildete Zink- oder Zinklegierungsüberzug eine Dicke von 2,0 bis 30,0 µm aufweist.
     
    11. Wässriges alkalisches Abscheidungsbad zur elektrolytischen Abscheidung eines Zink- oder Zinklegierungsüberzug auf einem Metallsubstrat, wobei das Bad Folgendes umfasst:

    a) eine Quelle von Zinkionen gemäß einem der Ansprüche 1 oder 2,

    b) eine Quelle von Hydroxidionen gemäß einem der Ansprüche 1 oder 3,

    c) ein Zinkabscheidungsbad-Additiv, bei dem es sich um mindestens eine Verbindung der allgemeinen Formel (I) handelt,



    wobei R für C4-C6-Alkyl steht; G1 aus Monosacchariden mit 4 bis 6 Kohlenstoffatomen ausgewählt ist; x im Bereich von 1 bis 4 liegt und sich auf auf Durchschnittswerte bezieht.
     
    12. Verwendung eines Zinkabscheidungsbad-Additivs zur Verbesserung der optischen und/oder mechanischen Oberflächeneigenschaften eines Zink- oder Zinklegierungsüberzugs auf einem Metallsubstrat, wobei es sich bei dem Zinkabscheidungsbad-Additiv mindestens um eine Verbindung der allgemeinen Formel (I)I handelt,

    wobei R für C4-C6-Alkyl steht; G1 aus Monosacchariden mit 4 bis 6 Kohlenstoffatomen ausgewählt ist; x im Bereich von 1 bis 4 liegt und sich auf auf Durchschnittswerte bezieht.
     
    13. Verwendung eines Zinkabscheidungsbad-Additivs nach Anspruch 12 auf einem Substrat aus Gusseisen.
     


    Revendications

    1. Procédé pour le dépôt électrolytique d'un revêtement de zinc ou d'alliage de zinc sur un substrat métallique, le procédé comprenant au moins les étapes de :

    a) mise à disposition d'un bain de placage alcalin aqueux comprenant

    i) une source d'ions zinc,

    ii) une source d'ions hydroxyde, et

    iii) un additif de bain de placage au zinc qui est au moins un composé de formule générale (I),



    R étant C4-6-alkyle ; G1 étant choisi parmi des monosaccharides comportant 4 à 6 atomes de carbone ; x étant dans la plage allant de 1 à 4 et faisant référence à des valeurs moyennes, et

    b) placement d'un substrat métallique dans le bain de placage alcalin aqueux de sorte qu'un revêtement de zinc ou d'alliage de zinc est formé sur le substrat métallique.


     
    2. Procédé selon la revendication 1, la source d'ions zinc étant l'oxyde de zinc et/ou les ions zinc étant présents dans le bain de placage alcalin aqueux en une quantité allant de 2,0 à 30,0 g/L de bain.
     
    3. Procédé selon la revendication 1 ou 2, la source d'ions hydroxyde étant l'hydroxyde de sodium et/ou les ions hydroxyde étant présents dans le bain de placage alcalin aqueux en une quantité allant de 50,0 à 250,0 g/L de bain.
     
    4. Procédé selon l'une quelconque des revendications 1 à 3, dans la formule générale (I), R étant C4-alkyle ; G1 étant le glucose et/ou le xylose et/ou l'arabinose et x étant dans la plage allant de 1 à 1,8.
     
    5. Procédé selon l'une quelconque des revendications 1 à 4, l'additif de bain de placage au zinc étant présent dans le bain de placage alcalin aqueux en une quantité allant de 0,1 à 10,0 g/L de bain.
     
    6. Procédé selon l'une quelconque des revendications 1 à 5, le bain de placage alcalin aqueux possédant un pH allant de 12,0 à 14,0.
     
    7. Procédé selon l'une quelconque des revendications 1 à 6, le bain de placage alcalin aqueux comprenant en outre au moins un additif habituel choisi dans le groupe comprenant un brillanteur, des polymères hydrosolubles, des agents nivelants, des adoucisseurs d'eau, des agents complexants, une source d'ions cyanure et des mélanges correspondants.
     
    8. Procédé selon l'une quelconque des revendications 1 à 7, l'étape de procédé b) étant mise en œuvre à une température allant de 10 à 40 °C.
     
    9. Procédé selon l'une quelconque des revendication 1 à 8, l'étape de procédé b) étant mise en œuvre à une densité de courant allant de 0,05 à 15,0 A/dm2.
     
    10. Procédé selon l'une quelconque des revendications 1 à 9, le revêtement de zinc ou d'alliage de zinc formé sur le substrat métallique possédant une épaisseur allant de 2,0 à 30,0 µm.
     
    11. Bain de placage alcalin aqueux pour le dépôt électrolytique d'un revêtement de zinc ou d'alliage de zinc sur un substrat métallique, le bain comprenant

    a) une source d'ions zinc telle que définie selon l'une quelconque des revendications 1 ou 2,

    b) une source d'ions hydroxyde telle que définie selon l'une quelconque des revendications 1 ou 3, et

    c) un additif de bain de placage au zinc qui est au moins un composé de formule générale (I),



    R étant C4-6-alkyle ; G1 étant choisi parmi des monosaccharides comportant 4 à 6 atomes de carbone ; x étant dans la plage allant de 1 à 4 et faisant référence à des valeurs moyennes.
     
    12. Utilisation d'un additif de bain de placage au zinc pour l'amélioration des propriétés optiques et/ou mécaniques de surface d'un revêtement de zinc ou d'alliage de zinc sur un substrat métallique, l'additif de bain de placage au zinc étant au moins un composé de formule générale (I),

    R étant C4-6-alkyle ; G1 étant choisi parmi des monosaccharides comportant 4 à 6 atomes de carbone ; x étant dans la plage allant de 1 à 4 et faisant référence à des valeurs moyennes.
     
    13. Utilisation d'un additif de bain de placage au zinc selon la revendication 12 sur un substrat de fer forgé.
     




    REFERENCES CITED IN THE DESCRIPTION



    This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

    Patent documents cited in the description




    Non-patent literature cited in the description